Bariatric Surgery Anaesthesia

Quick Answer

Bariatric surgery anaesthesia presents unique challenges due to obesity-related physiological changes. Key physiological alterations: Reduced functional residual capacity (FRC) by 30-50%, increased closing capacity leading to rapid desaturation, increased intra-abdominal pressure, and obstructive sleep apnoea (OSA) affecting 40-70% of morbidly obese patients. Airway management: Anticipate difficult mask ventilation and intubation; use ramped position with ear-to-sternal notch alignment; video laryngoscopy recommended as first-line. Ventilation strategy: Pressure-controlled ventilation, higher PEEP (10-15 cmH₂O), recruitment manoeuvres, and careful positioning to minimize atelectasis. VTE prophylaxis: Extended chemoprophylaxis with LMWH (enoxaparin 40mg twice daily for BMI ≥40) or unfractionated heparin 7,500 units three times daily, plus mechanical prophylaxis. Rhabdomyolysis prevention: Position padding, limit operative time, and monitor creatine kinase (CK) postoperatively if prolonged surgery. Propofol dosing: Use ideal body weight (IBW) for induction (1.5-2.5 mg/kg IBW) and lean body weight (LBW) for maintenance infusion (maximum 4 mg/kg/hr TBW to prevent PRIS). [1-25]

Physiological Changes in Obesity

Respiratory System Changes

Mechanical Alterations:

• Increased intra-abdominal pressure: Elevated by 5-15 mmHg above normal due to central adiposity, reducing diaphragmatic excursion
• Decreased FRC: Reduced by 30-50% in morbid obesity (BMI >40 kg/m²), predisposing to rapid oxygen desaturation
• Increased closing capacity: Exceeds FRC in supine position, leading to airway closure and ventilation-perfusion mismatch
• Reduced chest wall compliance: Up to 30% reduction due to increased weight on chest wall
• Decreased pulmonary compliance: Approximately 20-30% reduction in static compliance
• Increased work of breathing: Up to 3-5 times normal due to reduced compliance and increased airway resistance

Gas Exchange Abnormalities:

• Ventilation-perfusion mismatch: Predominant V/Q mismatch in dependent lung zones
• Increased physiological shunt: 5-10% increase due to atelectasis in supine position
• Decreased arterial oxygenation: Lower PaO₂ due to shunt and V/Q mismatch
• Normal or increased PaCO₂: Usually preserved unless severe obesity hypoventilation syndrome present

Atelectasis Development:

• Morbidly obese patients develop significant atelectasis within 5 minutes of induction
• Atelectasis affects 20-30% of total lung capacity in supine position
• Positive end-expiratory pressure (PEEP) of 10-15 cmH₂O required to maintain FRC

Cardiovascular System Changes

Cardiac Changes:

• Increased cardiac output: 0.1 L/min per kg of excess body weight to perfuse adipose tissue
• Increased blood volume: Total blood volume increases proportionally with body mass (60-70 mL/kg TBW)
• Hypertension: Present in 60-70% of morbidly obese patients
• Left ventricular hypertrophy: Due to chronic volume and pressure overload
• Diastolic dysfunction: Common even with preserved systolic function
• Cor pulmonale: Secondary to OSA and obesity hypoventilation syndrome

Vascular Changes:

• Increased preload: Enhanced venous return from expanded blood volume
• Increased afterload: Due to hypertension and increased peripheral resistance
• Higher incidence of coronary artery disease: 2-3 times higher than non-obese population
• Venous stasis and incompetence: Contributing to VTE risk

Hemodynamic Response to Anaesthesia:

• Greater hypotension with induction agents due to slower circulation time
• Cardiovascular depressant effects of anaesthetics more pronounced
• Difficult non-invasive blood pressure measurement due to arm circumference

Metabolic and Endocrine Changes

Metabolic Syndrome:

• Insulin resistance: Universal in morbid obesity, progressing to type 2 diabetes in 30-40%
• Dyslipidemia: Elevated triglycerides, decreased HDL cholesterol
• Fatty liver disease: Non-alcoholic fatty liver disease (NAFLD) in 70-90%
• Hypercoagulable state: Elevated fibrinogen, factor VII, PAI-1, and platelet activation

Gastrointestinal Changes:

• Increased gastric volume: Due to increased caloric intake and delayed gastric emptying
• Reduced lower oesophageal sphincter tone: Contributing to reflux risk
• Hiatal hernia: Present in 30-50% of morbidly obese patients
• Fatty infiltration of liver: Increases risk of drug metabolism abnormalities

Endocrine Considerations:

• Altered drug metabolism: Increased volume of distribution for lipophilic drugs, decreased for hydrophilic drugs
• Increased cortisol: Due to stress response and metabolic syndrome
• Altered thyroid function: Subclinical hypothyroidism common

Pharmacokinetic and Pharmacodynamic Changes

Volume of Distribution (Vd):

• Lipophilic drugs: Increased Vd (propofol, benzodiazepines, opioids) requiring adjusted dosing based on lean body weight (LBW)
• Hydrophilic drugs: Vd correlates with extracellular fluid volume and ideal body weight (IBW)
• Protein binding: Albumin normal, alpha-1-acid glycoprotein may be elevated

Drug Clearance:

• Increased cardiac output: Accelerates initial distribution but not clearance
• Hepatic metabolism: Variable effects depending on liver function (fatty liver)
• Renal clearance: Glomerular filtration rate often increased (obesity-related glomerulopathy)

Specific Drug Dosing Considerations:

Obstructive Sleep Apnoea (OSA)

Epidemiology and Diagnosis

Prevalence:

• OSA affects 40-70% of patients presenting for bariatric surgery
• Severe OSA (AHI >30) present in 20-30% of morbidly obese patients
• Many patients undiagnosed preoperatively (estimated 70-80%)

Screening Tools:

• STOP-BANG questionnaire: Most validated screening tool
  • Snoring, Tiredness, Observed apnoea, high blood Pressure
  • BMI >35, Age >50, Neck circumference >40cm, male Gender
  • Score ≥3 indicates high OSA risk (sensitivity 93-100%)
• Berlin Questionnaire: Alternative screening tool
• Nocturnal oximetry: If formal polysomnography unavailable

Risk Stratification:

• High risk: STOP-BANG ≥5, known severe OSA, oxygen-dependent
• Moderate risk: STOP-BANG 3-4, symptoms suggestive
• Low risk: STOP-BANG 0-2, no symptoms

Perioperative Implications

Airway Management:

• Difficult mask ventilation 3-5 times more likely
• Difficult intubation increased risk (relative risk 2-3)
• Rapid desaturation during apnoea (time to desaturation <3 minutes)
• CPAP/BiPAP availability in recovery essential

Anaesthetic Considerations:

• Induction: Ramped position, video laryngoscopy, preparation for difficult airway
• Maintenance: Careful opioid titration, multimodal analgesia to reduce opioid requirements
• Emergence: Extubate awake, sitting position if possible, CPAP immediately available
• Postoperative: High dependency care, CPAP continuation, prone positioning avoided

Specific Recommendations:

• Continue home CPAP/BiPAP therapy through perioperative period
• Bring CPAP machine to hospital for postoperative use
• Extubate only when fully awake with intact airway reflexes
• Avoid supine positioning postoperatively if possible

Airway Management in Bariatric Surgery

Preoperative Assessment

History:

• Previous difficult intubation or mask ventilation
• Snoring, witnessed apnoeas, morning headaches
• Neck circumference measurement (>40 cm high risk)
• Mallampati score (III or IV increased risk)

Physical Examination:

• Mouth opening: <3 finger breadths concerning
• Thyromental distance: <6 cm indicates potential difficulty
• Mandibular protrusion: Reduced prognathism predicts difficult intubation
• Neck circumference: >40 cm predicts difficult intubation (OR 4.5)
• Upper lip bite test: Inability to bite upper lip predicts difficult intubation

Airway Scoring Systems:

• LEMON assessment: Look externally, Evaluate 3-3-2, Mallampati, Obstruction, Neck mobility
• Obesity-specific: Consider higher Mallampati due to redundant pharyngeal tissue

Positioning for Airway Management

Ramped Position (Ear-to-Sternal Notch Alignment):

• Place multiple folded blankets or commercial ramp under patient
• Align external auditory meatus with sternal notch in horizontal plane
• This position improves laryngoscopic view by 1-2 grades in obese patients
• Reduces airway collapsibility and improves mask ventilation

Comparison of Positions:

Lateral Position:

• Alternative for mask ventilation if supine fails
• Improves FRC and compliance
• Consider for induction in high-risk patients

Intubation Techniques

First-Line: Video Laryngoscopy:

• C-MAC, GlideScope, McGrath or similar devices recommended
• Improves laryngoscopic view in 80-90% of difficult airways
• Reduces intubation attempts and airway trauma
• Stylet or bougie often required for tube delivery

Direct Laryngoscopy:

• Still requires mastery for potential device failure
• Macintosh blade may be suboptimal; Miller or McCoy may help
• External laryngeal manipulation (BURP - Backwards, Upwards, Rightwards Pressure) useful

Difficult Airway Algorithm in Obesity:

1. Preoxygenation with CPAP or 100% O₂ for 3-5 minutes
2. Ramped position
3. Video laryngoscopy first attempt
4. If failed: Second attempt with bougie/stylet optimization
5. If failed: Supraglottic airway (LMA) as rescue
6. If failed: Emergency front-of-neck access

Extubation Strategy

Awake Extubation:

• Patient must be fully awake with intact airway reflexes
• Extubate in semi-upright or sitting position if possible
• Have CPAP immediately available
• Consider airway exchange catheter for high-risk extubations

Risk Factors for Post-Extubation Obstruction:

• Severe OSA
• Difficult intubation
• Prolonged surgery
• Airway oedema
• Opioid-induced respiratory depression

Ventilatory Management

Intraoperative Ventilation Strategy

Mechanical Ventilation Settings:

• Mode: Pressure-controlled or volume-controlled with pressure limit
• Tidal volume: 6-8 mL/kg IBW (not TBW)
• Respiratory rate: 12-16 breaths/min to maintain normocapnia
• PEEP: 10-15 cmH₂O to maintain FRC and reduce atelectasis
• Inspiration:Expiration ratio: 1:2 or inverse ratio in severe cases
• FiO₂: Minimum to maintain SpO₂ >94% (reduces atelectasis vs 100%)

Recruitment Maneuvers:

• Apply 30-40 cmH₂O for 30-40 seconds after intubation
• Repeat after position changes
• Reduces atelectasis and improves oxygenation
• Monitor hemodynamics during recruitment (can cause hypotension)

Monitoring:

• EtCO₂: Higher PaCO₂-EtCO₂ gradient in obesity; may need arterial line
• Airway pressures: Monitor plateau pressure (<30 cmH₂O ideal)
• Arterial blood gas: Consider if prolonged surgery or respiratory issues
• SpO₂: Continuous monitoring essential

Positioning for Surgery

Common Positions in Bariatric Surgery:

1. Supine with Reverse Trendelenburg:

   • Reduces aspiration risk
   • Improves ventilation-perfusion matching
   • Decreases venous congestion

2. Lithotomy:

   • Further reduces FRC
   • Increased VTE risk
   • Careful padding of lower extremities

3. Left Lateral Decubitus:

   • Improves FRC compared to supine
   • Better oxygenation
   • Access challenges for surgeon

Pressure Injury Prevention:

• Gel pads or foam padding at all pressure points
• Special attention to occiput, scapulae, sacrum, heels
• Frequent repositioning if possible
• Check skin immediately postoperatively

Venous Thromboembolism (VTE) Prophylaxis

Risk Stratification

Highest Risk Population:

• Bariatric surgery patients at highest risk for VTE among all surgical patients
• Risk factors cumulative: obesity, immobility, pneumoperitoneum, Trendelenburg position
• Incidence of DVT: 1-3% without prophylaxis
• Incidence of PE: 0.5-2% without prophylaxis; fatal PE 0.1-0.3%

Risk Factors:

• BMI >50 kg/m² (highest risk)
• History of VTE
• Hypercoagulable disorders
• Prolonged operative time (>3 hours)
• Immobility postoperatively
• OSA with hypoventilation
• Heart failure

Pharmacological Prophylaxis

Low Molecular Weight Heparin (LMWH):

• Enoxaparin: 40 mg subcutaneous twice daily for BMI ≥40 kg/m² (higher dose than standard)
• Standard prophylaxis (40 mg daily) insufficient for morbid obesity
• Initiate 12-24 hours postoperatively (balance bleeding vs VTE risk)
• Continue for 7-10 days postoperatively (extended prophylaxis)

Unfractionated Heparin (UFH):

• 5,000 units subcutaneous three times daily
• Alternative if LMWH contraindicated (renal failure, bleeding risk)
• Shorter half-life, easier to reverse if bleeding
• High-dose UFH: 7,500 units three times daily for BMI >50 kg/m²

Warfarin:

• Rarely used for bariatric surgery prophylaxis
• If indicated for mechanical valves or prior VTE, bridge appropriately
• Vitamin K malabsorption after malabsorptive procedures affects dosing

Direct Oral Anticoagulants (DOACs):

• Limited data in bariatric surgery
• Avoided in immediate perioperative period
• May be considered for extended prophylaxis in high-risk patients

Mechanical Prophylaxis

Intraoperative:

• Sequential compression devices (SCDs) on lower extremities from induction
• Continue until patient ambulating

Postoperative:

• Continue SCDs until full ambulation achieved
• Early ambulation (within 4-6 hours postoperatively)
• Anti-embolism stockings (if calf circumference permits fitting)

Inferior Vena Cava (IVC) Filters:

• Controversial; not routine
• Consider only for very high-risk patients with contraindications to anticoagulation
• Increased risk of IVC thrombosis and filter complications
• Must be retrieved postoperatively

Guidelines Summary

Rhabdomyolysis Risk and Prevention

Pathophysiology

Risk Factors:

• Prolonged immobility: Pressure on muscle groups causes ischemia
• Pressure points: Sacrum, buttocks, calves most affected
• Operative time: Risk increases exponentially after 4-5 hours
• Patient positioning: Trendelenburg and lithotomy increase pressure
• Hypotension: Reduces muscle perfusion
• Male gender: Increased muscle mass higher risk

Incidence:

• Occurs in 0.5-2% of bariatric surgeries
• More common in super-obesity (BMI >50)
• Often asymptomatic, detected by elevated CK

Prevention Strategies

Positioning:

• Adequate padding: Use gel pads at all pressure points
• Frequent repositioning: Every 2 hours if surgery prolonged
• Avoid extreme Trendelenburg: Limit angle to <30 degrees when possible
• Lithotomy positioning: Limit time, ensure adequate padding
• Arm positioning: Avoid extreme abduction (>90 degrees)

Hemodynamic Management:

• Maintain adequate perfusion pressure
• Avoid prolonged hypotension
• Maintain urine output >0.5 mL/kg/hr
• Consider arterial line for beat-to-beat monitoring

Monitoring:

• Creatine kinase (CK): Check postoperatively if prolonged surgery (>4 hours) or high-risk positioning
• Urinalysis: Check for myoglobinuria (tea-colored urine)
• Renal function: Monitor creatinine if CK elevated

Treatment

Mild Rhabdomyolysis (CK <5,000 U/L):

• Aggressive hydration (maintain urine output >100 mL/hr)
• Monitor renal function
• Usually self-limiting

Severe Rhabdomyolysis (CK >5,000 U/L or with renal impairment):

• Aggressive intravenous crystalloid resuscitation
• Consider sodium bicarbonate (urine alkalinization target pH >6.5)
• Avoid loop diuretics (may worsen myoglobin precipitation)
• Consider mannitol (osmotic diuretic) after adequate hydration
• Nephrology consultation if renal impairment develops
• Compartment syndrome: Emergency fasciotomy if present

Propofol Infusion Syndrome (PRIS) in Obesity

Risk Factors and Mechanism

Pathophysiology:

• Mitochondrial dysfunction leading to lactic acidosis, cardiac failure, rhabdomyolysis, and renal failure
• Uncoupling of oxidative phosphorylation
• Inhibition of mitochondrial fatty acid oxidation
• Obesity increases risk due to altered drug metabolism and higher infusion requirements

Risk Factors:

• High-dose propofol infusions: >4 mg/kg/hr for >48 hours
• Duration: Risk increases significantly after 48-72 hours
• Catecholamine use: Concurrent vasopressor administration
• Corticosteroid use: Exacerbates metabolic derangements
• Age: Younger patients at higher risk
• Critical illness: Sepsis, multisystem organ failure
• Obesity: Higher absolute doses required; triglyceride abnormalities

Prevention in Bariatric Surgery

Dosing Strategies:

• Induction: 1.5-2.5 mg/kg ideal body weight (IBW)
• Maintenance: Maximum 4 mg/kg/hr total body weight (TBW) - never exceed this threshold
• Alternative agents: Consider desflurane or other agents for prolonged cases
• Multimodal approach: Combine with opioids, regional techniques to reduce propofol requirements

Monitoring:

• Triglycerides: Check every 12-24 hours if propofol infusion >48 hours
• CK: As baseline and every 24 hours with prolonged infusion
• Lactate: Monitor for unexplained metabolic acidosis
• ECG: Watch for arrhythmias, Brugada-like pattern, heart block
• Hemodynamics: Monitor for unexplained cardiac depression

Early Warning Signs:

• Unexplained metabolic acidosis
• Elevated lactate despite adequate oxygenation
• New arrhythmias or ECG changes
• Elevated triglycerides (>400 mg/dL or 4.5 mmol/L)
• Unexplained bradycardia

Management of Suspected PRIS

Immediate Actions:

1. Stop propofol infusion immediately
2. Switch to alternative sedation (benzodiazepines, dexmedetomidine, volatile agents)
3. Aggressive hemodynamic support
4. ECMO/CPR if cardiac failure refractory

Supportive Care:

• Inotropic support (dobutamine, milrinone preferred over catecholamines)
• Hemodialysis for refractory acidosis or renal failure
• Cardiac pacing if bradycardia unresponsive
• Glucose management (often hyperglycemia)
• Management of hyperkalemia

Prognosis:

• Mortality 30-50% if full syndrome develops
• Early recognition critical
• Most cases occur in ICU setting, not intraoperatively

Fluid and Hemodynamic Management

Fluid Requirements

Calculation:

• Maintenance: 1.5-2 mL/kg/hr lean body weight (not total body weight)
• Deficit: Standard Holliday-Segar formula based on IBW
• Third space losses: May be significant with extensive dissection
• Blood loss: Replace mL-for-mL with crystalloid (3:1 ratio) or colloid (1:1)

Monitoring:

• Arterial line for beat-to-beat monitoring recommended for BMI >50 or comorbidities
• Central venous catheter rarely needed unless major fluid shifts anticipated
• Urinary catheter essential; target urine output >0.5 mL/kg/hr
• Consider stroke volume variation (SVV) or pulse pressure variation (PPV) for fluid responsiveness

Fluid Choice:

• Balanced crystalloids preferred (Plasma-Lyte, Hartmann's)
• Avoid excessive normal saline (hyperchloremic acidosis)
• Colloids if significant volume required (albumin, gelatin-based)

Hemodynamic Goals

Blood Pressure Management:

• Target MAP: >65 mmHg, higher if chronic hypertension
• Avoid hypotension: Reduces tissue perfusion and increases complications
• Avoid hypertension: Increases bleeding risk and myocardial oxygen demand
• Positioning: Reverse Trendelenburg improves venous return and BP

Vasoactive Medications:

• Phenylephrine: Pure alpha-agonist; good for maintenance; may reduce cardiac output
• Metaraminol: Good for bolus dosing
• Noradrenaline: If significant cardiac output reduction; first-line for septic shock
• Vasopressin: Add-on if catecholamine-resistant hypotension

Postoperative Care and Complications

Immediate Postoperative Period

Recovery Room Priorities:

1. Airway: CPAP immediately available; OSA patients high priority for monitoring
2. Ventilation: Supplemental O₂; maintain SpO₂ >94% on room air before discharge to ward
3. Hemodynamics: Continue invasive monitoring if hemodynamically labile
4. Pain: Multimodal analgesia to minimize opioid requirements
5. Nausea: Aggressive antiemetic prophylaxis (high risk)

Extubation Criteria:

• Awake and following commands
• Return of airway reflexes
• Adequate oxygenation on minimal FiO₂
• Hemodynamically stable
• Pain adequately controlled
• Normothermic

Common Complications

Respiratory:

• Atelectasis: Universal; physiotherapy, incentive spirometry, early ambulation
• Pneumonia: Risk increased with aspiration, immobility
• Respiratory failure: May require non-invasive ventilation or reintubation
• Pulmonary embolism: Peak risk POD 2-5

Cardiovascular:

• Myocardial infarction: Monitor for chest pain, ECG changes, troponin elevation
• Arrhythmias: Atrial fibrillation common postoperatively
• Deep vein thrombosis: Regular lower extremity assessment
• Hypertension: Common; manage carefully to avoid stroke or cardiac events

Surgical:

• Anastomotic leak: Bariatric surgery specific; septic presentation
• Gastric pouch bleeding: May require reoperation
• Bowel obstruction: Internal hernias after bypass procedures
• Wound infection: Higher risk due to wound size and adipose tissue

Metabolic:

• Hypoglycemia: After malabsorptive procedures
• Thiamine deficiency: Wernicke's encephalopathy risk
• Dehydration: Reduced oral intake capacity
• Dumping syndrome: Post-gastric bypass

Indigenous Health Considerations

Aboriginal and Torres Strait Islander Peoples:

Indigenous Australians experience disproportionately high rates of obesity and obesity-related comorbidities compared to non-Indigenous populations. The prevalence of obesity in Aboriginal and Torres Strait Islander adults is approximately 40-45%, significantly higher than the general Australian population. This disparity is driven by complex interrelated factors including socioeconomic disadvantage, limited access to fresh nutritious food in remote communities, historical trauma, and ongoing systemic inequities in healthcare access.

Obstructive sleep apnoea (OSA) is particularly prevalent among Indigenous Australians, with studies suggesting rates 2-3 times higher than non-Indigenous populations. This increased prevalence is multifactorial, relating to higher rates of obesity, craniofacial characteristics including retrognathia, and increased rates of tobacco smoking. The high burden of cardiovascular disease, type 2 diabetes, and chronic kidney disease in this population creates compounding risks for perioperative complications.

Cultural safety in the perioperative setting requires specific considerations. Many Aboriginal patients may experience institutional barriers and historical distrust of healthcare systems. Engaging Aboriginal Health Workers (AHWs) and Aboriginal Liaison Officers (ALOs) is essential for effective communication, particularly when discussing sensitive topics such as obesity, sleep apnoea, and surgical interventions. These cultural brokers help navigate language barriers, cultural protocols, and family involvement in healthcare decision-making.

Remote and rural Indigenous communities face additional challenges in bariatric surgery pathways. Many patients must travel significant distances to access tertiary surgical services, requiring coordination with patient travel schemes and ensuring adequate social support accompanies the patient. Postoperative follow-up presents particular difficulties, with limited access to dietitian support, CPAP services, and ongoing metabolic monitoring in remote settings. Telehealth and outreach specialist services play a crucial role in maintaining surgical outcomes.

Genetic factors may influence drug metabolism in Indigenous populations, potentially affecting anaesthetic drug dosing and response. While specific data for bariatric surgery pharmacokinetics is limited, anaesthetists should be alert to potential variations in sensitivity to opioids and sedative agents. Close monitoring and individualised titration are essential.

Māori Health Considerations:

Māori experience similarly elevated rates of obesity, with approximately 48% of Māori adults classified as obese compared to 29% of non-Māori adults. This disparity reflects broader health inequities and the ongoing impacts of colonisation on determinants of health including food security, physical activity opportunities, and healthcare access.

Whānau involvement in healthcare decision-making is fundamental to Māori health models. Discussions regarding bariatric surgery require engagement with extended family networks, not solely the individual patient. Māori Health Workers and cultural advisors should be involved early in the surgical pathway to ensure culturally safe care.

The prevalence of OSA among Māori populations is significantly elevated, with studies indicating untreated rates as high as 30-40% in some communities. Socioeconomic barriers to CPAP access and adherence create ongoing challenges in optimizing patients for surgery. Preoperative CPAP optimization programs, often delivered through community-based models, have shown success in improving surgical readiness.

Postoperative nutrition support requires cultural adaptation. Traditional Māori foods and dietary patterns should be incorporated into post-bariatric nutrition counseling where appropriate. Understanding whānau food preparation practices and the social significance of shared meals is essential for effective dietary intervention.

Institutional racism and unconscious bias in healthcare settings remain barriers to equitable care. Anaesthesia departments should ensure cultural safety training for all staff and establish relationships with local Māori health providers to facilitate seamless care pathways.

ANZCA Exam Focus

Common Viva Topics

Physiology and Pharmacology:

• Explain the respiratory changes in obesity and their impact on anaesthesia
• Describe the pharmacokinetic changes in obesity and how they affect drug dosing
• Discuss the pathophysiology of OSA and its perioperative implications
• Explain the mechanisms of PRIS and prevention strategies

Clinical Management:

• How would you manage the airway of a 160kg patient with severe OSA for laparoscopic sleeve gastrectomy?
• Discuss your VTE prophylaxis strategy for a 45-year-old with BMI 52 undergoing gastric bypass
• Describe the positioning and ventilatory strategy for bariatric surgery
• How would you recognize and manage rhabdomyolysis postoperatively?

Assessment Content

SAQ 1: Pharmacology in Obesity (20 marks)

A 52-year-old male (weight 140kg, height 175cm, BMI 45.7) is scheduled for laparoscopic sleeve gastrectomy. He has a history of hypertension, type 2 diabetes, and severe OSA (AHI 45) on CPAP.

a) Calculate this patient's ideal body weight (IBW) and lean body weight (LBW) using the Janmahasatian equations. (4 marks)

b) For the following drugs, state which body weight you would use for dosing and calculate the appropriate dose: i) Propofol for induction ii) Suxamethonium iii) Rocuronium for intubation iv) Fentanyl for analgesia (8 marks)

c) Explain why propofol infusion syndrome (PRIS) is a particular concern in obese patients and outline your strategies to prevent it. (8 marks)

Model Answer:

a) IBW and LBW calculation:

• BMI = 45.7 kg/m²

• IBW (Janmahasatian) = 9270 × (1.75) / (6680 + (216 × BMI)) = 9270 × 1.75 / (6680 + 9871) = 16222.5 / 16551 = 70.3 kg (3 marks)

• LBW = (9270 × weight) / (6680 + (216 × BMI)) = (9270 × 140) / 16551 = 1297800 / 16551 = 78.4 kg (1 mark)

b) Drug dosing: i) Propofol induction: Use IBW Dose: 1.5-2.5 mg/kg × 70.3 = 105-176 mg (usually use 150-200 mg) (2 marks)

ii) Suxamethonium: Use TBW (pseudocholinesterase correlates with body mass) Dose: 1.0-1.5 mg/kg × 140 = 140-210 mg (2 marks)

iii) Rocuronium: Use IBW (distribution in extracellular fluid) Dose: 0.6-1.2 mg/kg × 70.3 = 42-84 mg (2 marks)

iv) Fentanyl: Use LBW (lipophilic, large Vd but central effects correlate with lean mass) Dose: 2-5 mcg/kg × 78.4 = 157-392 mcg (2 marks)

c) PRIS in obesity: Mechanism concerns:

• Obese patients may require higher absolute propofol doses for adequate sedation (2 marks)
• Altered lipid metabolism and mitochondrial function in obesity may increase susceptibility (2 marks)
• Higher baseline triglycerides may mask early signs of PRIS (1 mark)
• Difficult dosing calculations increase risk of overdose (1 mark)

Prevention strategies:

• Use IBW for induction, LBW for maintenance (not TBW) (1 mark)
• Maximum 4 mg/kg/hr TBW for prolonged infusions (1 mark)
• Limit propofol duration; use alternative agents for prolonged cases (1 mark)
• Monitor triglycerides every 12-24 hours with prolonged infusions (1 mark)
• Early recognition: monitor for unexplained lactic acidosis, arrhythmias, elevated CK (1 mark)
• Immediately stop propofol and switch to alternative if any signs develop (1 mark)

SAQ 2: VTE Prophylaxis (20 marks)

A 42-year-old female with BMI 52 kg/m² is scheduled for laparoscopic Roux-en-Y gastric bypass. She has a history of previous DVT following hip surgery 5 years ago.

a) Outline the specific risk factors for VTE in this patient. (6 marks)

b) Describe your pharmacological VTE prophylaxis strategy, including drug choice, dosing, timing, and duration. (8 marks)

c) What additional non-pharmacological measures would you implement, and why? (6 marks)

Model Answer:

a) Risk factors:

• BMI 52 kg/m² (very high risk category) (1 mark)
• Previous VTE history (strongest predictor of recurrence) (1 mark)
• Major abdominal surgery with pneumoperitoneum (1 mark)
• Prolonged operative time (bariatric procedures often >3 hours) (1 mark)
• Postoperative immobility during recovery (1 mark)
• Hypercoagulable state of obesity (increased fibrinogen, factor VII, PAI-1) (1 mark)
• Potential OSA (common in this BMI range) contributing to immobility (1 mark)

b) Pharmacological prophylaxis: Drug choice:

• LMWH (enoxaparin) preferred over UFH (1 mark)
• Higher dose required due to BMI >50: 40 mg twice daily (NOT standard 40 mg daily) (2 marks)
• Alternative: UFH 7,500 units three times daily if LMWH contraindicated (1 mark)

Timing:

• First dose: 12-24 hours postoperatively (balance bleeding vs thrombosis risk) (1 mark)
• Earlier if surgery uncomplicated and bleeding risk low (1 mark)

Duration:

• Extended prophylaxis: minimum 14 days (1 mark)
• Consider 30 days given prior VTE history and super-obesity (1 mark)
• May require transition to oral anticoagulant if prolonged needed (1 mark)

c) Non-pharmacological measures:

• Sequential compression devices (SCDs): Apply before induction, continue until ambulating (2 marks)
• Early ambulation: Mobilize within 4-6 hours postoperatively if possible (1 mark)
• Adequate hydration: Maintain euvolemia to prevent hemoconcentration (1 mark)
• Anti-embolism stockings: If calf circumference permits fitting (1 mark)
• Physical therapy: Respiratory and limb physiotherapy to promote circulation (1 mark)
• Consider IVC filter: Only if absolute contraindication to anticoagulation (controversial, not routine) (1 mark)

References

1. De Baerdemaeker L, Margarson M. Best evidence regarding the influence of obesity on perioperative outcomes. Best Pract Res Clin Anaesthesiol. 2011;25(3):299-308. PMID: 22103895
2. Pelosi P, Gregoretti C. Perioperative management of obese patients. Br J Anaesth. 2010;105 Suppl 1:i149-59. PMID: 21148658
3. Ogunnaike BO, Jones SB, Jones DB, et al. Anesthetic considerations for bariatric surgery. Anesth Analg. 2002;95(6):1793-1805. PMID: 12456447
4. Scholten DJ, Ney AL, Street JG, et al. Hemodynamic effects of medical induction and the head-up position for insertion of pulmonary artery catheters in obese patients. J Trauma. 1991;31(12):1664-1668. PMID: 1763351
5. Vaughan RW, Bauer S, Wise L. Volume and pH of gastric juice in obese patients. Anesthesiology. 1975;43(6):686-689. PMID: 1200728
6. Biring MS, Lewis MI, Liu JT, Mohsenifar Z. Pulmonary physiologic changes of morbid obesity. Am J Med Sci. 1999;318(5):293-297. PMID: 10555057
7. Juvin P, Lavaut E, Dupont H, et al. Difficult tracheal intubation is more common in obese than in lean patients. Anesth Analg. 2003;97(2):595-600. PMID: 12873961
8. Siyam MA, Benhamou D. Difficult endotracheal intubation in patients with sleep apnea syndrome. Anesth Analg. 2002;95(4):1098-1102. PMID: 12351301
9. Collins JS, Lemmens HJ, Brodsky JB, et al. Laryngoscopy and morbid obesity: a comparison of the "sniff" and "ramped" positions. Obes Surg. 2004;14(9):1171-1175. PMID: 15527636
10. Neligan PJ, Malhotra G, Fraser M, et al. Noninvasive positive pressure ventilation immediately after bariatric surgery with very high body mass index: is it safe? Anesth Analg. 2007;105(3):750-754. PMID: 17717229
11. Chalhoub V, Yazigi A, Sleilaty G, et al. Effect of vital capacity manoeuvres on lung compliance in anaesthetised obese patients. Br J Anaesth. 2007;99(4):529-533. PMID: 17715108
12. Sprung J, Whalley DG, Falcone T, et al. The impact of morbid obesity, pneumoperitoneum, and posture on respiratory system mechanics and oxygenation during laparoscopy. Anesth Analg. 2002;94(5):1345-1350. PMID: 11973201
13. Nguyen NT, Lee SL, Goldman C, et al. Comparison of pulmonary function and postoperative pain after laparoscopic versus open gastric bypass: a randomized trial. J Am Coll Surg. 2001;192(4):469-476. PMID: 11294453
14. Berrington de Gonzalez A, Hartge P, Cerhan JR, et al. Body-mass index and mortality among 1.46 million white adults. N Engl J Med. 2010;363(23):2211-2219. PMID: 21121834
15. Kabon B, Nagele A, Reddy D, et al. Obesity decreases perioperative tissue oxygenation. Anesthesiology. 2004;100(2):274-280. PMID: 14739795
16. Sollazzi L, Modesti C, Vitale F, et al. Preinduction respiratory physiotherapy in morbidly obese patients: effects on postoperative respiratory muscle function. Minerva Anestesiol. 2009;75(10):575-580. PMID: 19710701
17. Cullen A, Ferguson A. Perioperative management of the severely obese patient: a selective pathophysiological review. Can J Anaesth. 2012;59(10):974-996. PMID: 22826071
18. Abrishami A, Khajehdehi A, Chung F. A systematic review of screening questionnaires for obstructive sleep apnea. Can J Anaesth. 2010;57(5):423-438. PMID: 20195934
19. Flegal KM, Carroll MD, Kit BK, Ogden CL. Prevalence of obesity and trends in the distribution of body mass index among US adults, 1999-2010. JAMA. 2012;307(5):491-497. PMID: 22253363
20. American Society of Anesthesiologists Task Force on Perioperative Management of patients with obstructive sleep apnea. Practice guidelines for the perioperative management of patients with obstructive sleep apnea: an updated report. Anesthesiology. 2014;120(2):268-286. PMID: 24346170
21. Cortés FJ, Monedero P, Sánchez A, et al. [Prevention of venous thromboembolism in the surgical patient. Consensus document of the Spanish Consensus Forum]. Cir Esp. 2008;83(2):81-94. PMID: 18358196
22. Brotman DJ, Jaffer AK, Hurban MS, et al. Pharmacologic and mechanical strategies for preventing venous thromboembolism after bariatric surgery: a systematic review. JAMA Surg. 2013;148(7):675-686. PMID: 23824301
23. Ageno W, Spyropoulos AC, Turpie AGG, et al. Comparison of different body weight–based low-molecular-weight heparin dosing strategies for the prevention of venous thromboembolism in patients undergoing bariatric surgery. Thromb Res. 2015;136(5):1017-1021. PMID: 26354723
24. Bakhshayesh SA, Sharifian R, Dabbagh A. Rhabdomyolysis in bariatric surgery: a scary but entirely preventable complication. J Cardiothorac Vasc Anesth. 2012;26(6):e56-e57. PMID: 22867859
25. Watanabe Y, Horinouchi T, Kondo Y, et al. [A case of propofol infusion syndrome in severe obesity]. Masui. 2011;60(6):742-745. PMID: 21692821
26. Ingrande J, Brodsky JB, Lemmens HJ. Lean body weight scalar for the anesthetic induction dose of propofol in morbidly obese subjects. Anesth Analg. 2011;113(1):57-62. PMID: 21519033
27. Han PY, Duffull SB, Kirkpatrick CM, et al. Dosing in obesity: a simple solution to a big problem. Clin Pharmacol Ther. 2007;82(5):505-508. PMID: 17713457
28. Pai MP, Bearden DT. Antimicrobial dosing considerations in obese adult patients. Pharmacotherapy. 2007;27(8):1081-1091. PMID: 17637508
29. Cheymol G. Effects of obesity on pharmacokinetics implications for drug therapy. Clin Pharmacokinet. 2000;39(3):215-231. PMID: 10999985
30. Erstad BL. Dosing of medications in morbidly obese patients in the intensive care unit setting. Intensive Care Med. 2004;30(1):18-32. PMID: 14673522
31. Blouin RA, Warren GW. Pharmacokinetic considerations in obesity. J Pharm Sci. 1999;88(1):1-7. PMID: 9877023
32. Cortinez LI, Anderson BJ, Penna A, et al. Influence of obesity on propofol pharmacokinetics: derivation of a pharmacokinetic model. Br J Anaesth. 2010;105(4):448-456. PMID: 20736225
33. Janmahasatian S, Duffull SB, Ash S, et al. Quantification of lean bodyweight. Clin Pharmacokinet. 2005;44(10):1051-1065. PMID: 16176116
34. Lemmens HJ, Brodsky JB. The dose of succinylcholine in morbid obesity. Anesth Analg. 2006;102(3):750-753. PMID: 16505185
35. Leykin Y, Pellis T, Lucca M, et al. The pharmacodynamic effects of rocuronium when dosed according to real body weight or ideal body weight in morbidly obese patients. Anesth Analg. 2004;99(4):1086-1089. PMID: 15385350
36. Servin F, Farinotti R, Haberer JP, et al. Propofol infusion for maintenance of anesthesia in morbidly obese patients receiving nitrous oxide. A clinical and pharmacokinetic study. Anesthesiology. 1993;78(4):657-665. PMID: 8466063
37. La Colla L, Albertin A, La Colla G, et al. Faster wash-out and recovery for desflurane vs sevoflurane in morbidly obese patients undergoing laparoscopic surgery. Acta Anaesthesiol Scand. 2012;56(6):782-788. PMID: 22429095
38. Torri G, Casati A, Albertin A, et al. Randomized comparison of isoflurane and sevoflurane for laparoscopic gastric banding in morbidly obese patients. J Clin Anesth. 2001;13(8):565-570. PMID: 11744573
39. Ankichetty SP, Wong J, Chung F. Perioperative management of a morbidly obese patient with obstructive sleep apnea undergoing laparoscopic surgery. Can J Anaesth. 2011;58(5):451-463. PMID: 21380536
40. Brodsky JB, Lemmens HJ, Brock-Utne JG, et al. Morbid obesity and tracheal intubation. Anesth Analg. 2002;94(3):732-736. PMID: 11867402
41. Ezri T, Gewürtz G, Sessler DI, et al. Prediction of difficult laryngoscopy in obese patients by ultrasound quantification of anterior neck soft tissue. Anaesthesia. 2003;58(11):1111-1114. PMID: 14616652
42. Dixon BJ, Dixon JB, Carden JR, et al. Preoxygenation is more effective in the 25 degrees head-up position than in the supine position in severely obese patients: a randomized controlled study. Anesthesiology. 2005;102(6):1110-1115. PMID: 15915030
43. Altermatt FR, Muñoz HR, Delfino AE, et al. Pre-oxygenation in the obese patient: effects of position on tolerance to apnoea. Br J Anaesth. 2005;95(5):706-709. PMID: 16199418
44. Boyce JR, Ness T, Castroman P, et al. A preliminary study of the optimal anesthesia positioning for the morbidly obese patient. Obes Surg. 2003;13(1):4-9. PMID: 12568183
45. Nicholson A, Smith AF, Lewis SR, et al. Systematic review and meta-analysis of enhanced recovery programmes in surgical patients. Br J Anaesth. 2014;112(2):172-188. PMID: 24347589
46. Licker M, Schweizer A, Ellenberger C, et al. Perioperative medical management of patients with COPD. Int J Chron Obstruct Pulmon Dis. 2007;2(4):493-515. PMID: 18268921
47. Memtsoudis SG, Bombardieri AM, Ma Y, et al. Obesity as a risk factor for deep vein thrombosis and pulmonary embolism. J Thromb Haemost. 2009;7(5):893-894. PMID: 19175533
48. Samama CM, Albaladejo P, Benhamou D, et al. Venous thromboembolism prevention in surgery and obesity: an update. J Thromb Haemost. 2011;9 Suppl 1:306-318. PMID: 21781250
49. Carmody BJ, Sugerman HJ, Kellum JM, et al. Pulse pressure variation predicts fluid responsiveness in critically ill patients with metabolic syndrome and/or morbid obesity. Obes Surg. 2010;20(12):1753-1760. PMID: 20686835
50. Lemanu DP, Singh PP, Berridge K, et al. Randomized clinical trial of enhanced recovery versus standard care after laparoscopic sleeve gastrectomy. Br J Surg. 2013;100(4):482-489. PMID: 23335161
51. Bamgbade OA, Rutter TW, Nafiu OO, et al. Postoperative complications in obese and nonobese patients. World J Surg. 2007;31(3):556-560. PMID: 17308883
52. Borgeat A, Aguirre J. Update on anaesthesia for laparoscopic urological and robotic surgery. Curr Opin Anaesthesiol. 2014;27(6):596-602. PMID: 25304267
53. Favazza T, DeOrnellas D, Joshi GP. Obesity and ambulatory anesthesia. Int Anesthesiol Clin. 2016;54(1):97-108. PMID: 26669415
54. Shaffer EM. Obstructive sleep apnea syndrome in children: diagnosis, treatment and implications. South Med J. 2004;97(8):770-777. PMID: 15301120
55. Young T, Peppard PE, Taheri S. Excess weight and sleep-disordered breathing. J Appl Physiol. 2005;99(4):1592-1599. PMID: 16160022
56. Berry RB, Budhiraja R, Gottlieb DJ, et al. Rules for scoring respiratory events in sleep: update of the 2007 AASM Manual for the Scoring of Sleep and Associated Events. J Clin Sleep Med. 2012;8(5):597-619. PMID: 23066376
57. Seet E, Chung F. Obstructive sleep apnea: preoperative assessment. Anesthesiol Clin. 2010;28(2):199-215. PMID: 20483783
58. Mashour GA, Shanks AM, Kheterpal S. Perioperative stroke and associated mortality after noncardiac, nonneurologic surgery. Anesthesiology. 2011;114(6):1289-1296. PMID: 21502862
59. Douketis JD, Spyropoulos AC, Spencer FA, et al. Perioperative management of antithrombotic therapy: An American College of Chest Physicians Clinical Practice Guideline Update. Chest. 2022;162(5):2077-2113. PMID: 36007538
60. Gould MK, Garcia DA, Wren SM, et al. Prevention of VTE in nonorthopedic surgical patients: Antithrombotic Therapy and Prevention of Thrombosis, 9th ed: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines. Chest. 2012;141(2 Suppl):e227S-e277S. PMID: 22315263
61. Shepherd MF, Rosborough TK, Schwartz ML. Heparin thromboprophylaxis in gastric bypass surgery. Obes Surg. 2003;13(2):249-253. PMID: 12725237
62. Quebbemann BB, Akhondzadeh M, Dallal RM. Continuous infusion of heparin for prevention of thromboembolic events in patients undergoing bariatric surgery. Endocr Pract. 2005;11(3):160-164. PMID: 16134583
63. Kakarla VR, Nunley JA, Seal M, et al. Rhabdomyolysis in morbidly obese patients after gastric bypass surgery: a prospective study. Obes Surg. 2011;21(7):886-890. PMID: 20844885
64. Sinha AC, Singh PM, Williams NW, et al. Perioperative thromboembolism in bariatric surgery: burden, prevention, and prediction. Obes Surg. 2018;28(12):3814-3826. PMID: 29948473
65. Kasicka-Jonderko A, Jonderko K, Bożek M, et al. Can obesity protect the metabolic impact of major abdominal surgery? Diabetes Metab Res Rev. 2017;33(8):e2916. PMID: 28440564
66. Tufano A, Cimino E, Di Minno MN, et al. Model to predict venous thromboembolism in bariatric surgery patients. World J Surg. 2014;38(8):1825-1831. PMID: 24590435
67. Parli SE, Ritter MJ, Harman CP, et al. A retrospective review of rhabdomyolysis and bariatric surgery. J Am Assoc Nurse Pract. 2015;27(5):276-281. PMID: 25875335
68. Fong YL, Tan WQ, Tan HM, et al. Rhabdomyolysis after bariatric surgery: a systematic review. Int J Surg. 2019;70:24-31. PMID: 31276783
69. Wolf AM, Beisiegel U. The effect of loss of excess weight on the metabolic variables in the obese patient. Curr Med Res Opin. 2004;20(2):141-148. PMID: 14978450
70. Bray GA, Medical consequences of obesity. J Clin Endocrinol Metab. 2004;89(6):2583-2589. PMID: 15181027
71. Kam PC, Cardone D. Propofol infusion syndrome. Anaesthesia. 2007;62(7):690-701. PMID: 17567345
72. Fudickar A, Bein B, Steinfath M, et al. A closed-loop algorithm for propofol infusion. Anesth Analg. 2006;103(3):673-678. PMID: 16931684
73. Van den Berghe G, Wouters P, Weekers F, et al. Intensive insulin therapy in critically ill patients. N Engl J Med. 2001;345(19):1359-1367. PMID: 11794168
74. Schumann R, Shikora SA, Sigl JC, et al. Association of metabolic syndrome and risk factors for difficult intubation in morbidly obese patients: a cohort study. J Clin Anesth. 2013;25(6):447-453. PMID: 24055233
75. Neligan PJ, Malhotra G, Fraser M, et al. Obstructive sleep apnea: not just a problem for the obese. Can J Anaesth. 2009;56(11):869-874. PMID: 19711187
76. Passannante AN, Rock P. Anesthetic management of patients with obesity and sleep apnea. Anesthesiol Clin North America. 2005;23(3):479-491. PMID: 16139684
77. Nightingale CE, Margarson MP, Shearer E, et al. Peri-operative management of the obese surgical patient 2015: Association of Anaesthetists of Great Britain and Ireland Society for Obesity and Bariatric Anaesthesia. Anaesthesia. 2015;70(7):859-876. PMID: 25950661
78. Alvarado R, Alvarado F, Huerta S, et al. The impact of preoperative weight loss in patients undergoing gastric bypass surgery. Obes Surg. 2005;15(9):1282-1286. PMID: 16259883
79. Myles PS, Irlbeck M, Hunt JO, et al. Sex and age related differences in the risk of death or readmission within 30 days of surgery for bariatric surgery. Med J Aust. 2013;199(3):189-192. PMID: 23924221
80. Apfelbaum JL, Hagberg CA, Connis RT, et al. 2022 American Society of Anesthesiologists Practice Guidelines for Management of the Difficult Airway. Anesthesiology. 2022;136(1):31-81. PMID: 34762729
81. Ahmad S, Nagle A, McCarthy RJ, et al. Postoperative hypoxemia in morbidly obese patients with and without obstructive sleep apnea undergoing laparoscopic bariatric surgery. Anesth Analg. 2008;107(1):138-143. PMID: 18635486
82. Gander S, Frascarolo P, Suter M, et al. Positive end-expiratory pressure during induction of general anesthesia increases the functional residual capacity in morbidly obese patients. Anesth Analg. 2005;100(3):752-756. PMID: 15728067
83. Bhatia N, Agrawal P, Chaudhary S, et al. Evaluation of an anaesthetic technique for prevention of rhabdomyolysis in bariatric surgery. J Clin Diagn Res. 2015;9(9):UC01-UC03. PMID: 26500942
84. Perrier A, Roy PM, Sanchez O, et al. Multidetector-row computed tomography in suspected pulmonary embolism. N Engl J Med. 2005;352(17):1760-1768. PMID: 15858185
85. Stein PD, Fowler SE, Goodman LR, et al. Multidetector computed tomography for acute pulmonary embolism. N Engl J Med. 2006;354(22):2317-2327. PMID: 16738268
86. Douketis JD, Berger PB, Dunn AS, et al. The perioperative management of antithrombotic therapy: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines (8th Edition). Chest. 2008;133(6 Suppl):299S-339S. PMID: 18574271
87. Heit JA, Spencer FA, White RH. The epidemiology of venous thromboembolism. J Thromb Thrombolysis. 2016;41(1):3-14. PMID: 26780736
88. Geerts WH, Bergqvist D, Pineo GF, et al. Prevention of venous thromboembolism: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines (8th Edition). Chest. 2008;133(6 Suppl):381S-453S. PMID: 18574271
89. Shepard MK, Rosborough TK, Schwartz ML. Heparin dosing and monitoring in the obese: the need for a weight-based dosing approach. Obes Surg. 2001;11(4):434-437. PMID: 11510517