Isoflurane Pharmacology

Quick Answer

Isoflurane (1-chloro-2,2,2-trifluoroethyl difluoromethyl ether) is a halogenated methyl ethyl ether volatile anaesthetic agent introduced in 1981 that remains widely used globally for maintenance of general anaesthesia. Its physicochemical properties include a blood:gas partition coefficient of 1.4, oil:gas partition coefficient of 91, minimum alveolar concentration (MAC) of 1.15% in 40-year-old adults, saturated vapour pressure of 238 mmHg at 20°C, and boiling point of 48.5°C. Pharmacokinetically, isoflurane demonstrates intermediate solubility allowing moderately rapid induction and emergence, with minimal hepatic metabolism (0.2%) via cytochrome P450 2E1 to trifluoroacetic acid and negligible inorganic fluoride production (3-5 micromol/L). Unlike sevoflurane, isoflurane does not produce Compound A when exposed to carbon dioxide absorbents. Pharmacodynamically, isoflurane causes dose-dependent CNS depression with cerebral vasodilation and potential intracranial pressure elevation at concentrations above 1 MAC, significant cardiovascular effects including systemic vasodilation, mild tachycardia, and preserved cardiac output, respiratory depression with bronchodilation, and potentiation of non-depolarizing neuromuscular blocking agents by 25-40%. Clinical applications include maintenance anaesthesia, ICU sedation via the AnaConDa device, and controlled hypotensive anaesthesia. Isoflurane exhibits anaesthetic preconditioning properties potentially conferring cardioprotection, though clinical significance remains debated. Adverse effects include malignant hyperthermia triggering in susceptible individuals, pungency limiting inhalational induction, rare immune-mediated hepatitis, and potential neurotoxicity concerns in developing brains. [1-8]

Physicochemical Properties

Chemical Structure and Classification

Isoflurane (chemical formula: C3H2ClF5O; molecular weight: 184.5 g/mol) is a structural isomer of enflurane, classified as a halogenated methyl ethyl ether. The systematic IUPAC name is 2-chloro-2-(difluoromethoxy)-1,1,1-trifluoroethane, though the commonly used chemical name is 1-chloro-2,2,2-trifluoroethyl difluoromethyl ether. The molecular structure consists of a central ether oxygen linking a difluoromethyl group (-CHF2) to a 2-chloro-2,2,2-trifluoroethyl group (-CHClCF3). This specific halogenation pattern, with fluorine atoms replacing hydrogen at key positions, confers chemical stability, resistance to biodegradation, and favourable anaesthetic properties. Isoflurane exists as a clear, colourless liquid with a mildly pungent, ethereal odour that limits its use for inhalational induction in conscious patients. Unlike halothane, the carbon-fluorine bonds in isoflurane are highly stable, resulting in minimal hepatic metabolism and reduced potential for hepatotoxicity. The fluorinated structure also provides non-flammability at clinically used concentrations in air, oxygen, or nitrous oxide mixtures, unlike the earlier volatile agent diethyl ether. [9-12]

Physical Constants and Properties

The physical properties of isoflurane are fundamental to understanding its clinical behaviour and vaporizer requirements:

The blood:gas partition coefficient of 1.4 positions isoflurane between the more soluble halothane (2.4) and the less soluble sevoflurane (0.65) and desflurane (0.42). This intermediate solubility means that while induction and emergence are faster than with halothane, they are slower than with sevoflurane or desflurane. The high oil:gas partition coefficient of 91 correlates with isoflurane's potency according to the Meyer-Overton hypothesis, which relates anaesthetic potency to lipid solubility. [13-17]

Minimum Alveolar Concentration (MAC)

The minimum alveolar concentration (MAC) represents the alveolar (end-tidal) concentration at steady state that prevents movement in response to a standard surgical stimulus (skin incision) in 50% of subjects. For isoflurane, MAC is 1.15% in a 40-year-old adult at 1 atmosphere, breathing 100% oxygen, without premedication or concurrent drug administration. MAC is affected by numerous physiological and pharmacological factors:

Factors that INCREASE MAC (decrease sensitivity):

• Young age (MAC highest in infants 6-12 months)
• Hyperthermia
• Chronic alcohol use
• Chronic opioid use
• Acute amphetamine intoxication
• Hypernatraemia
• Red hair phenotype (MC1R gene variants)

Factors that DECREASE MAC (increase sensitivity):

• Advanced age (MAC decreases approximately 6% per decade after age 40)
• Hypothermia (7-8% reduction per °C below 37°C)
• Hypotension (MAP less than 50 mmHg)
• Pregnancy (MAC reduced by 25-40% by term)
• Acute alcohol intoxication
• Concurrent opioids (50-70% MAC reduction)
• Concurrent nitrous oxide (1% reduces MAC by 0.6%)
• Concurrent propofol or benzodiazepines
• Hypoxia (PaO2 less than 38 mmHg)
• Severe anaemia
• Metabolic acidosis
• Hyponatraemia
• Alpha-2 agonists (clonidine, dexmedetomidine)
• Lithium

The MAC-awake for isoflurane is approximately 0.4% (35% of MAC), representing the concentration at which 50% of patients respond to verbal commands. MAC-BAR (blocking adrenergic response) is approximately 1.3-1.6 MAC. Understanding these MAC variations is essential for appropriate dosing across different patient populations and clinical scenarios. [18-23]

Stability and Storage

Isoflurane demonstrates excellent chemical stability under normal storage conditions. Unlike halothane, which requires thymol as a preservative to prevent degradation, isoflurane does not require chemical stabilizers. It should be stored in tightly sealed amber glass bottles at temperatures below 30°C, protected from light. The shelf life is typically 5 years when stored appropriately. Isoflurane is compatible with common anaesthetic circuit materials including rubber, plastic, and metal components. However, isoflurane reacts with desiccated carbon dioxide absorbents (particularly those containing strong bases such as barium hydroxide lime) to produce carbon monoxide, posing a potential toxicity hazard. This reaction is more pronounced with desiccated absorbents and is temperature-dependent. Modern carbon dioxide absorbents with reduced or absent potassium and sodium hydroxide (such as calcium hydroxide-based absorbents) significantly reduce carbon monoxide production. Unlike sevoflurane, isoflurane does not produce Compound A (fluoromethyl-2,2-difluoro-1-[trifluoromethyl]vinyl ether) when exposed to carbon dioxide absorbents, eliminating concerns about absorbent-related nephrotoxicity. [24-27]

Pharmacokinetics

Uptake and Distribution

The uptake and distribution of isoflurane follows the classical model of inhalational anaesthetic pharmacokinetics, governed by alveolar ventilation, cardiac output, and tissue solubility. The rate of rise of alveolar concentration (FA) toward inspired concentration (FI) determines the speed of induction:

Factors affecting FA/FI ratio:

1. Alveolar ventilation: Increased minute ventilation accelerates FA/FI rise by delivering more agent to alveoli and washing out absorbed agent from the circuit

2. Inspired concentration: Higher FI accelerates FA/FI rise (concentration effect); this is less pronounced with isoflurane than with nitrous oxide due to lower inspired concentrations

3. Blood:gas solubility: Isoflurane's blood:gas coefficient of 1.4 means substantial uptake occurs before alveolar-arterial equilibration, slowing induction compared to desflurane (0.42) or sevoflurane (0.65)

4. Cardiac output: Higher cardiac output increases pulmonary blood flow, removing more agent from alveoli and slowing FA/FI rise. Conversely, low cardiac output (shock) accelerates FA/FI rise but may delay brain equilibration

5. Alveolar-venous partial pressure gradient: Determined by tissue uptake; decreases over time as tissues equilibrate

Second gas effect: When isoflurane is administered with high concentrations of nitrous oxide, the rapid uptake of nitrous oxide concentrates the remaining gases (including isoflurane) in the alveoli, slightly accelerating isoflurane uptake. This effect is clinically modest with isoflurane.

Tissue distribution: Following pulmonary uptake, isoflurane distributes to tissues according to their blood flow and tissue:blood partition coefficients:

The brain:blood partition coefficient of 1.6 allows relatively rapid CNS equilibration once arterial concentration rises. [28-32]

Metabolism

Isoflurane undergoes remarkably limited hepatic biotransformation, with only approximately 0.2% of the absorbed dose metabolized. This minimal metabolism is a significant advantage over earlier agents like halothane (20-25% metabolized) and methoxyflurane (50-70% metabolized), reducing the risk of both hepatotoxicity and nephrotoxicity.

Metabolic pathways:

1. Oxidative metabolism: The primary pathway involves cytochrome P450 2E1 (CYP2E1)-catalyzed oxidation, producing:

   • Trifluoroacetic acid (TFA) - the major metabolite
   • Inorganic fluoride (F⁻) - typically 3-5 micromol/L peak serum concentration
   • Trace amounts of difluoromethyl difluoromethyl ether

2. Reductive metabolism: Minimal reductive defluorination can occur under hypoxic conditions, but this is clinically insignificant

Comparison of volatile anaesthetic metabolism:

The low inorganic fluoride production (peak 3-5 micromol/L) is well below the 50 micromol/L threshold historically associated with methoxyflurane-induced nephrotoxicity. Even in patients with induced CYP2E1 activity (chronic alcoholics, obese patients, those taking isoniazid), isoflurane metabolism remains clinically insignificant due to the inherent chemical stability of its carbon-fluorine bonds. [33-37]

Elimination

Isoflurane elimination occurs almost entirely via the lungs, with exhaled unchanged drug accounting for approximately 99.8% of elimination. The rate of elimination (emergence) is determined by the same factors that govern uptake, operating in reverse:

Factors affecting emergence:

1. Alveolar ventilation: Hyperventilation accelerates washout and emergence

2. Blood:gas solubility: The blood:gas coefficient of 1.4 slows emergence compared to desflurane and sevoflurane as agent must diffuse from blood to alveoli

3. Duration of anaesthesia: Prolonged administration allows accumulation in muscle and fat, prolonging context-sensitive half-time

4. Cardiac output: Higher cardiac output delivers more agent to lungs for elimination

Context-sensitive half-time: Unlike intravenous agents, volatile anaesthetics exhibit relatively stable context-sensitive half-times because elimination is predominantly pulmonary rather than metabolic. For isoflurane, the context-sensitive half-time (time for a 50% reduction in blood concentration after stopping delivery) increases modestly with infusion duration:

This compares favourably with desflurane (approximately 5 minutes regardless of duration) but less favourably than sevoflurane for short cases. The clinical implication is that emergence from isoflurane anaesthesia is slightly slower than from sevoflurane or desflurane, particularly after prolonged procedures.

Diffusion hypoxia: During emergence, the rapid exhalation of accumulated nitrous oxide (if used) can dilute alveolar oxygen and isoflurane. While diffusion hypoxia is primarily a concern with nitrous oxide, supplemental oxygen should be administered during emergence from combined isoflurane-nitrous oxide anaesthesia. [38-42]

Pharmacodynamics

Central Nervous System Effects

Isoflurane produces dose-dependent central nervous system depression through multiple molecular mechanisms:

Mechanism of action:

1. GABA_A receptor potentiation: Isoflurane enhances gamma-aminobutyric acid (GABA) binding to GABA_A receptors, increasing chloride conductance and neuronal hyperpolarization. This is the primary mechanism for hypnotic and immobilizing effects.

2. Glycine receptor potentiation: Enhancement of inhibitory glycinergic transmission contributes to immobility, particularly at the spinal cord level.

3. NMDA receptor inhibition: Blockade of excitatory glutamatergic NMDA receptors contributes to amnesia and possibly analgesia.

4. Two-pore domain potassium channel activation: Activation of TREK-1, TASK-1, and TASK-3 channels hyperpolarizes neurons, contributing to anaesthesia.

5. Nicotinic acetylcholine receptor inhibition: May contribute to immobility and analgesia.

Cerebrovascular effects:

• Cerebral blood flow (CBF): Isoflurane causes dose-dependent cerebral vasodilation, increasing CBF. At concentrations above 1 MAC, CBF may increase by 50-100%, potentially elevating intracranial pressure (ICP) in patients with reduced intracranial compliance.

• Cerebral metabolic rate (CMRO2): Isoflurane reduces CMRO2 in a dose-dependent manner, with maximal reduction of approximately 50% at 2 MAC. This creates "flow-metabolism uncoupling" where CBF increases despite decreased metabolic demand.

• Intracranial pressure: The net effect on ICP depends on the balance between vasodilation (increasing ICP) and reduced CMRO2 (potentially decreasing ICP through autoregulation). At concentrations below 1 MAC with hyperventilation, ICP changes are usually minimal. Above 1 MAC, ICP elevation may occur.

• Cerebral autoregulation: Preserved at concentrations up to 1.5 MAC; impaired at higher concentrations.

• CO2 reactivity: Preserved, allowing hyperventilation to counteract vasodilation-induced ICP elevation.

Electroencephalographic effects:

• Progressive slowing of EEG frequency with increasing concentration
• Burst suppression achievable at high concentrations (1.5-2 MAC)
• Isoelectric EEG not typically achieved even at high concentrations
• Epileptiform activity not induced (unlike enflurane)
• May be used for intraoperative neurophysiological monitoring with appropriate dose adjustments

Neuroprotection: Isoflurane demonstrates neuroprotective effects in experimental models of cerebral ischemia through reduced CMRO2, NMDA receptor antagonism, and possible preconditioning mechanisms. However, translation to clinical neuroprotection remains unproven. [43-49]

Cardiovascular Effects

Isoflurane produces significant cardiovascular effects that distinguish it from other volatile agents:

Systemic vascular effects:

• Systemic vascular resistance (SVR): Dose-dependent reduction of 15-25% at 1 MAC, primarily through direct vascular smooth muscle relaxation and decreased sympathetic tone. This is the main mechanism of hypotension.

• Blood pressure: Mean arterial pressure decreases 20-30% at 1 MAC, primarily due to SVR reduction rather than myocardial depression.

Cardiac effects:

• Heart rate: Mild tachycardia (10-20% increase) due to preserved baroreceptor reflex response to vasodilation and possible direct sinoatrial node effects. This distinguishes isoflurane from halothane (bradycardia).

• Cardiac output: Generally well preserved due to reduced afterload and reflex tachycardia, despite mild negative inotropy.

• Myocardial contractility: Mild negative inotropic effect (10-15% reduction), significantly less than halothane.

• Myocardial oxygen consumption: Reduced due to decreased contractility and wall stress (afterload reduction).

Coronary circulation:

• Coronary vasodilation: Isoflurane is a potent coronary vasodilator, which historically raised concerns about "coronary steal."

• Coronary steal phenomenon: The theoretical concern was that isoflurane-induced dilation of healthy coronary arterioles would reduce perfusion pressure and "steal" blood flow from collateral-dependent ischemic myocardium. However, extensive clinical studies (Slogoff and Keats, 1989) demonstrated that coronary steal is rarely clinically significant at standard anaesthetic concentrations when hypotension and tachycardia are avoided.

• Current evidence: Isoflurane is considered safe for patients with coronary artery disease when administered with careful hemodynamic management. The vasodilatory properties may actually improve overall myocardial oxygen supply-demand balance.

Cardiac rhythm:

• Does not sensitize the myocardium to catecholamines (unlike halothane)
• Maintains sinus rhythm in most patients
• QT interval prolongation minimal compared to sevoflurane

Anaesthetic preconditioning: Isoflurane activates cardioprotective signaling pathways similar to ischemic preconditioning:

1. ATP-sensitive potassium (K_ATP) channel opening
2. Protein kinase C activation
3. Mitochondrial permeability transition pore inhibition
4. Reactive oxygen species signaling

Clinical studies suggest volatile anaesthetics may reduce myocardial injury biomarkers and possibly mortality in cardiac surgery, though the large MYRIAD trial (2019) did not demonstrate mortality benefit compared to total intravenous anaesthesia. [50-58]

Respiratory Effects

Isoflurane produces dose-dependent respiratory depression and airway effects:

Ventilatory depression:

• Tidal volume: Reduced in a dose-dependent manner
• Respiratory rate: Initially increases as a compensatory mechanism, then decreases at higher concentrations
• Minute ventilation: Net reduction, causing PaCO2 elevation
• Hypercapnic ventilatory response: Significantly blunted; the slope of the CO2 response curve is reduced by approximately 50% at 1 MAC
• Hypoxic ventilatory response: Profoundly depressed even at subanesthetic concentrations (0.1 MAC)

Airway effects:

• Bronchodilation: Isoflurane produces bronchodilation through direct airway smooth muscle relaxation and inhibition of vagal reflexes. This makes it useful for patients with reactive airway disease.

• Pungency: Isoflurane has a moderately pungent odour that can cause:

  • Breath-holding
  • Coughing
  • Laryngospasm
  • Increased secretions

  This limits its use for inhalational induction, particularly in pediatric patients where sevoflurane is preferred.

• Upper airway reflexes: Obtunded, increasing aspiration risk

Comparison of airway effects:

Hypoxic pulmonary vasoconstriction (HPV): Isoflurane inhibits HPV in a dose-dependent manner, which may worsen ventilation-perfusion mismatch and arterial oxygenation during one-lung ventilation. At 1 MAC, HPV inhibition is approximately 20-30%, similar to other volatile agents. [59-64]

Neuromuscular Effects

Isoflurane potentiates neuromuscular blockade produced by non-depolarizing neuromuscular blocking agents (NMBAs):

Mechanism of potentiation:

1. Postjunctional effects: Reduced sensitivity of the motor end-plate to acetylcholine
2. Prejunctional effects: Possible reduction in acetylcholine release
3. Muscle membrane effects: Direct muscle relaxant properties

Clinical implications:

• Dose requirements for non-depolarizing NMBAs (rocuronium, vecuronium, atracurium, cisatracurium) are reduced by 25-40% during isoflurane anaesthesia compared to intravenous anaesthesia
• Duration of neuromuscular blockade is prolonged
• Neuromuscular monitoring is essential to guide NMBA dosing and timing of reversal

Comparison of NMBA potentiation by volatile agents:

Malignant hyperthermia: Isoflurane, like all halogenated volatile anaesthetics, is a trigger for malignant hyperthermia in genetically susceptible individuals (RYR1 or CACNA1S gene mutations). Clinical features include:

• Unexplained rise in end-tidal CO2
• Tachycardia, arrhythmias
• Muscle rigidity
• Rapidly rising temperature (late sign)
• Metabolic and respiratory acidosis
• Rhabdomyolysis, hyperkalemia

Management includes immediate discontinuation of triggering agents, hyperventilation with 100% oxygen, dantrolene sodium (2.5 mg/kg IV, repeated as needed), and supportive care. All anaesthesia facilities using volatile agents must have dantrolene immediately available. [65-69]

Hepatic Effects

Isoflurane has a favourable hepatic safety profile compared to earlier volatile agents:

Hepatic blood flow:

• Isoflurane reduces hepatic blood flow in proportion to the decrease in cardiac output and blood pressure
• Hepatic arterial buffer response is preserved, partially compensating for reduced portal venous flow
• At clinical concentrations, hepatic oxygen delivery-consumption balance is generally maintained

Hepatotoxicity:

• Incidence: Extremely rare; significantly less than halothane hepatitis
• Mechanism: The minimal metabolism of isoflurane (0.2%) produces small amounts of trifluoroacetylated (TFA) protein adducts that can theoretically trigger immune-mediated hepatitis in susceptible individuals
• Cross-sensitivity: Patients with documented halothane hepatitis may have cross-sensitivity to isoflurane due to structural similarity of TFA metabolites; avoidance is recommended in these patients
• Risk factors: Previous halothane exposure, multiple halogenated anaesthetic exposures, obesity, female sex, middle age

Clinical significance: Isoflurane-induced hepatitis is rare enough that routine liver function monitoring is not required after isoflurane anaesthesia. The drug is considered safe for patients with pre-existing liver disease, though careful attention to hemodynamic stability is important to maintain hepatic perfusion. [70-73]

Renal Effects

Isoflurane has minimal direct renal toxicity:

Renal blood flow:

• Decreases in proportion to reduced cardiac output and blood pressure
• Autoregulation preserved at moderate anaesthetic depths
• Glomerular filtration rate may decrease during anaesthesia but recovers postoperatively

Nephrotoxicity:

• Fluoride ions: Peak serum fluoride concentrations of 3-5 micromol/L are well below the 50 micromol/L threshold associated with methoxyflurane nephrotoxicity
• Compound A: Unlike sevoflurane, isoflurane does not produce Compound A (fluoromethyl-2,2-difluoro-1-[trifluoromethyl]vinyl ether) when exposed to carbon dioxide absorbents, eliminating this potential source of nephrotoxicity
• Clinical studies: No evidence of clinically significant nephrotoxicity even with prolonged isoflurane exposure

Safe in renal impairment: Isoflurane is considered safe for patients with pre-existing renal impairment as elimination is pulmonary rather than renal, and toxic metabolite production is negligible. [74-77]

Clinical Applications

Maintenance of General Anaesthesia

Isoflurane remains one of the most widely used agents globally for maintenance of general anaesthesia:

Typical administration:

• Vaporizer setting: 0.8-1.5% (approximately 0.7-1.3 MAC) when combined with opioids and/or nitrous oxide
• Higher concentrations (1.5-2%) may be required for surgical stimulation without adjuncts
• Delivered via agent-specific calibrated vaporizer in a fresh gas flow of 1-3 L/min for low-flow anaesthesia

Advantages for maintenance anaesthesia:

1. Excellent muscle relaxation, reducing NMBA requirements
2. Cardiovascular stability compared to halothane
3. Bronchodilatory properties beneficial in reactive airway disease
4. No Compound A production, allowing safe use with all CO2 absorbents
5. Lower cost than sevoflurane and desflurane
6. Extensive clinical experience and safety record

Disadvantages:

1. Slower emergence than sevoflurane or desflurane
2. Pungency limits use for inhalational induction
3. Cerebral vasodilation may be problematic in neurosurgery
4. Requires agent-specific vaporizer (not interchangeable)

Low-flow anaesthesia: Isoflurane is well-suited for low-flow techniques (fresh gas flow 0.5-1 L/min) due to:

• Minimal metabolism reducing accumulation of degradation products
• No Compound A concerns
• Cost-effective drug utilization
• Reduced environmental pollution

Balanced anaesthesia: Isoflurane is commonly used as part of balanced anaesthetic technique with:

• Opioids (fentanyl, remifentanil) for analgesia
• Neuromuscular blocking agents for paralysis
• Nitrous oxide (optional) for MAC reduction
• Propofol for induction

This combination allows lower isoflurane concentrations while achieving adequate anaesthesia. [78-82]

ICU Sedation

Isoflurane can be delivered for ICU sedation via specialized devices, most notably the Anaesthetic Conserving Device (AnaConDa or Sedaconda):

Delivery system:

• The AnaConDa is placed between the ventilator Y-piece and patient
• Contains an evaporator rod and activated carbon reflector
• Liquid isoflurane is infused via syringe pump
• The reflector captures approximately 90% of exhaled agent for re-inhalation
• Requires end-tidal agent monitoring and active/passive scavenging

Evidence for ICU sedation:

• The SEDAC trial (2021) demonstrated non-inferiority of isoflurane to propofol for ICU sedation
• Faster time to extubation compared to intravenous sedatives
• Particularly useful when rapid neurological assessment is required
• May provide organ-protective preconditioning effects

Advantages:

1. Organ-independent elimination (beneficial in multi-organ failure)
2. Predictable emergence for daily sedation holds
3. Bronchodilation beneficial in ARDS and asthma
4. Reduced propofol/benzodiazepine exposure and associated complications
5. Potential cardioprotective effects

Disadvantages/Cautions:

1. Requires specialized equipment and training
2. Malignant hyperthermia trigger (rare in ICU setting)
3. May increase ICP in acute brain injury
4. Requires scavenging systems
5. Staff exposure concerns

Dosing for ICU sedation:

• Target end-tidal concentration: 0.3-0.8%
• Titrate to RASS or other sedation scale targets
• Liquid isoflurane infusion: 2-6 mL/hour typically [83-87]

Hypotensive Anaesthesia

Isoflurane's potent vasodilatory properties make it useful for controlled hypotension:

Applications:

• Middle ear surgery (bloodless surgical field)
• Spinal surgery (reduced blood loss)
• Orthopedic surgery (improved surgical conditions)
• Plastic and reconstructive surgery

Technique:

• Increase isoflurane concentration to 1.5-2 MAC
• Target MAP reduction of 20-30% from baseline
• Combine with head-up positioning to enhance effect
• May supplement with beta-blockers or other antihypertensives

Contraindications to deliberate hypotension:

• Coronary artery disease
• Cerebrovascular disease
• Uncontrolled hypertension
• Severe anemia
• Hypovolemia

Monitoring requirements:

• Invasive arterial blood pressure monitoring
• Adequate IV access for fluid resuscitation
• Close attention to perfusion indicators

The predictable, titratable hypotension with isoflurane and rapid recovery of blood pressure with dose reduction make it suitable for this application. [88-91]

Comparison with Other Volatile Agents

Isoflurane vs Sevoflurane

Clinical choice: Sevoflurane is preferred for inhalational induction and pediatric anaesthesia due to minimal pungency. Isoflurane remains widely used for maintenance anaesthesia where its lower cost and lack of Compound A production are advantageous. [92-95]

Isoflurane vs Desflurane

Clinical choice: Desflurane is preferred when rapid emergence is critical (ambulatory surgery, neurosurgery requiring rapid neurological assessment). Isoflurane's lower cost, standard vaporizer requirements, and lesser airway irritation make it preferable for many routine cases. Desflurane's high global warming potential (GWP) has led to reduced use in some institutions. [96-99]

Comparison Table Summary

Adverse Effects

Common Adverse Effects

1. Cardiovascular: Hypotension (dose-dependent), mild tachycardia
2. Respiratory: Respiratory depression, apnea at high concentrations, potential airway irritation
3. Neurological: Postoperative nausea and vomiting (less than with nitrous oxide), emergence delirium (rare), potential neurocognitive effects
4. Musculoskeletal: Postoperative shivering
5. Other: Hypothermia (due to vasodilation and cold inspired gases)

Serious Adverse Effects

1. Malignant hyperthermia: Life-threatening hypermetabolic crisis in genetically susceptible individuals (incidence 1:5,000 to 1:50,000)

2. Hepatotoxicity: Very rare immune-mediated hepatitis (significantly less than halothane)

3. Carbon monoxide toxicity: Potential CO production with desiccated CO2 absorbents

4. Neurotoxicity: Concerns regarding neurodevelopmental effects in young children based on animal studies; FDA warning for repeated or prolonged exposures in children under 3 years

5. Environmental concerns: Isoflurane is a potent greenhouse gas with atmospheric persistence; global warming potential approximately 510 times that of CO2

Contraindications

Absolute:

• Known or suspected malignant hyperthermia susceptibility
• Known hypersensitivity to isoflurane or halogenated agents

Relative:

• Elevated intracranial pressure (use with caution, hyperventilate)
• Severe hypovolemia (correct before induction)
• History of halothane hepatitis (cross-sensitivity possible)
• Myasthenia gravis (enhanced NMBA sensitivity)
• Porphyria (theoretical; limited data) [100-104]

Indigenous Health Considerations

Pharmacokinetic and Pharmacodynamic Considerations in Aboriginal and Torres Strait Islander Populations

Aboriginal and Torres Strait Islander peoples experience higher rates of chronic diseases that may affect isoflurane pharmacology and anaesthetic management. The prevalence of type 2 diabetes mellitus is 3-4 times higher than in non-Indigenous Australians, while chronic kidney disease affects Indigenous Australians at 3.5 times the national rate. These conditions have important implications for volatile anaesthetic administration.

Diabetes-associated autonomic neuropathy may blunt the compensatory tachycardic response to isoflurane-induced vasodilation, increasing susceptibility to profound hypotension. Diabetic patients may also demonstrate altered pharmacodynamics due to changes in blood-brain barrier permeability and neuronal sensitivity. Careful haemodynamic monitoring and reduced induction rates are recommended.

Chronic kidney disease, while not directly affecting isoflurane elimination (which is primarily pulmonary), may alter volume of distribution and protein binding. Uraemic patients may demonstrate increased sensitivity to the CNS-depressant effects of volatile agents. The minimal renal metabolism of isoflurane (compared to methoxyflurane or sevoflurane) makes it a relatively safe choice in renal impairment.

Obesity prevalence varies across Indigenous communities but is elevated in many urban and regional areas. For obese patients, the increased adipose tissue acts as a reservoir for lipophilic isoflurane, potentially prolonging emergence after prolonged administration. However, for induction, dosing should be based on lean body mass rather than total body weight.

Chronic respiratory disease, including bronchiectasis and chronic obstructive pulmonary disease, is more prevalent in some Indigenous communities due to historical factors including childhood infections, overcrowded housing, and smoking rates. Isoflurane's bronchodilatory properties may be advantageous in these patients, though careful attention to oxygenation and ventilation is essential.

Cultural Safety in Anaesthetic Care

Culturally safe anaesthetic practice for Aboriginal and Torres Strait Islander patients requires understanding and respecting traditional health beliefs, family structures, and communication preferences. Many Indigenous patients prefer family involvement in medical decision-making, and consent discussions should accommodate extended family where culturally appropriate. Aboriginal Health Workers and Hospital Liaison Officers are invaluable resources for facilitating culturally safe communication.

Preoperative anxiety may be heightened for Indigenous patients due to historical experiences of discrimination in healthcare settings, unfamiliarity with hospital environments (particularly for patients from remote communities), and separation from family and country. Non-pharmacological anxiolysis strategies, including allowing family presence during induction where safe and practical, may reduce anxiolytic medication requirements and improve patient experience.

Postoperative care should consider potential language barriers, the importance of maintaining connection to family and country during recovery, and the need for culturally appropriate pain assessment tools. Discharge planning must account for access to follow-up care, particularly for patients from remote communities who may require extended stays or retrieval service coordination through organisations such as the Royal Flying Doctor Service.

Research specifically examining volatile anaesthetic pharmacology in Indigenous Australian populations is limited, highlighting the need for culturally appropriate clinical research to address knowledge gaps and ensure evidence-based care for all Australians. Healthcare providers should advocate for inclusive research while applying general pharmacological principles with cultural sensitivity and individualised patient assessment. [105-108]

SAQ Practice Question (15 marks)

Question: A 45-year-old man with a history of recent severe traumatic brain injury (GCS 8, ICP 22 mmHg with EVD in situ) requires emergency laparotomy for splenic rupture following a motor vehicle accident.

a) Discuss the effects of isoflurane on cerebral physiology and the implications for this patient. (6 marks)

b) Compare the cerebral effects of isoflurane with those of propofol and explain your choice of anaesthetic technique for this case. (6 marks)

c) If isoflurane were to be used, outline the strategies to minimise its adverse cerebral effects. (3 marks)

---

Model Answer:

a) Effects of isoflurane on cerebral physiology (6 marks)

Cerebral blood flow (CBF): [2 marks]

• Isoflurane causes dose-dependent cerebral vasodilation, increasing CBF by 15-20% at 1 MAC and up to 50-100% at concentrations above 1.5 MAC
• This vasodilation is a direct effect on cerebral vascular smooth muscle
• In this patient with already elevated ICP (22 mmHg), increased CBF would increase cerebral blood volume and further elevate ICP

Cerebral metabolic rate (CMRO2): [1 mark]

• Isoflurane reduces CMRO2 by approximately 50% at anaesthetic concentrations
• This creates "flow-metabolism uncoupling" where CBF increases despite decreased metabolic demand
• The CMRO2 reduction alone would be potentially neuroprotective, but is offset by increased CBF

Intracranial pressure (ICP): [2 marks]

• The net effect of isoflurane on ICP is usually an increase at concentrations above 1 MAC due to cerebral vasodilation exceeding the effects of reduced CMRO2
• In this patient with reduced intracranial compliance (elevated baseline ICP), even modest increases in cerebral blood volume could cause dangerous ICP elevation
• This could lead to reduced cerebral perfusion pressure (CPP = MAP - ICP) and secondary brain injury

Autoregulation and CO2 reactivity: [1 mark]

• Cerebral autoregulation is preserved up to 1.5 MAC but impaired at higher concentrations
• CO2 reactivity is preserved, which is therapeutically useful (hyperventilation can counteract vasodilation)
• The already injured brain may have impaired autoregulation, making isoflurane effects less predictable

---

b) Comparison with propofol and anaesthetic choice (6 marks)

Propofol cerebral effects: [3 marks]

• Propofol reduces both CBF and CMRO2 in a coupled manner (maintains flow-metabolism coupling)
• CBF reduction of approximately 30-50% at anaesthetic doses
• CMRO2 reduction of approximately 30-50%
• ICP consistently reduced due to cerebral vasoconstriction
• Autoregulation and CO2 reactivity preserved
• Neuroprotective in experimental models

Comparison table:

Anaesthetic choice for this case: [3 marks]

• Total intravenous anaesthesia (TIVA) with propofol is preferred for this patient with elevated ICP
• Propofol will reduce CBF and ICP while maintaining cerebral perfusion
• Induction: Propofol 1.5-2 mg/kg (titrated to effect given TBI)
• Maintenance: Propofol infusion 100-200 mcg/kg/min with remifentanil 0.1-0.25 mcg/kg/min
• Target CPP more than 60 mmHg by maintaining adequate MAP while propofol reduces ICP
• BIS monitoring recommended to guide depth of anaesthesia
• Avoid hypotension (propofol causes vasodilation) which would reduce CPP

---

c) Strategies to minimise isoflurane cerebral effects if used (3 marks)

If isoflurane must be used (e.g., equipment limitations, prolonged case):

1. Limit concentration: Use less than 1 MAC isoflurane (ideally less than 0.5 MAC) supplemented with opioids and/or nitrous oxide to provide adequate anaesthesia while minimising cerebral vasodilation [1 mark]

2. Hyperventilation: Maintain PaCO2 at 30-35 mmHg to induce cerebral vasoconstriction, counteracting isoflurane-induced vasodilation. CO2 reactivity is preserved with isoflurane. [1 mark]

3. Timing and ICP control:

   • Ensure ICP is optimally controlled (CSF drainage via EVD) before isoflurane introduction
   • Elevate head of bed 15-30 degrees
   • Avoid introducing isoflurane during periods of surgical stimulation that may independently raise ICP
   • Have propofol or thiopentone available for ICP rescue [1 mark]

Total: 15 marks

---

Viva Scenario (20 marks)

Examiner: You are the anaesthetist for a 52-year-old woman scheduled for laparoscopic cholecystectomy. She weighs 85 kg and has well-controlled asthma. You plan to use isoflurane for maintenance. Tell me about isoflurane's physicochemical properties.

---

Candidate: Isoflurane is a halogenated methyl ethyl ether with the chemical formula C3H2ClF5O and molecular weight of 184.5 g/mol. It's a clear, colourless liquid with a mildly pungent odour.

The key physical properties include:

• Blood:gas partition coefficient of 1.4, which indicates intermediate solubility
• Oil:gas partition coefficient of 91, correlating with its potency
• MAC of 1.15% in a 40-year-old adult
• Boiling point of 48.5°C
• Saturated vapour pressure of 238 mmHg at 20°C

Examiner: Good. How does the blood:gas partition coefficient affect the speed of induction? [3 marks]

Candidate: The blood:gas partition coefficient determines how much anaesthetic dissolves in blood relative to the alveolar gas phase. A coefficient of 1.4 means that at equilibrium, 1.4 times more isoflurane dissolves in blood than remains in the alveolar gas.

This intermediate solubility has important implications:

• During induction, substantial uptake into blood occurs before alveolar concentration rises significantly
• This slows the rate of rise of FA/FI (alveolar to inspired concentration ratio)
• Compared to sevoflurane (blood:gas 0.65) or desflurane (0.42), induction and emergence are slower
• However, induction is faster than with halothane (blood:gas 2.4)

Clinically, this means isoflurane is suitable for maintenance but not ideal for inhalational induction.

Examiner: What factors could speed up induction with isoflurane? [3 marks]

Candidate: Several factors can accelerate FA/FI rise:

1. Increased alveolar ventilation: Higher minute ventilation delivers more agent and washes out absorbed agent from alveoli

2. Higher inspired concentration: Increasing the dial setting compensates for tissue uptake; we can use the "overpressure" technique initially

3. Reduced cardiac output: Lower pulmonary blood flow means less agent is removed from alveoli, accelerating FA rise - though this also slows brain equilibration

4. Second gas effect: Concurrent use of high-concentration nitrous oxide concentrates the remaining gases including isoflurane

5. Reduced functional residual capacity: Smaller lung volumes equilibrate faster

Examiner: This patient has asthma. Is isoflurane a good choice? Why? [3 marks]

Candidate: Yes, isoflurane is an excellent choice for patients with asthma for several reasons:

1. Bronchodilation: Isoflurane produces direct bronchodilation through relaxation of airway smooth muscle and inhibition of vagal reflexes. This can reduce airway resistance and improve ventilation.

2. Reduction in airway reactivity: Volatile agents decrease the bronchoconstrictive response to stimuli like intubation and surgical manipulation

3. Comparison with alternatives:

   • Better than desflurane, which can cause airway irritation and bronchoconstriction at high concentrations
   • Similar bronchodilatory effects to sevoflurane
   • Propofol also has bronchodilatory properties but doesn't provide the sustained airway relaxation of volatile agents

The main limitation is isoflurane's pungency, which could trigger bronchospasm during inhalational induction - but for maintenance after IV induction, this is not a concern.

Examiner: Describe the cardiovascular effects of isoflurane at 1 MAC. [4 marks]

Candidate: At 1 MAC isoflurane, the cardiovascular effects include:

Blood pressure:

• Mean arterial pressure decreases by 20-30%
• Primary mechanism is reduction in systemic vascular resistance (15-25% decrease)
• This is due to direct vascular smooth muscle relaxation and decreased sympathetic tone

Heart rate:

• Mild tachycardia of 10-20% increase
• This is a baroreceptor-mediated response to vasodilation
• Distinguishes isoflurane from halothane, which causes bradycardia

Cardiac output:

• Generally well preserved despite mild negative inotropy
• The reduced afterload and compensatory tachycardia maintain cardiac output

Myocardial effects:

• Mild negative inotropy (10-15% reduction in contractility)
• Does not sensitise the myocardium to catecholamines (unlike halothane)
• Coronary vasodilation occurs but clinically significant "coronary steal" is rare at standard doses

Examiner: What is "coronary steal" and is it clinically significant? [3 marks]

Candidate: Coronary steal is the theoretical phenomenon where isoflurane-induced coronary arteriolar vasodilation could:

1. Reduce coronary perfusion pressure
2. Redirect blood flow away from ischemic myocardial regions (which have maximally dilated vessels) toward non-ischemic regions (which can dilate further)
3. Worsen ischemia in collateral-dependent myocardium

This was a significant concern in the 1980s following laboratory studies.

Current clinical consensus:

• Extensive clinical studies by Slogoff and Keats (1989) showed that coronary steal is rarely clinically significant at standard anaesthetic doses
• The phenomenon may occur in specific circumstances (significant coronary artery disease with steal-prone anatomy) but is not a general contraindication
• Isoflurane is considered safe for patients with coronary artery disease when:
  • Hypotension is avoided (maintain coronary perfusion pressure)
  • Tachycardia is controlled (reduces myocardial oxygen demand)
  • Hemodynamic stability is maintained

Examiner: How is isoflurane metabolised and why is this clinically important? [4 marks]

Candidate: Isoflurane undergoes remarkably limited hepatic metabolism - only approximately 0.2% of the absorbed dose is metabolised. This is clinically important for several reasons:

Metabolic pathway:

• Primary metabolism is via cytochrome P450 2E1 (CYP2E1) oxidation
• Produces trifluoroacetic acid (TFA) and minimal inorganic fluoride (3-5 micromol/L peak)

Clinical significance:

1. Hepatotoxicity: The minimal metabolism means very little TFA-protein adduct formation, making immune-mediated hepatitis extremely rare (much less than halothane at 20% metabolism)

2. Nephrotoxicity: Peak fluoride of 3-5 micromol/L is well below the 50 micromol/L threshold associated with methoxyflurane nephrotoxicity - no renal concerns

3. No Compound A: Unlike sevoflurane, isoflurane does not produce Compound A with CO2 absorbents, eliminating absorbent-related nephrotoxicity concerns

4. Organ-independent elimination: The 99.8% pulmonary elimination means isoflurane can be safely used in patients with hepatic or renal impairment

5. Drug interactions: The non-reliance on hepatic metabolism means fewer drug interactions compared to agents with significant CYP-mediated metabolism

Examiner: Thank you. That concludes the viva.

Total: 20 marks

---

References

1. Eger EI 2nd. Isoflurane: a review. Anesthesiology. 1981;55(5):559-576. PMID: 6795417

2. Eger EI 2nd. The pharmacology of isoflurane. Br J Anaesth. 1984;56 Suppl 1:71S-99S. PMID: 6715886

3. Patel SS, Goa KL. Isoflurane. A review of its pharmacodynamic and pharmacokinetic properties and its clinical use in general anaesthesia. Drugs. 1996;52(4):574-594. PMID: 8891469

4. Ebert TJ, Muzi M. Sympathetic hyperactivity during desflurane anesthesia in healthy volunteers. A comparison with isoflurane. Anesthesiology. 1993;79(3):444-453. PMID: 8363069

5. Matta BF, Heath KJ, Tipping K, Summors AC. Direct cerebral vasodilatory effects of sevoflurane and isoflurane. Anesthesiology. 1999;91(3):677-680. PMID: 10485778

6. Slogoff S, Keats AS. Randomized trial of primary anesthetic agents on outcome of coronary artery bypass operations. Anesthesiology. 1989;70(2):179-188. PMID: 2913857

7. Kharasch ED, Thummel KE. Identification of cytochrome P450 2E1 as the predominant enzyme catalyzing human liver microsomal defluorination of sevoflurane, isoflurane, and methoxyflurane. Anesthesiology. 1993;79(4):795-807. PMID: 8214760

8. Frink EJ Jr, Malan TP, Isner RJ, Brown EA, Morgan SE, Brown BR Jr. Renal concentrating function with prolonged sevoflurane or enflurane anesthesia in volunteers. Anesthesiology. 1994;80(5):1019-1025. PMID: 8017640

9. Strum DP, Eger EI 2nd. Partition coefficients for sevoflurane in human blood, saline, and olive oil. Anesth Analg. 1987;66(7):654-656. PMID: 3605674

10. Taheri S, Halsey MJ, Liu J, Eger EI 2nd, Koblin DD, Laster MJ. What solvent best represents the site of action of inhaled anesthetics in humans, rats, and dogs? Anesth Analg. 1991;72(5):627-634. PMID: 2018220

11. Stevens WC, Dolan WM, Gibbons RT, et al. Minimum alveolar concentrations (MAC) of isoflurane with and without nitrous oxide in patients of various ages. Anesthesiology. 1975;42(2):197-200. PMID: 1115369

12. Fragen RJ, Dunn KL. The minimum alveolar concentration (MAC) of sevoflurane with and without nitrous oxide in elderly versus young adults. J Clin Anesth. 1996;8(5):352-356. PMID: 8832444

13. Nickalls RW, Mapleson WW. Age-related iso-MAC charts for isoflurane, sevoflurane and desflurane in man. Br J Anaesth. 2003;91(2):170-174. PMID: 12878613

14. Fang ZX, Eger EI 2nd, Laster MJ, Chortkoff BS, Kandel L, Ionescu P. Carbon monoxide production from degradation of desflurane, enflurane, isoflurane, halothane, and sevoflurane by soda lime and Baralyme. Anesth Analg. 1995;80(6):1187-1193. PMID: 7762849

15. Baxter PJ, Garton K, Kharasch ED. Mechanistic aspects of carbon monoxide formation from volatile anesthetics. Anesthesiology. 1998;89(4):929-941. PMID: 9778011

16. Belelli D, Lambert JJ, Peters JA, Wafford K, Whiting PJ. The interaction of the general anesthetic etomidate with the gamma-aminobutyric acid type A receptor is influenced by a single amino acid. Proc Natl Acad Sci USA. 1997;94(20):11031-11036. PMID: 9380754

17. Franks NP, Lieb WR. Molecular and cellular mechanisms of general anaesthesia. Nature. 1994;367(6464):607-614. PMID: 7509043

18. Eger EI 2nd, Koblin DD, Bowland T, et al. Nephrotoxicity of sevoflurane versus desflurane anesthesia in volunteers. Anesth Analg. 1997;84(1):160-168. PMID: 8989018

19. Kersten JR, Schmeling TJ, Pagel PS, Gross GJ, Warltier DC. Isoflurane mimics ischemic preconditioning via activation of K(ATP) channels: reduction of myocardial infarct size with an acute memory phase. Anesthesiology. 1997;87(2):361-370. PMID: 9286901

20. De Hert SG, Van der Linden PJ, Cromheecke S, et al. Cardioprotective properties of sevoflurane in patients undergoing coronary surgery with cardiopulmonary bypass are related to the modalities of its administration. Anesthesiology. 2004;101(2):299-310. PMID: 15277911

21. Landoni G, Biondi-Zoccai GG, Zangrillo A, et al. Desflurane and sevoflurane in cardiac surgery: a meta-analysis of randomized clinical trials. J Cardiothorac Vasc Anesth. 2007;21(4):502-511. PMID: 17678775

22. Landoni G, Lomivorotov VV, Nigro Neto C, et al. Volatile Anesthetics versus Total Intravenous Anesthesia for Cardiac Surgery. N Engl J Med. 2019;380(13):1214-1225. PMID: 30888743

23. Rosenberg H, Pollock N, Schiemann A, Bulger T, Stowell K. Malignant hyperthermia: a review. Orphanet J Rare Dis. 2015;10:93. PMID: 26238698

24. Hopkins PM, Rüffert H, Snoeck MM, et al. European Malignant Hyperthermia Group guidelines for investigation of malignant hyperthermia susceptibility. Br J Anaesth. 2015;115(4):531-539. PMID: 26188342

25. Jerath A, Panckhurst J, Parotto M, et al. Safety and Efficacy of Volatile Anesthetic Agents Compared With Standard Intravenous Midazolam/Propofol Sedation in Ventilated Critical Care Patients: A Meta-analysis and Systematic Review of Prospective Trials. Anesth Analg. 2017;124(4):1190-1199. PMID: 26176510

26. Meiser A, Laubenthal H. Inhalational anaesthetics in the ICU: theory and practice of inhalational sedation in the ICU, economics, risk-benefit. Best Pract Res Clin Anaesthesiol. 2005;19(3):523-538. PMID: 16013698

27. Mesnil M, Capdevila X, Bringuier S, et al. Long-term sedation in intensive care unit: a randomized comparison between inhaled sevoflurane and intravenous propofol or midazolam. Intensive Care Med. 2011;37(6):933-941. PMID: 21445642

28. Sackey PV, Martling CR, Granath F, Radell PJ. Prolonged isoflurane sedation of intensive care unit patients with the Anesthetic Conserving Device. Crit Care Med. 2004;32(11):2241-2246. PMID: 15640636

29. Röhm KD, Wolf MW, Schöllhorn T, Schellhaass A, Boldt J, Piper SN. Short-term sevoflurane sedation using the Anaesthetic Conserving Device after cardiothoracic surgery. Intensive Care Med. 2008;34(9):1683-1689. PMID: 30335029

30. Jerath A, Beattie SW, Engelman DT, et al. Volatile anesthetics and perioperative outcomes: is there any evidence? An expert opinion. J Cardiothorac Vasc Anesth. 2019;33(12):3406-3411. PMID: 34119011

31. Zaugg M, Schaub MC, Pasch T, Spahn DR. Modulation of beta-adrenergic receptor subtype activities in perioperative medicine: mechanisms and sites of action. Br J Anaesth. 2002;88(1):101-123. PMID: 12933399

32. Tanaka K, Ludwig LM, Kersten JR, Pagel PS, Warltier DC. Mechanisms of cardioprotection by volatile anesthetics. Anesthesiology. 2004;100(3):707-721. PMID: 15504100