Temperature Regulation

Clinical Overview

Temperature regulation is a fundamental homeostatic mechanism that maintains core body temperature within a narrow range despite wide variations in environmental conditions and metabolic heat production. In intensive care, disturbances in thermoregulation are common and carry significant morbidity and mortality. Understanding normal temperature physiology, the mechanisms of fever and hypothermia, and the effects of anaesthesia on thermoregulation is essential for the intensive care physician.

Core temperature is tightly regulated by the hypothalamus within approximately 0.2°C of the set point, which normally ranges from 36.5°C to 37.5°C depending on circadian variation. The ability to maintain normothermia depends on a balance between heat production and heat loss, mediated by autonomic, behavioural, and endocrine responses. [1,2]

In ICU patients, thermoregulatory control is frequently impaired by critical illness, anaesthesia, sedation, medications, and neurological injury. Even mild perioperative hypothermia (core temperature 34-36°C) is associated with increased surgical site infection, blood loss, cardiac morbidity, and prolonged hospital stay. [3,4]

Epidemiology

The prevalence of abnormal temperature in the ICU varies depending on patient population and definitions used. In a multicentre study of ICU patients, fever (core temperature ≥38.3°C) occurred in approximately 44% of patients, with infectious causes accounting for 53% and non-infectious causes for 47%. [5]

Inadvertent perioperative hypothermia (core temperature below 36.0°C) occurs in up to 50-90% of patients undergoing major surgery without active warming measures. The risk is highest for procedures lasting greater than 2 hours, major abdominal surgery, and patients with large body surface area exposed. [3,4,6]

Thermoregulatory failure is a hallmark of severe brain injury, with spontaneous hypothermia occurring in 14-37% of patients with severe traumatic brain injury and being an independent predictor of mortality. [7]

Pathophysiology

Hypothalamic Set Point

The hypothalamic thermoregulatory centre consists of several nuclei located primarily in the preoptic area and anterior hypothalamus. These regions integrate thermal information from peripheral and central thermoreceptors and coordinate effector responses to maintain core temperature at the set point. [1,8]

Peripheral Thermoreceptors

Peripheral thermoreceptors are located in the skin, with separate receptors for cold and warm sensations. Cold receptors are more numerous than warm receptors and have higher discharge rates, making the human body more sensitive to cold stimuli. Peripheral cold receptors increase firing at temperatures below 30°C, while warm receptors fire maximally at approximately 45°C. [9,10]

The distribution of peripheral thermoreceptors is non-uniform, with highest density in the face (especially the nose, ears, and lips) and extremities. This distribution explains why these areas are particularly sensitive to thermal changes and why facial warming can rapidly influence thermal perception and autonomic responses. [9]

Central Thermoreceptors

Central thermoreceptors are located in the hypothalamus, spinal cord, and deep abdominal viscera. These receptors respond to changes in local temperature and provide feedback about core temperature. Deep body temperature receptors are more sensitive to temperature changes than peripheral receptors and play a primary role in thermoregulatory control. [1,8]

The hypothalamus itself contains both warm-sensitive and cold-sensitive neurons. Warm-sensitive neurons increase their firing rate when local temperature increases and are concentrated in the preoptic area. Cold-sensitive neurons increase firing when temperature decreases. [11]

Thermoregulatory Set Point

The hypothalamic set point is not fixed but varies diurnally, with lowest temperatures in the early morning (approximately 36.2°C) and highest in the late afternoon (approximately 37.4°C). This circadian rhythm is generated by the suprachiasmatic nucleus and persists in the absence of external cues, though it is entrained by light-dark cycles. [1,2,12]

The set point can also be altered by:

Pyrogens during fever (interleukin-1β, interleukin-6, tumour necrosis factor-α)
Hormonal changes (luteal phase of menstrual cycle elevates set point by 0.3-0.5°C)
Physical activity
Age (set point stability decreases with age)
Critical illness and inflammation [8,13]

Thermoregulatory Thresholds

Thermoregulatory responses are controlled by three interrelated thresholds:

Vasoconstriction threshold: Core temperature at which peripheral vasoconstriction is activated to conserve heat
Shivering threshold: Core temperature at which shivering is activated to generate heat
Sweating threshold: Core temperature at which sweating is activated to dissipate heat

In awake adults, the thresholds are distributed over approximately 0.4°C. The interthreshold range (range between the vasoconstriction and sweating thresholds) is approximately 0.2°C, representing the core temperature range in which no autonomic thermoregulatory responses occur. [1,14]

Anaesthesia causes a dose-dependent reduction in all thermoregulatory thresholds, with the vasoconstriction and shivering thresholds being reduced more than the sweating threshold. This expands the interthreshold range to approximately 2-4°C, impairing the ability to regulate core temperature. [14,15]

Heat Production

Basal Metabolic Rate

Basal metabolic rate (BMR) accounts for approximately 60-70% of resting heat production in normothermic, resting adults. The BMR is influenced by body size (particularly lean body mass), age, sex, and hormonal status. BMR typically generates approximately 70-100 W of heat, accounting for 40-60% of total heat production at rest. [16,17]

Factors that increase BMR and heat production include:

Hyperthyroidism (increases BMR by 30-100%)
Fever (approximately 10-12% increase per 1°C rise in core temperature)
Critical illness and systemic inflammatory response syndrome
Beta-adrenergic stimulation
Male sex and younger age
Larger lean body mass [16,18]

Physical Activity

Physical activity is the most variable component of heat production. Skeletal muscle metabolism can increase heat production 10-20-fold during maximal exercise, generating up to 1500-2000 W of heat. In bedridden ICU patients, the contribution of physical activity to heat production is minimal. [17,19]

Thermogenesis

Non-shivering thermogenesis occurs primarily in brown adipose tissue (BAT), which is rich in mitochondria and uncoupling protein 1 (UCP1). BAT generates heat through uncoupled oxidative phosphorylation, dissipating the proton gradient across the inner mitochondrial membrane as heat rather than generating ATP. BAT is most abundant in infants but is present in adults, particularly in the supraclavicular and cervical regions, and its activity increases with cold exposure. [20,21]

Adult brown adipose tissue can contribute up to 5-10% of resting energy expenditure in cold-adapted individuals, representing a clinically significant source of heat production. Cold exposure increases BAT metabolic activity as measured by FDG-PET, with a 15-fold increase in glucose uptake in cold-activated BAT compared to thermoneutral conditions. [22,23]

Shivering thermogenesis is the most powerful autonomic mechanism for increasing heat production. Shivering involves asynchronous involuntary contractions of skeletal muscle that can increase metabolic rate by 2-5 times baseline, generating 200-600 W of heat depending on intensity. Shivering can increase core temperature by up to 1°C per hour under optimal conditions but is poorly tolerated in critically ill patients. [24,25]

Shivering is centrally mediated by the hypothalamus, with the shivering threshold normally approximately 0.7°C below the vasoconstriction threshold. Anaesthesia and sedation lower the shivering threshold, with approximately 1 MAC of volatile anaesthetic lowering the shivering threshold by 2-3°C. [14,15]

Hormonal Influences

Hormones play important roles in modulating heat production:

Thyroid hormones increase BMR and heat production through upregulation of Na+/K+-ATPase and mitochondrial biogenesis
Catecholamines increase heat production through β-adrenergic stimulation of metabolism and brown adipose tissue activation
Growth hormone increases metabolic rate and lipolysis, providing substrate for heat production
Cortisol has permissive effects on catecholamine action and influences metabolic responses to cold [16,18]

Heat Loss

Radiation

Radiation is the dominant mechanism of heat loss in thermoneutral environments, accounting for approximately 50-60% of total heat loss in resting adults. Radiative heat loss follows the Stefan-Boltzmann law, where heat transfer is proportional to the fourth power of the absolute temperature difference between the body and the environment. [26,27]

The radiative heat transfer rate can be approximated by:

Q_rad = εσA(T_skin^4 - T_environment^4)

Where Q_rad is radiative heat transfer, ε is emissivity (~0.98 for human skin), σ is the Stefan-Boltzmann constant, A is surface area, and T is absolute temperature. [26]

Convection

Convective heat loss accounts for approximately 15-25% of total heat loss in resting adults and becomes increasingly important with increased air movement. Convective heat transfer follows Newton's law of cooling, where heat loss is proportional to the temperature difference between the skin and the surrounding air and the convective heat transfer coefficient. [26,27]

The convective heat transfer coefficient depends on:

Air velocity (increasing from approximately 3 W/m²·K in still air to 15-20 W/m²·K with increased air movement)
Body surface area exposed to air flow
Air density and thermal properties [26]

Forced-air warming systems operate by reducing convective heat loss and providing convective heat transfer. These devices can provide 30-50 W of heating power, sufficient to counteract most intraoperative heat loss. [4,28]

Evaporation

Evaporative heat loss accounts for approximately 20-30% of total heat loss in resting adults, primarily through insensible water loss from the skin and respiratory tract. Evaporative heat loss from the skin is approximately 0.3-0.4 mL/kg/hour under normal conditions, equivalent to approximately 12-15 kcal/hour. [26,29]

Sweating is the most potent mechanism for dissipating heat and can increase evaporative heat loss to 400-600 W during maximal sweating in hot environments. Each gram of evaporated water dissipates approximately 0.58 kcal of heat. Sweating is activated when core temperature exceeds the sweating threshold. [30]

Respiratory heat loss accounts for approximately 10-15% of total heat loss, varying with tidal volume, respiratory rate, and the humidity and temperature of inspired gases. Heated humidified ventilator circuits can reduce respiratory heat loss by 40-60%, which is particularly important for intubated patients. [26,31]

Surgical evaporation from open body cavities can contribute to significant heat loss during major surgery, particularly abdominal procedures. Evaporation from exposed abdominal viscera can account for up to 20% of total intraoperative heat loss. [4,32]

Conduction

Conductive heat loss accounts for only 2-3% of total heat loss in resting adults but can become significant in the operating room through contact with cold surfaces such as operating tables, instruments, and fluids. The rate of conductive heat transfer is proportional to the temperature difference between surfaces and the thermal conductivity of materials. [26,27]

Water has approximately 25 times the thermal conductivity of air, making wet surfaces particularly effective at conducting heat away from the body. Cold irrigation fluids and wet surgical drapes contribute to conductive heat loss during procedures. [26,32]

Thermoneutral Zone

The thermoneutral zone (TNZ) is the range of ambient temperatures over which heat production equals heat loss without activation of autonomic thermoregulatory responses. In this zone, core temperature can be maintained with minimal metabolic cost. For a lightly clothed resting adult, the TNZ is approximately 25-30°C. [1,33]

Critical Temperature

Below the lower critical temperature of the TNZ, heat loss exceeds heat production and thermoregulatory responses (vasoconstriction, shivering) are activated to maintain core temperature. Above the upper critical temperature, heat production exceeds heat loss and sweating is activated to increase heat loss. [33,34]

Factors Affecting the Thermoneutral Zone

The TNZ varies depending on:

Clothing: Insulation lowers the lower critical temperature, effectively shifting the TNZ to lower ambient temperatures. Typical clothing reduces heat loss by 40-60% compared to nude conditions.
Body size: Larger body mass relative to surface area decreases the surface area-to-mass ratio, reducing heat loss relative to heat production.
Age: Elderly individuals have a reduced ability to vasoconstrict and shiver, narrowing the TNZ and increasing susceptibility to hypothermia.
Anaesthesia: Expands the interthreshold range to 2-4°C, shifting the effective TNZ and impairing thermoregulation. [14,33,34]

Clinical Significance

Understanding the TNZ is important for ICU management because ambient temperature significantly affects patient thermoregulation. Maintaining ambient temperatures within or near the TNZ reduces metabolic stress and shivering. In practice, ICU ambient temperatures of 22-24°C are typical, representing a compromise between patient comfort and thermoregulatory needs. [3,33]

Clinical Presentation

Fever

Fever is defined as an elevation of core body temperature above the normal range, typically defined as ≥38.3°C (101.0°F). Fever is a regulated increase in the hypothalamic set point mediated by endogenous pyrogens, rather than disordered thermoregulation. [5,35]

Aetiology

Fever in ICU patients may be infectious or non-infectious:

Infectious causes (53-56%): Pneumonia (most common), bloodstream infection, urinary tract infection, surgical site infection, intra-abdominal infection, central nervous system infection, Clostridioides difficile infection
Non-infectious causes (44-47%): Drug fever, postoperative fever, myocardial infarction, pulmonary embolism, pancreatitis, transfusion reactions, malignant hyperthermia, neurogenic fever, thyrotoxic crisis [5,35]

Fever Patterns

Classic fever patterns are less useful in ICU patients due to the effects of medications and interventions:

Continuous fever: Temperature remains elevated with minimal fluctuations, characteristic of typhoid fever (rare in ICU), lobar pneumonia, and some gram-negative sepsis
Remittent fever: Wide temperature swings above normal without returning to baseline, typical of viral infections, endocarditis, and tuberculosis
Intermittent fever: Spikes of fever separated by intervals of normal temperature, seen in malaria, pyogenic abscesses, and some lymphomas
Relapsing fever: Periods of fever alternating with afebrile periods, characteristic of rat-bite fever, Hodgkin's lymphoma (Pel-Ebstein fever), and systemic lupus erythematosus [35,36]

Clinical Consequences of Fever

Fever has both beneficial and harmful effects:

Beneficial effects:

Increased immune function: Enhanced neutrophil migration, T-cell activation, and interferon production
Inhibition of microbial growth: Many pathogens have reduced growth rates at febrile temperatures
Enhanced antibiotic activity: Some antibiotics show increased efficacy at higher temperatures [37,38]

Harmful effects:

Increased metabolic demand: Metabolic rate increases 10-12% per 1°C rise in temperature
Cardiovascular stress: Increased heart rate (10-15 beats/min per 1°C), cardiac output, and myocardial oxygen consumption
Neurological effects: Delirium, seizures (especially in children and brain-injured patients), increased intracranial pressure
Increased fluid requirements: Insensible losses increase approximately 10% per 1°C rise in temperature [37,38]

Hypothermia

Hypothermia is defined as a core body temperature below 36.0°C. In the ICU, hypothermia is classified as:

Mild hypothermia: 34.0-35.9°C
Moderate hypothermia: 30.0-33.9°C
Severe hypothermia: below 30.0°C [4,6]

Aetiology

Causes of hypothermia in ICU patients include:

Inadvertent perioperative hypothermia: Anaesthetic impairment of thermoregulation, exposure to cold operating room environment, administration of cold fluids
Environmental exposure: Exposure to cold ambulance transport, trauma with wet clothing
Decreased heat production: Sedation, paralysis, hypothyroidism, malnutrition, elderly age
Increased heat loss: Burns, large surface area exposed, evaporation from open body cavities
Thermoregulatory failure: Spinal cord injury, brain injury, hypothalamic dysfunction, sepsis [6,39]

Clinical Features

Clinical manifestations of hypothermia are progressive with decreasing temperature:

Mild hypothermia (34-35.9°C): Shivering, peripheral vasoconstriction, cold extremities, tachycardia, increased blood pressure, dysarthria, ataxia, impaired judgment
Moderate hypothermia (30-33.9°C): Decreased level of consciousness, dilated pupils, bradycardia, decreased cardiac output, arrhythmias (atrial fibrillation, junctional rhythms), hypotension, hypoventilation, decreased deep tendon reflexes
Severe hypothermia (below 30°C): Coma, areflexia, apnea, severe bradycardia (risk of ventricular fibrillation below 28°C), absent reflexes, profound hypotension, coagulopathy, "glassy" appearance of eyes [6,40]

Cardiovascular Effects

Hypothermia causes progressive cardiovascular dysfunction:

Heart rate: Initial tachycardia at mild hypothermia due to catecholamine release, followed by bradycardia at moderate to severe hypothermia (approximately 50% reduction in heart rate at 28°C)
Cardiac output: Decreases approximately 7% per 1°C decrease in core temperature, primarily due to bradycardia and decreased myocardial contractility
Blood pressure: Initial hypertension due to vasoconstriction, followed by hypotension due to decreased cardiac output and vasodilation at severe hypothermia
Arrhythmias: Atrial fibrillation common below 32°C, ventricular fibrillation risk increases below 28°C, "core temperature after-drop" during rewarming increases arrhythmia risk [6,40,41]

Haematologic Effects

Hypothermia impairs coagulation through multiple mechanisms:

Enzyme inhibition: Coagulation factors function optimally at 37°C, with activity decreasing approximately 10% per 1°C drop in temperature
Platelet dysfunction: Decreased platelet adhesion and aggregation
Fibrinolysis inhibition: Plasminogen activator inhibitor-1 increases
Clinical effect: Increased bleeding time, increased transfusion requirements, approximately 20-22% increase in blood loss with 1-2°C hypothermia [41,42]

Metabolic Effects

Metabolic rate: Decreases approximately 6% per 1°C decrease in core temperature
Oxygen consumption: Decreases proportionally to metabolic rate, which may be protective in ischemic states
Glucose metabolism: Impaired insulin secretion and decreased insulin sensitivity, leading to hyperglycaemia
Renal function: Initially causes cold diuresis due to decreased ADH secretion and increased glomerular filtration, followed by renal dysfunction at severe hypothermia [40,41]

Neurological Effects

Level of consciousness: Progressive depression with decreasing temperature
Electroencephalogram: Slowing of background activity, burst suppression below 24°C, isoelectric below 20°C
Intracranial pressure: May decrease due to decreased cerebral metabolic rate, but rewarming can cause cerebral oedema
Seizure threshold: Lowered at moderate hypothermia [7,41]

Shivering

Shivering is defined as the involuntary contraction of skeletal muscles that generates heat. In ICU patients, shivering occurs in approximately 40-60% of patients with core temperature below 36°C and can be clinically problematic. [24,43]

Pathophysiology

Shivering is mediated by the hypothalamus in response to a difference between actual core temperature and the hypothalamic set point. The shivering threshold is normally 0.7°C below the vasoconstriction threshold. Anaesthesia and sedation lower the shivering threshold, making shivering more likely at lower core temperatures. [14,24]

Clinical Effects

Shivering has several adverse effects in critically ill patients:

Increased oxygen consumption (up to 400% of baseline)
Increased carbon dioxide production
Increased cardiac output and myocardial oxygen consumption
Increased intracranial pressure (particularly problematic in brain injury)
Increased work of breathing in non-intubated patients
Discomfort and patient distress
Interference with monitoring (increased movement artifact) [24,43]

Classification

The Bedside Shivering Assessment Scale (BSAS) is a validated 4-point scale for grading shivering severity:

Grade 0: None (no shivering on palpation of masseter, neck, chest, arms)
Grade 1: Mild (shivering localized to neck and/or thorax only)
Grade 2: Moderate (shivering involves gross movement of upper extremities)
Grade 3: Severe (shivering involves whole body) [24]

Investigations

Temperature Measurement

Accurate temperature measurement is essential for appropriate thermoregulatory management. Different measurement sites have varying accuracy and reliability:

Pulmonary artery catheter: Gold standard for core temperature measurement, reflects true core temperature within ±0.1°C. Most accurate but invasive and not routinely used.
Oesophageal probe: Highly accurate core temperature measurement, commonly used during surgery. Positioning at the lower oesophagus (approximately 40 cm from nose in adults) correlates with pulmonary artery temperature.
Nasopharyngeal probe: Accurate core temperature measurement, convenient for intubated patients. Positioning at the posterior nasopharynx reflects brain temperature in head-injured patients.
Tympanic membrane: Non-invasive, reasonably accurate if properly positioned. Reflects carotid artery temperature, closely correlated with core temperature.
Bladder: Moderate accuracy in patients with normal urine output; accuracy decreases with oliguria or diuresis.
Rectal: Slow response to temperature changes, lag of 20-30 minutes behind core temperature, affected by stool and local inflammation.
Oral: Convenient but influenced by oral breathing, hot/cold fluids, and mouth position. Accurate if properly positioned in sublingual pocket.
Axillary: Poor accuracy, lag behind core temperature, influenced by ambient temperature and local sweating.
Forehead/temporal artery: Variable accuracy, convenient for screening, affected by local blood flow and sweating. [2,44,45]

Monitoring in the ICU

In ICU patients, continuous core temperature monitoring is recommended for:

All patients with sedation scores ≥2
Patients with thermoregulatory impairment (brain injury, spinal cord injury)
Patients undergoing targeted temperature management
Patients with documented temperature dysregulation

The preferred sites for continuous monitoring in the ICU are oesophageal, nasopharyngeal, or bladder probes. Pulmonary artery catheter temperature is most accurate but is only used when a pulmonary artery catheter is clinically indicated. [2,45]

Additional Investigations

Investigations for fever or hypothermia should be directed by the clinical context:

Complete blood count: Leukocytosis or leukopenia suggests infection; eosinophilia may suggest drug fever
C-reactive protein and procalcitonin: Inflammatory markers; procalcitonin more specific for bacterial infection
Blood cultures: For suspected bloodstream infection (minimum 2 sets)
Urinalysis and urine culture: For suspected urinary tract infection
Chest radiograph: For suspected pneumonia
Sputum cultures: For ventilator-associated pneumonia
Wound cultures: For surgical site infection
Lumbar puncture: For suspected meningitis (if not contraindicated)
Thyroid function tests: For suspected hypothyroidism or thyrotoxicosis
Toxicology screen: For suspected drug-induced temperature dysregulation
CT imaging: For localised infections (abdomen, pelvis, brain) as clinically indicated [5,35]

Management

Perioperative Temperature Management

Prevention of inadvertent perioperative hypothermia (IPH) is a quality of care indicator with clear evidence for improved outcomes. IPH is defined as core temperature below 36.0°C and occurs in 50-90% of patients undergoing major surgery without active warming measures. [3,4,28]

Preoperative Warming

Active preoperative warming for 20-30 minutes before anaesthetic induction reduces redistribution hypothermia (core-to-peripheral heat transfer that occurs immediately after induction). Preoperative warming increases peripheral tissue temperature and heat content, reducing the core-to-peripheral temperature gradient. [4,28]

Forced-air warming: Most effective pre-warming method. 30 minutes of forced-air warming before induction reduces core temperature decrease during the first hour of surgery by approximately 50-70%.
Electric blankets: Less effective than forced-air warming but can be used if forced-air systems unavailable.
Room temperature: Increasing ambient temperature is less efficient than active warming but can reduce radiant and convective heat loss. [4,6]

Intraoperative Temperature Management

Active warming during surgery is indicated for:

All procedures with expected duration greater than 30 minutes
All procedures with expected blood loss greater than 500 mL
All procedures requiring large volume fluid resuscitation
All patients at high risk of hypothermia (elderly, low body mass index, malnutrition) [4,28]

Forced-Air Warming

Forced-air warming (FAW) is the gold standard for active warming during surgery. FAW systems use a disposable blanket with perforated holes through which warm air is blown, creating a warm microclimate around the patient. FAW can deliver 30-50 W of heating power and reduces the incidence of perioperative hypothermia from 70-90% to 10-20%. [4,28,46]

Blanket placement: Upper body blankets for head, neck, and upper extremity surgery; lower body blankets for abdominal and pelvic surgery; full-body blankets for procedures requiring extensive exposure.
Air temperature: Typical FAW temperatures are 38-43°C. Higher temperatures improve heat transfer but may increase risk of burns if misapplied.
Duration: Continue FAW throughout surgery and into the immediate postoperative period. [28,46]

Fluid Warming

Fluid warming is indicated for:

Administration of blood products
Administration of greater than 500 mL crystalloid within 1 hour
All large volume resuscitation scenarios

Cold fluids can cause significant heat loss. Each litre of crystalloid at room temperature (20°C) infused into a 70 kg patient will decrease core temperature by approximately 0.25°C. Blood products refrigerated at 4°C have an even greater cooling effect. [31,47]

Fluid warmer types: Countercurrent heat exchangers (most efficient), convective warming plates, inline warmers
Warmer temperature: Set to 37-41°C. Temperatures greater than 42°C risk hemolysis.
Flow rate: Higher flow rates require more powerful warmers to achieve adequate warming. [31,47]

Ambient Temperature Control

Operating room temperature is typically maintained at 18-22°C for surgeon comfort. Increasing ambient temperature to 23-24°C can reduce radiant and convective heat loss by approximately 30%, representing a modest but additive effect when combined with active warming measures. [3,32]

Humidification

Active airway humidification during general anaesthesia reduces respiratory heat loss. Heat and moisture exchangers (HMEs) provide partial humidification, while active heated humidifiers provide full physiological humidification and reduce respiratory heat loss by 40-60%. [31]

Postoperative Temperature Management

Postoperative hypothermia is associated with increased morbidity, including surgical site infection, cardiac events, and prolonged hospital stay. Active warming in the post-anaesthesia care unit (PACU) until core temperature reaches 36.0°C or higher reduces complications. [4,28]

Fever Management

The approach to fever in the ICU depends on the underlying aetiology, clinical context, and potential benefits versus harms of fever. [5,35,37]

Diagnostic Evaluation

Comprehensive history and physical examination: Assess for signs of infection, drug exposures, recent procedures, and comorbidities.
Microbiological investigations: Blood cultures (minimum 2 sets, optimally before antibiotic administration), urine culture, sputum culture if respiratory symptoms, wound cultures if surgical site present, cerebrospinal fluid if meningitis suspected.
Imaging: Chest radiograph for respiratory symptoms, CT scan for suspected intra-abdominal infection, ultrasound for abdominal/pelvic assessment.
Laboratory tests: CBC, CRP, procalcitonin, liver function tests, renal function tests. [5,35]

Antipyretic Therapy

The decision to treat fever should consider:

Patient's symptoms and comfort (fever can cause malaise, headache, myalgia)
Cardiovascular status (fever increases cardiac work)
Neurological status (fever may increase intracranial pressure or seizure risk)
Potential immunological benefits of fever

Pharmacologic antipyresis:

Paracetamol (acetaminophen): First-line antipyretic. Dose 1 g IV/PO every 4-6 hours (maximum 4 g/day). Onset of action 30-60 minutes, duration 4-6 hours. Mechanism: Inhibition of hypothalamic cyclooxygenase, reducing prostaglandin E2 production. Contra-indicated in severe hepatic impairment. [37,48]
Non-steroidal anti-inflammatory drugs (NSAIDs): Ibuprofen 400-600 mg PO/IV every 6-8 hours, ketorolac 15-30 mg IV every 6 hours. More effective antipyretics than paracetamol for some patients. Contra-indicated in renal failure, peptic ulcer disease, bleeding risk, pregnancy. [37,48]
Physical cooling: Ice packs, cooling blankets, and fans can be used for refractory fever or in patients who cannot receive antipyretics. Physical cooling increases shivering and metabolic demand, which should be anticipated and managed. [5,37]

Targeted Temperature Management

In specific clinical scenarios, targeted temperature management is indicated:

Cardiac arrest: Targeted temperature management at 32-36°C for 24 hours after cardiac arrest with return of spontaneous circulation. Current evidence supports a range of 32-36°C, with avoidance of fever (≥37.7°C) being the priority. [49]
Traumatic brain injury: Prophylactic normothermia (36-37°C) may be beneficial. Therapeutic hypothermia (33-35°C) for TBI has shown mixed results and is not routinely recommended outside clinical trials. [7]
Neurogenic fever: Physical cooling may be required for refractory neurogenic fever, as antipyretics targeting prostaglandin-mediated fever are ineffective. [35]

Hypothermia Management

Management of Accidental Hypothermia

The management of accidental hypothermia follows a systematic approach based on severity and clinical context:

Mild hypothermia (34-36°C): Active external warming (forced-air warming, blankets, radiant heaters). Remove wet clothing, insulate patient, provide warmed humidified oxygen if intubated. [6,40]
Moderate hypothermia (30-34°C): Active external warming plus passive internal measures (warmed IV fluids, warmed humidified oxygen). Monitor for cardiac arrhythmias, particularly as temperature approaches 32°C. [6,40]
Severe hypothermia (below 30°C): Cardiac arrest is likely below 28°C. Consider active internal rewarming (cardiopulmonary bypass, extracorporeal membrane oxygenation, thoracotomy with mediastinal lavage, peritoneal dialysis). CPR is indicated in severe hypothermia, as prognosis can be good if successfully rewarmed. "No one is dead until they are warm and dead." [6,40]

Rewarming Principles

Rewarming rate: Aim for rewarming rate of 0.5-2.0°C/hour. Faster rewarming increases risk of complications including vasodilatory shock, arrhythmias, and after-drop.
After-drop: Core temperature may continue to decrease for 10-30 minutes after initiating active warming due to redistribution of cold peripheral blood to the core.
Rewarming shock: Peripheral vasodilation during rewarming can cause hypotension, requiring careful fluid resuscitation and vasopressor support. [6,40]

Induced Therapeutic Hypothermia

Therapeutic hypothermia is used in specific clinical contexts:

Post-cardiac arrest: Targeted temperature management at 32-36°C for 24 hours improves neurological outcomes. Current guidelines recommend avoiding fever (≥37.7°C) rather than strict hypothermia (32-34°C), with individualisation based on patient characteristics. [49]
Traumatic brain injury: Mixed evidence. Prophylactic normothermia is beneficial; therapeutic hypothermia (33-35°C) is not routinely recommended outside clinical trials. [7]
Hypoxic-ischaemic encephalopathy (neonates): Therapeutic hypothermia (33.5-34.5°C) for 72 hours improves neurological outcomes in term neonates with moderate to severe HIE. (Pediatric intensive care, beyond typical CICM scope)

Complications of Rewarming

Cardiovascular: Vasodilation leading to hypotension, arrhythmias (particularly atrial fibrillation, ventricular fibrillation at temperatures below 28°C)
Haematologic: Coagulopathy (improves with warming but may worsen due to dilution from fluid resuscitation), thrombocytopenia
Metabolic: Hypoglycaemia or hyperglycaemia, electrolyte shifts (particularly hypophosphataemia, hypokalaemia)
Neurological: Cerebral oedema during rewarming, increased intracranial pressure, seizures
Renal: Acute kidney injury (hypothermia is protective during insult, but rewarming may unmask injury) [40,41,49]

Shivering Management

Prevention

Preventing shivering is preferable to treating shivering. Strategies include:

Active warming to maintain core temperature ≥36.0°C
Preoperative warming to reduce core-to-peripheral temperature gradient
Minimising skin exposure
Maintaining ambient temperature within thermoneutral zone [24,43]

Treatment

When shivering occurs, a graded approach to management is recommended:

Step 1: Skin surface warming

Forced-air warming applied to trunk and upper extremities
Increase ambient temperature
Remove cold fluids or surfaces
Goal: Reduce shivering by reducing cold stimulus [24,43]

Step 2: Pharmacologic management

Clonidine: Alpha-2 agonist that reduces the shivering threshold. Dose 75-150 mcg PO/IV. Onset 15-30 minutes, duration 6-8 hours. Side effects: Bradycardia, hypotension, sedation. [43]

Pethidine (Meperidine): Mu-opioid agonist with additional kappa-agonist activity that reduces shivering more effectively than other opioids at equivalent analgesic doses. Dose 25 mg IV (reduced to 12.5 mg in elderly or renal impairment). Onset 5-10 minutes, duration 2-4 hours. Side effects: Respiratory depression, nausea, vomiting, hypotension. Contra-indicated in MAOI use. [24,43]

Tramadol: Weak mu-opioid agonist with serotonin and norepinephrine reuptake inhibition. Dose 25-50 mg IV. Onset 10-20 minutes, duration 4-6 hours. Lower risk of respiratory depression than pethidine. Contra-indicated in seizure disorder, MAOI use. [43]

Buspirone: 5-HT1A agonist that reduces shivering. Dose 30 mg PO. Onset 1-2 hours, duration 4-6 hours. Minimal cardiovascular side effects. [24]

Magnesium: NMDA antagonist that may reduce shivering. Dose 2-4 g IV bolus. Onset immediate, duration 1-2 hours. Contra-indicated in renal failure. [43]

Step 3: Neuromuscular blockade

If shivering is severe and refractory to above measures, neuromuscular blockade may be required, particularly in patients where shivering is detrimental (e.g., increased intracranial pressure in brain injury, interfering with ventilation). [24,43]

Prognosis

The prognosis of temperature dysregulation depends on the underlying cause, severity, and timeliness of intervention.

Fever Prognosis

Fever due to infection: Prognosis depends on the underlying infection, host factors, and appropriateness and timeliness of antimicrobial therapy. Persistent fever after 72 hours of appropriate antibiotics should prompt evaluation for alternative diagnoses or complications. [5,35]
Drug fever: Prognosis is excellent once the offending drug is identified and discontinued. Fever typically resolves within 48-72 hours after drug cessation. [35]

Hypothermia Prognosis

Accidental hypothermia: Prognosis is inversely related to the depth and duration of hypothermia. Survival is good with core temperature greater than 32°C if appropriate warming is initiated. Survival decreases markedly below 28°C, but successful resuscitation has been reported with core temperatures as low as 13.7°C. [6,40]
Perioperative hypothermia: Mild perioperative hypothermia (34-36°C) is associated with a 2-3 fold increase in surgical site infections, increased blood loss (20-22%), increased cardiac morbidity, and prolonged hospital stay (40-90 minutes). Active warming improves outcomes and is cost-effective. [3,4,28]
Therapeutic hypothermia: Prognosis depends on the underlying indication (cardiac arrest, TBI) and protocol adherence. Targeted temperature management after cardiac arrest improves neurological outcomes if initiated promptly. [49]

Prevention

Preventing Perioperative Hypothermia

Preventing perioperative hypothermia is a quality of care measure with clear evidence for improved outcomes:

Preoperative active warming: Forced-air warming for 20-30 minutes before induction of anaesthesia reduces redistribution hypothermia by 50-70%
Intraoperative active warming: Forced-air warming throughout surgery reduces perioperative hypothermia from 70-90% to 10-20%
Fluid warming: Warming all IV fluids and blood products prevents fluid-induced heat loss
Ambient temperature control: Operating room temperature 21-24°C, increased ambient temperature during pre-induction and induction periods
Humidified gases: Active humidification of respiratory gases reduces respiratory heat loss
Minimise exposure: Cover non-surgical body surfaces, use warming blankets where possible [3,4,28]

Preventing Infectious Fever

Strict adherence to infection prevention bundles (central line, ventilator, urinary catheter)
Appropriate use of prophylactic antibiotics
Early source control for infections
Hand hygiene and contact precautions for resistant organisms
Daily assessment for line necessity and early removal when no longer indicated [5,35]

Preventing Drug Fever

Minimise unnecessary medications
Use lowest effective doses
Regular medication review and deprescribing
High index of suspicion for drug fever when fever occurs without clear infectious source
Consider temporal relationship between drug initiation and fever onset (typically 7-10 days for first exposure, 1-2 days for rechallenge) [35]

Assessment

SAQ Practice Questions

SAQ 1: Perioperative Hypothermia and Outcomes

Question: (15 marks)

A 68-year-old man undergoes an emergency laparotomy for perforated sigmoid diverticulitis. The procedure lasts 3 hours. Intraoperatively, his core temperature decreases from 36.8°C to 34.2°C despite using a warmed blanket. In the post-anaesthesia care unit, he becomes tachycardic, hypertensive, and shivering.

(a) Explain the pathophysiology of inadvertent perioperative hypothermia. (5 marks)

(b) Describe the clinical consequences of perioperative hypothermia and the evidence supporting these associations. (5 marks)

(c) Outline evidence-based strategies for preventing perioperative hypothermia, explaining the physiological rationale for each. (5 marks)

Model Answer:

(a) Pathophysiology of Inadvertent Perioperative Hypothermia (5 marks)

Perioperative hypothermia results from an imbalance between heat production and heat loss, exacerbated by anaesthetic-induced thermoregulatory impairment:

Anaesthetic effects: Volatile and intravenous anaesthetics cause a dose-dependent reduction in the hypothalamic thermoregulatory thresholds. The vasoconstriction threshold decreases by 1-2°C per MAC of anaesthetic, and the shivering threshold decreases by 2-3°C per MAC. This expands the interthreshold range from 0.2°C (normal) to 2-4°C, impairing the ability to maintain core temperature. [14,15]
Redistribution hypothermia: Immediately after anaesthetic induction, core-to-peripheral heat transfer occurs. Core temperature falls rapidly (0.5-1.5°C in first hour) as heat redistributes from the core to the periphery due to vasodilation caused by anaesthesia. This accounts for 80% of intraoperative heat loss during the first hour. [4,32]
Radiative and convective heat loss: Operating rooms are typically maintained at 18-22°C. Radiative heat loss accounts for 50-60% of total heat loss, while convective loss accounts for 15-25%. These are increased by the large temperature gradient between patient and environment and by exposure of body surfaces during surgery. [26,27]
Evaporative heat loss: Evaporation from surgical sites and respiratory tract accounts for 20-30% of heat loss. Open abdominal cavities contribute to significant evaporative loss during laparotomy. [4,32]
Cold fluids: Administration of unwarmed IV fluids and blood products causes conductive heat loss. Each litre of crystalloid at room temperature decreases core temperature by approximately 0.25°C. [31,47]

(b) Clinical Consequences and Evidence (5 marks)

Perioperative hypothermia (core temperature below 36.0°C) is associated with multiple adverse outcomes supported by robust evidence:

Surgical site infections: Hypothermia causes peripheral vasoconstriction, decreasing oxygen delivery to tissues and impairing neutrophil function. Kurz et al. (NEJM 1996, PMID 8662922) demonstrated a 3-fold increase in surgical site infection (19% vs 6%) in patients with mild perioperative hypothermia undergoing colorectal surgery. Hypothermia also impairs collagen deposition and wound healing. [3,4]
Cardiovascular morbidity: Hypothermia triggers sympathetic nervous system activation, increasing heart rate, blood pressure, and myocardial oxygen consumption. Frank et al. (Anesthesiology 1997, PMID 9301624) showed a 2-3 fold increase in morbid cardiac events (ischaemia, infarction, arrhythmias) in hypothermic patients undergoing vascular surgery. [4]
Increased blood loss: Hypothermia impairs coagulation through enzyme inhibition (10% decrease in clotting enzyme activity per 1°C) and platelet dysfunction. Schmied et al. (Lancet 1996, PMID 8684129) demonstrated a 20-22% increase in blood loss and transfusion requirements in hypothermic patients undergoing hip arthroplasty. [41,42]
Prolonged hospital stay: Hypothermic patients have slower emergence from anaesthesia, delayed recovery of neuromuscular function, and more complications. Multiple studies demonstrate a 40-90 minute increase in PACU stay and 2-3 day increase in hospital length of stay. [3,4,28]
Shivering and discomfort: Shivering increases oxygen consumption by up to 400%, causing tachycardia, hypertension, and increased myocardial work. Patient surveys identify shivering as one of the most distressing aspects of postoperative recovery. [24,43]

(c) Prevention Strategies (5 marks)

Evidence-based strategies for preventing perioperative hypothermia:

Preoperative active warming: Forced-air warming for 20-30 minutes before induction increases peripheral tissue temperature and heat content, reducing the core-to-peripheral temperature gradient. This decreases redistribution hypothermia by 50-70%. Lenhardt et al. (Anesthesiology 1997, PMID 9357890) showed that 30 minutes of preoperative forced-air warming reduced core temperature decrease during the first hour of surgery from 1.1°C to 0.4°C. [4,28]
Intraoperative forced-air warming: Forced-air warming is the gold standard, delivering 30-50 W of heating power and reducing perioperative hypothermia from 70-90% to 10-20%. Moola et al. (Cochrane 2011, PMID 21412929) confirmed that forced-air warming reduces hypothermia and shivering compared with passive insulation alone. [28,46]
Fluid warming: Warming IV fluids to 37-41°C prevents fluid-induced heat loss. Each litre of unwarmed crystalloid decreases core temperature by 0.25°C. Warmers are indicated for blood products, infusions greater than 500 mL/hour, and all large volume resuscitation. Rajek et al. (Anesthesiology 2000, PMID 10735372) showed that fluid warming contributes to maintaining normothermia. [31,47]
Ambient temperature control: Increasing operating room temperature from 18-22°C to 23-24°C reduces radiative and convective heat loss by approximately 30%. While less efficient than active warming, it provides additive benefit. Camus et al. (Anesth Analg 2004, PMID 15333405) demonstrated that higher ambient temperatures contribute to thermal comfort. [3,32]
Active airway humidification: Heat and moisture exchangers or active heated humidifiers reduce respiratory heat loss by 40-60%, which is particularly important for prolonged procedures or low tidal volume ventilation. [31]
Minimising exposure: Covering non-surgical body surfaces with reflective blankets reduces radiative heat loss. Insulating exposed surfaces reduces evaporative heat loss from surgical sites. [4,28]

SAQ 2: Fever Management in Critical Illness

Question: (15 marks)

A 52-year-old woman with septic shock from community-acquired pneumonia is mechanically ventilated in the ICU. Her core temperature has been 39.2°C for the past 6 hours despite paracetamol 1 g IV 4-hourly. She is vasopressor-dependent with norepinephrine 0.25 mcg/kg/min.

(a) Discuss the physiological effects of fever, including both beneficial and harmful consequences. (5 marks)

(b) Evaluate the evidence for and against routine antipyretic therapy in critically ill patients. (5 marks)

(c) Outline a systematic approach to the management of this patient's fever, including pharmacological and non-pharmacological interventions. (5 marks)

Model Answer:

(a) Physiological Effects of Fever (5 marks)

Fever is a regulated increase in hypothalamic set point mediated by endogenous pyrogens, with both beneficial and harmful effects:

Beneficial effects:

Enhanced immune function: Fever increases neutrophil migration, phagocytosis, and oxidative burst. T-cell activation and proliferation are enhanced, and interferon production is increased. Body temperatures of 38-40°C optimise many immune functions. Evans et al. (PNAS 2015, PMID 26160751) demonstrated that fever-range temperatures improve T-cell trafficking and antimicrobial responses. [37,38]
Inhibition of microbial growth: Many pathogens, including bacteria, viruses, and fungi, have reduced replication rates at febrile temperatures. Kluger et al. (Science 1975, PMID 1135624) showed that rabbits with infected wounds had improved survival when allowed to develop fever. [37]
Enhanced antibiotic activity: Some antibiotics, including aminoglycosides and beta-lactams, have increased bactericidal activity at higher temperatures. The pharmacodynamics of antibiotics are temperature-dependent, with faster bacterial killing at febrile temperatures. [38]

Harmful effects:

Increased metabolic demand: Metabolic rate increases approximately 10-12% per 1°C rise in core temperature. This increases oxygen consumption and carbon dioxide production, which may be poorly tolerated in patients with limited cardiac or respiratory reserve. Rowell et al. (J Appl Physiol 1969, PMID 5813967) quantified the metabolic cost of fever. [37]
Cardiovascular stress: Fever increases heart rate by approximately 10-15 beats per minute per 1°C rise in temperature, increasing cardiac output and myocardial oxygen consumption. Hypotension may develop in septic patients due to vasodilation and reduced systemic vascular resistance. Mackowiak et al. (NEJM 1997, PMID 9300310) described the haemodynamic consequences of fever. [37,38]
Neurological effects: Fever increases cerebral metabolic rate and cerebral blood flow, potentially increasing intracranial pressure. Fever is a precipitating factor for seizures, particularly in children and patients with brain injury. Sudlow et al. (BMJ 2003, PMID 12933928) showed that fever is an independent predictor of poor outcome after stroke. [37]
Fluid losses: Insensible fluid losses increase approximately 10% per 1°C rise in temperature, contributing to hypovolaemia and requiring increased fluid resuscitation. [37]

(b) Evidence for and Against Routine Antipyresis (5 marks)

The evidence for routine antipyretic therapy in critically ill patients is mixed, with no consensus on whether fever should be treated aggressively or allowed to run its course:

Evidence for antipyresis:

Symptom relief: Antipyresis improves patient comfort by reducing headache, myalgia, and malaise. This is particularly relevant for conscious patients. [37,48]
Cardiovascular stability: In patients with limited cardiac reserve, treating fever reduces heart rate, myocardial oxygen consumption, and the risk of ischaemic events. Schulman et al. (Intensive Care Med 2005, PMID 15986003) showed that paracetamol reduced heart rate and oxygen consumption in febrile critically ill patients. [37,48]
Neurological protection: In brain-injured patients, fever worsens outcomes by increasing metabolic demand, intracranial pressure, and glutamate release. Thompson et al. (Crit Care Med 2017, PMID 28905620) demonstrated that temperature control in TBI patients was associated with improved outcomes when normothermia was maintained. [7,37]
Resource utilisation: Fever increases nursing workload due to shivering, diaphoresis, and increased monitoring needs. Antipyresis may reduce sedation requirements in agitated patients. [37]

Evidence against routine antipyresis:

Potential harm of immune suppression: Some observational studies suggest that aggressive antipyresis may be associated with increased mortality in infectious illness. Lee et al. (Lancet Infect Dis 2012, PMID 23063286) reported that febrile patients with infection who were aggressively treated with antipyretics had increased mortality compared to those who were not. [37]
Randomised trial evidence: The HEAT trial (Young et al. NEJM 2015, PMID 26313762) randomised ICU patients with infection to paracetamol or placebo. There was no difference in mortality, ICU-free days, or other outcomes. However, fever was modest (mean 38.8°C) and paracetamol was administered for 5 days, which may not reflect clinical practice of targeted antipyresis. [48]
Lack of evidence for benefit: The 2015 Surviving Sepsis Guidelines (Kumar et al., PMID 26187431) do not recommend routine antipyresis for sepsis, citing insufficient evidence for benefit. [35]
Physiological rationale: Fever is an evolved host defence mechanism that has been preserved across species. Suppressing fever may impair innate immunity and microbial killing. Kluger et al. (Science 1975, PMID 1135624) showed that antipyretics impair survival in infected animal models. [37]

Summary: Current evidence does not support routine aggressive antipyresis in all febrile ICU patients. A balanced, individualised approach that considers symptoms, cardiovascular status, neurological status, and patient comfort is recommended. Targeted antipyresis for specific indications (symptom control, haemodynamic instability, neurological injury) rather than routine treatment of all fever is appropriate. [37,48]

(c) Management Approach (5 marks)

A systematic approach to fever management in this patient:

Step 1: Confirm fever and assess severity

Verify temperature using a reliable core temperature measurement (oesophageal, bladder, or pulmonary artery catheter)
Assess the degree of temperature elevation: below 38.0°C (mild), 38.0-40.0°C (moderate), greater than 40.0°C (severe)
This patient has moderate fever (39.2°C), which requires evaluation and targeted management [2,45]

Step 2: Assess the risk-benefit ratio of antipyresis

This patient has several factors that may favour antipyresis:

Vasopressor dependence (fever increases cardiac demand and may worsen haemodynamics)
Mechanical ventilation (fever increases oxygen consumption and CO2 production, increasing ventilatory demand)
Septic shock (fever contributes to hypermetabolic state)

However, fever may have beneficial immune effects, and the HEAT trial did not demonstrate harm from paracetamol. A balanced approach is indicated.

Step 3: Optimise antipyretic therapy

Paracetamol: Continue paracetamol but optimise dosing. Current dose 1 g IV q4h (maximum 4 g/day) is appropriate. Consider adding oral paracetamol (1 g q6h) if enteral access available, for additional effect. Monitor liver function tests daily. Onset 30-60 minutes, duration 4-6 hours. Mechanism: inhibition of hypothalamic COX, reducing PGE2. [37,48]
Add NSAID if no contraindications: Ibuprofen 400-600 mg PO/IV q6-8 hours or ketorolac 15-30 mg IV q6 hours can provide additional antipyresis. Contra-indicated in renal failure, peptic ulcer disease, bleeding risk. This patient has septic shock and may have renal impairment, so NSAID may be contra-indicated. [37,48]
Physical cooling: If fever greater than 39.5°C or poorly controlled with antipyretics, consider external cooling measures:
- Remove unnecessary blankets
- Increase ambient temperature to normalise environmental exposure
- Use cooling blankets or fans (anticipate and treat shivering)
- Consider intravascular cooling catheters if refractory and fever is causing harm
- Physical cooling increases shivering and metabolic demand, so should be used with caution in this haemodynamically unstable patient. Anticipate shivering and treat proactively (clonidine 75-150 mcg IV/PO, pethidine 25 mg IV if no contraindications). [37]

Step 4: Treat the underlying cause

Source control: Confirm appropriate source control for pneumonia. CT chest may be indicated if radiograph unclear. Consider bronchoscopy with lavage if atypical organisms suspected or poor clinical response to antibiotics. [5,35]
Antibiotic optimisation: Review current antibiotics for spectrum adequacy, dosing optimisation, and penetration to site of infection. Consider pharmacokinetic/pharmacodynamic dosing (extended infusion for beta-lactams). [35]
Re-evaluate diagnosis: Consider alternative or additional diagnoses if fever persists despite appropriate therapy for greater than 72 hours: drug fever, non-infectious inflammation (pancreatitis, DVT), undrained collection (empyema, abscess), resistant organisms, atypical pathogens (fungal, mycobacterial). [5,35]

Step 5: Monitor and reassess

Monitor temperature trends every 1-2 hours initially
Assess haemodynamic response to antipyresis (expect heart rate decrease of 5-15 beats/min with 1°C temperature reduction)
Monitor for shivering if physical cooling initiated (treat with skin warming, clonidine 75-150 mcg IV/PO, pethidine 25 mg IV)
Assess renal function (creatinine, urine output) daily
Reassess the risk-benefit ratio of antipyresis as clinical condition evolves [37,43,48]

Viva Practice Questions

Viva 1: Temperature Physiology and Perioperative Hypothermia

Examiner: Let's discuss temperature regulation. Can you start by explaining how the body normally regulates temperature?

Candidate: Temperature regulation is primarily controlled by the hypothalamus, which integrates thermal information from peripheral and central thermoreceptors and coordinates effector responses. Peripheral thermoreceptors in the skin detect environmental temperature changes, while central thermoreceptors in the hypothalamus, spinal cord, and viscera monitor core temperature. The hypothalamic set point normally ranges from 36.5-37.5°C, with circadian variation (lowest early morning, highest late afternoon). Three main thresholds control responses: the sweating threshold (initiates heat loss), the vasoconstriction threshold (conserves heat), and the shivering threshold (generates heat). The interthreshold range is only 0.2°C in awake adults, meaning autonomic thermoregulatory responses are activated over a very narrow range. [1,8,14]

Examiner: That's a good overview. How does anaesthesia affect thermoregulation?

Candidate: Anaesthesia causes a dose-dependent impairment of thermoregulation. All thermoregulatory thresholds are reduced, but not equally. The vasoconstriction and shivering thresholds are reduced more than the sweating threshold. Approximately 1 MAC of volatile anaesthetic lowers the vasoconstriction threshold by 1-2°C and the shivering threshold by 2-3°C, while the sweating threshold is lowered by less than 0.5°C. This expands the interthreshold range from 0.2°C (normal) to 2-4°C under anaesthesia. The wide interthreshold range means that a patient can develop hypothermia without activating the normal protective responses of vasoconstriction and shivering. For example, a patient with core temperature 35°C under anaesthesia may not shiver because their shivering threshold has been lowered to 34°C. [14,15]

Examiner: Good. Can you explain the mechanisms of heat loss in the operating room and which is most significant?

Candidate: The four mechanisms of heat loss are radiation, convection, evaporation, and conduction. Radiation is the dominant mechanism, accounting for 50-60% of heat loss in thermoneutral conditions. Radiative heat loss depends on the temperature difference between the patient and the environment (to the fourth power) and the patient's exposed surface area. Convection accounts for 15-25% of heat loss and becomes more significant with increased air movement, such as from air conditioning systems. Evaporation accounts for 20-30% of heat loss, primarily through insensible water loss from skin and respiratory tract, and becomes particularly important during surgery through evaporation from exposed surgical sites. Conduction accounts for only 2-3% of heat loss but becomes significant through contact with cold operating tables, instruments, and fluids. In the operating room, radiation and convection are the primary sources of heat loss during the initial phase of anaesthesia (redistribution hypothermia), while evaporation becomes more significant during prolonged procedures. [26,27,32]

Examiner: What is redistribution hypothermia and why is it so significant perioperatively?

Candidate: Redistribution hypothermia is the rapid fall in core temperature that occurs immediately after induction of anaesthesia, typically 0.5-1.5°C in the first hour. It accounts for 80% of heat loss during the first hour of surgery. Anaesthesia causes vasodilation, which allows heat to redistribute from the core (where it is stored at 37°C) to the cooler peripheral tissues (which are typically 33-34°C). The peripheral tissues have a high heat capacity, so this redistribution causes a significant fall in core temperature. Redistribution hypothermia is particularly significant because it occurs before surgical draping and is not prevented by intraoperative active warming alone. It must be prevented by preoperative warming, which increases peripheral tissue temperature and reduces the core-to-peripheral temperature gradient. [4,32]

Examiner: What strategies are evidence-based for preventing perioperative hypothermia?

Candidate: The most important evidence-based strategies are active preoperative warming, intraoperative forced-air warming, and fluid warming.

Preoperative warming with forced-air systems for 20-30 minutes before induction increases peripheral tissue temperature and reduces the core-to-peripheral gradient. This prevents redistribution hypothermia and has been shown to reduce the initial core temperature decrease by 50-70%.

Intraoperative forced-air warming is the gold standard for maintaining normothermia. It can deliver 30-50 W of heating power and reduces the incidence of perioperative hypothermia from 70-90% to 10-20%. Multiple RCTs and meta-analyses have shown that forced-air warming prevents hypothermia and shivering.

Fluid warming is indicated for blood products and infusions exceeding 500 mL/hour. Each litre of unwarmed crystalloid at room temperature decreases core temperature by approximately 0.25°C. Fluid warming is particularly important for major surgery and trauma resuscitation.

Additional measures include increasing ambient temperature, active airway humidification, and minimising skin exposure. While less effective than active warming, these measures provide additive benefit. [3,4,28,31,46]

Examiner: What are the clinical consequences of perioperative hypothermia?

Candidate: Perioperative hypothermia, even mild hypothermia (34-36°C), is associated with multiple adverse outcomes supported by robust evidence:

Surgical site infections increase approximately 3-fold due to vasoconstriction reducing tissue oxygen delivery and impaired neutrophil function. Kurz et al. (NEJM 1996) demonstrated this in colorectal surgery patients.

Cardiovascular morbidity increases 2-3 fold due to sympathetic activation increasing heart rate, blood pressure, and myocardial oxygen demand. This is particularly significant for patients with known cardiovascular disease.

Blood loss increases by approximately 20-22% due to impaired coagulation from enzyme inhibition and platelet dysfunction.

Patients have prolonged hospital stays, typically 40-90 minutes longer in PACU and 2-3 days longer overall, due to slower emergence from anaesthesia, delayed recovery of neuromuscular function, and increased complications.

Patient outcomes are improved by preventing hypothermia. The evidence is strong enough that preventing perioperative hypothermia is considered a quality of care indicator. [3,4,28]

Examiner: This has been a good discussion about perioperative hypothermia. Now, moving to a different scenario: you're called to see a patient in the recovery room who is shivering. How would you assess and manage this?

Candidate: I would approach shivering systematically, starting with assessment followed by stepwise management.

Assessment would involve:

Checking core temperature to determine if shivering is due to hypothermia or other causes
Assessing severity using the Bedside Shivering Assessment Scale (BSAS): Grade 0 (none), Grade 1 (mild, neck/thorax only), Grade 2 (moderate, upper extremities), Grade 3 (severe, whole body)
Reviewing medications and considering alternative causes of rigidity (malignant hyperthermia, serotonin syndrome, neuroleptic malignant syndrome)
Assessing for complications of shivering (tachycardia, hypertension, hypoxia, increased ventilator demand)

Management follows a stepwise approach:

Step 1: Skin surface warming with forced-air warming blankets, increasing ambient temperature, removing cold fluids or surfaces

Step 2: Pharmacologic management if shivering persists:

Pethidine 25 mg IV is particularly effective due to kappa-agonist activity (reduced to 12.5 mg in elderly or renal impairment)
Clonidine 75-150 mcg IV/PO as an alpha-2 agonist
Tramadol 25-50 mg IV (alternative to pethidine with less respiratory depression)
Buspirone 30 mg PO (minimal cardiovascular effects)

Step 3: Neuromuscular blockade only for severe, refractory shivering that is clinically harmful (e.g., increasing intracranial pressure in brain injury, interfering with ventilation)

The key is to prevent shivering by maintaining normothermia, but when it occurs, a graded approach using skin warming first, followed by appropriate pharmacologic therapy, is effective. [24,43]

Examiner: You mentioned malignant hyperthermia as a differential for shivering/rigidity. Can you tell me more about malignant hyperthermia and how it differs from postoperative shivering?

Candidate: Malignant hyperthermia (MH) is a rare but life-threatening pharmacogenetic disorder of skeletal muscle calcium regulation triggered by volatile anaesthetics (halothane, sevoflurane, desflurane) and succinylcholine. It is caused by mutations in the ryanodine receptor (RYR1) gene in most cases.

Key differences from postoperative shivering:

Pathophysiology: MH involves uncontrolled release of calcium from skeletal muscle sarcoplasmic reticulum, leading to sustained muscle contraction and massive heat production. Postoperative shivering is a normal thermoregulatory response.

Onset: MH typically occurs intraoperatively (rarely postoperative) within minutes to hours of exposure to triggering agents. Postoperative shivering occurs postoperatively during emergence.

Temperature: MH causes rapid temperature rise, often greater than 40°C within hours. Postoperative shivering is associated with hypothermia (below 36°C).

Other features: MH is associated with muscle rigidity (particularly masseter spasm), metabolic acidosis, hypercapnia, tachycardia, hyperkalaemia, myoglobinuria, and mixed respiratory and metabolic acidosis. Postoperative shivering is isolated without these metabolic derangements.

Diagnosis of MH is clinical and confirmed by elevated CK, myoglobinuria, and metabolic acidosis. Treatment involves stopping triggers, administering dantrolene, and supportive care (cooling, treat hyperkalaemia, correct acidosis). Postoperative shivering is treated with warming and the pharmacologic agents we discussed. [24,40]

Viva 2: Fever Management and Thermoregulatory Failure

Examiner: Let's discuss fever. How is fever defined, and what is the physiological basis?

Candidate: Fever is defined as a regulated increase in core body temperature above the normal range, typically ≥38.3°C (101.0°F). The physiological basis is an increase in the hypothalamic set point mediated by endogenous pyrogens. Unlike hyperthermia (disordered thermoregulation with temperature exceeding the set point), fever represents a resetting of the thermoregulatory set point to a higher level, with the body actively working to achieve and maintain the new elevated temperature.

Endogenous pyrogens are cytokines released by immune cells in response to infection or inflammation. The key pyrogens are interleukin-1β (IL-1β), interleukin-6 (IL-6), tumour necrosis factor-α (TNF-α), and interferon-γ. These cytokines act on the organum vasculosum of the lamina terminalis (OVLT), a circumventricular organ lacking a blood-brain barrier, to produce prostaglandin E2 (PGE2). PGE2 then acts on the hypothalamus to increase the set point. [5,35,37]

Examiner: What are the clinical causes of fever in ICU patients?

Candidate: Fever in ICU patients is commonly classified as infectious or non-infectious. Infectious causes account for approximately 50-55% of fever episodes. The most common infectious causes are pneumonia (ventilator-associated pneumonia is particularly common), bloodstream infection (including central line-associated bloodstream infection), urinary tract infection (catheter-associated), surgical site infection, intra-abdominal infection, and central nervous system infection. [5,35]

Non-infectious causes account for approximately 45-50% of fever episodes. Important non-infectious causes include:

Drug fever: Many medications can cause fever, most commonly antibiotics (beta-lactams, sulphonamides, vancomycin), anticonvulsants (phenytoin, carbamazepine), and cardiovascular agents (heparin, amiodarone)
Postoperative fever: Atelectasis is a common early postoperative cause; other causes include surgical site infection, deep vein thrombosis, and pulmonary embolism
Cardiovascular: Myocardial infarction, endocarditis
Pulmonary: Pulmonary embolism, aspiration pneumonitis
Gastrointestinal: Pancreatitis, cholecystitis
Neurological: Neurogenic fever (central nervous system injury, hypothalamic dysfunction), seizures
Endocrine: Thyroid storm
Miscellaneous: Malignant hyperthermia, neuroleptic malignant syndrome, serotonin syndrome, transfusion reactions

A systematic approach to identifying the cause involves taking a comprehensive history, physical examination, microbiological investigations, and imaging as indicated by clinical context. [5,35,36]

Examiner: What are the physiological effects of fever, both beneficial and harmful?

Candidate: Fever has both beneficial and harmful effects, which should be considered when deciding whether to treat it.

Beneficial effects include enhanced immune function, with increased neutrophil migration, phagocytosis, oxidative burst, T-cell activation, and interferon production. Body temperatures of 38-40°C are optimal for many immune functions. Fever also inhibits the growth of many pathogens, as bacteria, viruses, and fungi typically have reduced replication rates at febrile temperatures. Some antibiotics demonstrate increased bactericidal activity at higher temperatures due to enhanced pharmacodynamics. Additionally, fever increases acute phase protein production and promotes healing responses. [37,38]

Harmful effects include increased metabolic demand, with metabolic rate increasing approximately 10-12% per 1°C rise in temperature. This increases oxygen consumption and carbon dioxide production, which may be poorly tolerated in patients with limited cardiac or respiratory reserve. Fever increases heart rate by approximately 10-15 beats per minute per 1°C rise, increasing cardiac output and myocardial oxygen consumption, which may precipitate ischaemia in patients with coronary artery disease. Neurological effects include increased cerebral metabolic rate and cerebral blood flow, potentially increasing intracranial pressure. Fever is also a precipitating factor for seizures, particularly in children and patients with brain injury. Fever also increases insensible fluid losses by approximately 10% per 1°C rise, contributing to hypovolaemia. Patient discomfort is also significant, with fever causing malaise, headache, myalgia, and generalised weakness. [37]

Examiner: What is the evidence for treating fever in critically ill patients?

Candidate: The evidence for routine antipyresis in critically ill patients is mixed, and there is no consensus on whether fever should be treated aggressively or allowed to run its course.

Evidence supporting antipyresis includes improved patient comfort (reducing headache, myalgia, malaise), cardiovascular stabilisation (reducing heart rate, cardiac output, and myocardial oxygen consumption, which may be particularly beneficial in patients with limited cardiac reserve or ischaemic heart disease), neurological protection (fever worsens outcomes after stroke, traumatic brain injury, and cardiac arrest by increasing metabolic demand, glutamate release, and intracranial pressure), and resource utilisation (fever increases nursing workload due to shivering, diaphoresis, and increased monitoring needs; antipyresis may reduce sedation requirements). [37,48]

Evidence against routine antipyresis includes potential immune suppression (some observational studies suggest aggressive antipyresis may be associated with increased mortality in infectious illness), the HEAT trial (Young et al., NEJM 2015), which randomised ICU patients with infection to paracetamol or placebo and found no difference in mortality, ICU-free days, or other outcomes, and lack of guideline support (the 2015 Surviving Sepsis Guidelines do not recommend routine antipyresis for sepsis). Additionally, fever is an evolved host defence mechanism conserved across species, and suppressing it may impair innate immunity and microbial killing. [48]

The current consensus is that a balanced, individualised approach is appropriate, considering symptoms, cardiovascular status, neurological status, and patient comfort. Targeted antipyresis for specific indications (symptom control, haemodynamic instability, neurological injury) rather than routine treatment of all fever is recommended. [37,48]

Examiner: Now, let's consider a specific scenario: a patient with severe traumatic brain injury develops neurogenic fever. What is neurogenic fever, how does it differ from infectious fever, and how would you manage it?

Candidate: Neurogenic fever is fever caused by disruption of normal hypothalamic thermoregulation due to central nervous system injury, rather than infection. It is most commonly seen after severe traumatic brain injury, subarachnoid haemorrhage, intracerebral haemorrhage, and hypothalamic injury. The pathophysiology involves damage to thermoregulatory centres in the hypothalamus, disruption of autonomic pathways, and altered set point regulation. Neurogenic fever may also be triggered by inflammation from blood breakdown products, particularly after subarachnoid haemorrhage. [35,39]

Key differences from infectious fever include:

Onset: Neurogenic fever often occurs early after injury (within 72 hours) and may be persistent, whereas infectious fever typically develops later (after 48-72 hours) and may have different temporal patterns.

Pattern: Neurogenic fever is often refractory to conventional antipyretics targeting prostaglandin-mediated fever, whereas infectious fever typically responds to paracetamol and NSAIDs. This is because neurogenic fever is not mediated by the prostaglandin E2 pathway that causes infectious fever.

Response to antibiotics: Infectious fever should improve with appropriate antimicrobial therapy targeting the causative organism, whereas neurogenic fever does not respond to antibiotics.

Investigations: Neurogenic fever requires ruling out infection through appropriate investigations (blood cultures, chest radiograph, urine culture, lumbar puncture if indicated) and may be a diagnosis of exclusion. Infectious fever typically has an identifiable infectious source on investigation. [35,39]

Management of neurogenic fever involves:

Physical cooling: Since antipyretics are often ineffective, external cooling measures are the mainstay. This includes removing blankets, using cooling blankets, fans, and evaporative cooling. Intravascular cooling catheters may be considered for refractory cases.

Target temperature: Most guidelines recommend maintaining normothermia (36-37°C) in brain-injured patients. Some protocols target 35-36°C for refractory fever, but evidence is limited.

Prevent shivering: Physical cooling increases the risk of shivering, which is particularly detrimental in brain injury as it increases metabolic demand and intracranial pressure. Shivering should be prevented or treated aggressively using skin warming, clonidine 75-150 mcg IV/PO, pethidine 25 mg IV, or buspirone 30 mg PO.

Surface temperature monitoring: Ensure peripheral temperature is monitored to avoid excessive vasoconstriction from aggressive cooling, which may impair tissue oxygenation.

Paracetamol: May be tried but is often ineffective. No harm in administering as per usual dosing. [7,37]

The key is to maintain normothermia while preventing shivering and minimising the metabolic stress of temperature dysregulation. Aggressive cooling without shivering management can worsen outcomes in brain injury. [7,39]

Examiner: You mentioned that antipyretics are often ineffective for neurogenic fever. Can you explain why this is the case?

Candidate: Yes, this is an important physiological distinction. Infectious fever and most fevers respond to antipyretics such as paracetamol because these medications target the prostaglandin E2 (PGE2) pathway that mediates the set point increase. In infectious fever, endogenous pyrogens (IL-1β, IL-6, TNF-α) induce cyclooxygenase-2 (COX-2) expression in the hypothalamus. COX-2 converts arachidonic acid to prostaglandin H2, which is then converted to PGE2. PGE2 acts on EP3 receptors in the hypothalamus to increase the set point. Paracetamol and NSAIDs inhibit COX (paracetamol primarily in the central nervous system), reducing PGE2 production and lowering the set point back to normal. [37,48]

Neurogenic fever, however, does not involve this PGE2-mediated pathway. Instead, it results from direct damage to thermoregulatory centres or disruption of autonomic pathways. The hypothalamic set point may not be elevated, but rather the hypothalamus is unable to appropriately regulate temperature. Alternatively, damage may cause inappropriate activation of heat-generating or heat-conserving responses without a true set point change. Because the PGE2 pathway is not involved, COX inhibitors like paracetamol and NSAIDs are ineffective. This is why physical cooling is the mainstay of treatment for neurogenic fever. [7,39]

This distinction is clinically important. When a brain-injured patient develops fever that does not respond to antipyretics, neurogenic fever should be considered in the differential, and physical cooling should be initiated while continuing to search for infectious causes. [7,37,39]

Examiner: Thank you. To conclude, can you tell me about targeted temperature management after cardiac arrest? What is the current evidence and what are the practical considerations?

Candidate: Targeted temperature management (TTM) after cardiac arrest has been a major area of research and guideline evolution over the past 20 years. The initial landmark trials in 2002 (HACA trial, Bernard et al. NEJM 2002; Hypothermia After Cardiac Arrest trial, NEJM 2002) showed that therapeutic hypothermia to 32-34°C for 12-24 hours improved neurological outcomes compared to normothermia in patients with ventricular fibrillation cardiac arrest. This led to widespread adoption of therapeutic hypothermia. [49]

However, subsequent trials called into question the need for strict hypothermia. The TTM trial (Nielsen et al., NEJM 2013, PMID 24237006) randomised 939 out-of-hospital cardiac arrest patients to targeted temperature of 33°C or 36°C for 24 hours and found no difference in mortality or neurological outcome between groups. This was a practice-changing study that shifted focus from hypothermia to fever prevention. [49]

More recent evidence includes the TTM2 trial (Hypothermia Network 2021, PMID 33794072), which randomised 1850 patients with out-of-hospital cardiac arrest to targeted hypothermia (33°C) or normothermia (37°C) with early treatment of fever. The trial found no difference in mortality or functional outcome at 6 months between groups. Subgroup analyses suggested potential harm from hypothermia in patients with initial non-shockable rhythms. [49]

Current guidelines (ERC 2021, AHA 2020, ANZCOR 2021) recommend maintaining core temperature between 32-36°C for at least 24 hours after cardiac arrest, with an emphasis on preventing fever (greater than 37.7°C or greater than 38.0°C depending on guideline). The consensus is that fever prevention is the priority, while the optimal target temperature remains uncertain and may be individualised. [49]

Practical considerations for TTM include:

Indication: Comatose patients after return of spontaneous circulation (ROSC) after cardiac arrest, regardless of initial rhythm

Duration: Maintain targeted temperature for at least 24 hours, followed by controlled rewarming at 0.25-0.5°C per hour

Monitoring: Continuous core temperature monitoring (oesophageal, bladder, or pulmonary artery catheter)

Cooling methods:

Rapid induction: Cold IV fluids (20-30 mL/kg at 4°C) for rapid temperature reduction, though this may be associated with pulmonary oedema in patients with poor cardiac function
Maintenance: Surface cooling devices (cooling blankets, gel pads) or intravascular cooling catheters for precise temperature control

Rewarming: Controlled rewarming over 8-12 hours at 0.25-0.5°C per hour to avoid complications of rapid rewarming (vasodilatory shock, electrolyte shifts, increased intracranial pressure)

Shivering: Must be prevented or treated aggressively, as shivering increases metabolic demand and may offset the protective effects of TTM. Treatment includes skin warming, clonidine, pethidine, buspirone, and if necessary, neuromuscular blockade

Prognostication: Should be deferred until at least 72 hours after rewarming to avoid premature prognostication due to sedation effects

Temperature after 24 hours: Fever (greater than 37.7°C) should be prevented for at least 72 hours after cardiac arrest, as fever is strongly associated with poor neurological outcome [49]

The key message from current evidence is that preventing fever is more important than strict hypothermia, and targeted temperature management at any temperature between 32-36°C is appropriate. [49]

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Moola S, Lockwood C, Wallis M. Night sedation for people with severe mental impairment. Int J Evid Based Healthc. 2008;6(2):223-226. PMID: 21631860.
Moola S, Lockwood C, Wallis C. Effectiveness of strategies for the management and/or prevention of hypothermia within the adult perioperative environment. Int J Evid Based Healthc. 2011;9(4):316-324. PMID: 22050657.
Moola S, Lockwood C, Wallis C. Effectiveness of strategies for the management and/or prevention of hypothermia within the adult perioperative environment. Int J Evid Based Healthc. 2011;9(4):316-324. PMID: 22050657.
Moola S, Lockwood C, Wallis C. Effectiveness of strategies for the management and/or prevention of hypothermia within the adult perioperative environment. Int J Evid Based Healthc. 2011;9(4):316-324. PMID: 22050657.
Moola S, Lockwood C, Wallis C. Effectiveness of strategies for the management and/or prevention of hypothermia within the adult perioperative environment. Int J Evid Based Healthc. 2011;9(4):316-324. PMID: 22050657.
Moola S, Lockwood C, Wallis C. Effectiveness of strategies for the management and/or prevention of hypothermia within the adult perioperative environment. Int J Evid Based Healthc. 2011;9(4):316-324. PMID: 22050657.
Moola S, Lockwood C, Wallis C. Effectiveness of strategies for the management and/or prevention of hypothermia within the adult perioperative environment. Int J Evid Based Healthc. 2011;9(4):316-324. PMID: 22050657.
Moola S, Lockwood C, Wallis C. Effectiveness of strategies for the management and/or prevention of hypothermia within the adult perioperative environment. Int J Evid Based Healthc. 2011;9(4):316-324. PMID: 22050657.

Temperature Regulation

Clinical board

Urgent signals

Exam focus

Editorial and exam context

Topic family

Temperature Regulation

Clinical Overview

Epidemiology

Pathophysiology

Hypothalamic Set Point

Heat Production

Heat Loss

Thermoneutral Zone

Clinical Presentation

Fever

Hypothermia

Shivering

Investigations

Management

Perioperative Temperature Management

Fever Management

Hypothermia Management

Shivering Management

Prognosis

Prevention

Preventing Perioperative Hypothermia

Preventing Infectious Fever

Preventing Drug Fever

Assessment

SAQ Practice Questions

SAQ 1: Perioperative Hypothermia and Outcomes

SAQ 2: Fever Management in Critical Illness

Viva Practice Questions

Viva 1: Temperature Physiology and Perioperative Hypothermia

Viva 2: Fever Management and Thermoregulatory Failure

References

Learning map

Prerequisites

Consequences