What are the complications and challenges of anesthesia in patients with morbid obesity?

Anesthesia in Morbid Obesity: Complications and Challenges

Morbidly obese patients present multiple life-threatening anesthetic challenges that require experienced staff, specialized equipment, and meticulous planning to prevent rapid desaturation, difficult airway management, accidental awareness, and cardiovascular collapse. 1

Why Morbid Obesity Complicates Anesthesia

Airway Management Crisis

• Desaturation occurs rapidly in obese patients due to reduced functional residual capacity and increased oxygen consumption, creating a narrow safety margin during induction 1, 2
• Difficult airway management occurs in 13-20% of obese patients, with complications developing rapidly and potentially catastrophically 2, 3
• A robust airway strategy must be planned and discussed before induction, as you may have only seconds before critical hypoxia develops 1
• The ramped or sitting position is mandatory for both induction and recovery to optimize respiratory mechanics 1, 2

Respiratory Compromise

• Obese patients have markedly reduced chest wall compliance and increased intra-abdominal pressure, especially in prone positioning 2
• Peak inspiratory pressures must remain <35 cmH₂O to prevent barotrauma 2
• Postoperative obstructive sleep apnea affects 50% of morbidly obese patients, creating high risk for respiratory failure 1, 3
• Pulmonary atelectasis (5%) and acute pulmonary embolism (5-12%) are common postoperative complications 3

Cardiovascular Instability

• Obesity increases cardiac output, blood pressure, and cardiac workload, with untreated sleep apnea causing pulmonary hypertension and heart failure 1
• Atrial fibrillation risk increases 1.5-fold, with markedly elevated sudden cardiac death risk 1
• Prolonged QT interval prevalence increases with BMI, creating arrhythmia risk with drugs like ondansetron 1
• Heart failure is the predominant risk factor for postoperative complications in this population 1

Drug Dosing Complexity and Awareness Risk

• Dosing to total body weight causes significant hypotension and overdose - use lean body weight (maximum 100 kg in men, 70 kg in women) or adjusted body weight instead 1, 4
• The NAP5 audit found a disproportionate number of obese patients experienced accidental awareness under anesthesia, with 93% involving neuromuscular blocking drugs 1
• The critical error: small induction doses based on lean body weight followed by delayed maintenance anesthesia initiation, creating an awareness window 1
• More rapid redistribution into larger fat mass means patients wake up faster after single bolus doses compared to non-obese patients 1

Target-Controlled Infusion Limitations

• Marsh and Schnider formulae become unreliable above 140-150 kg 1
• Commercial pumps do not allow weights >150 kg (Marsh) or BMI >35 kg/m² (female) and >42 kg/m² (male) using Schnider 1
• When using TCI with neuromuscular blocking drugs, depth of anesthesia monitoring is strongly recommended to prevent awareness 1

Metabolic and Thrombotic Risks

• VTE incidence may be 10 times higher in obese women compared to normal weight counterparts 1
• Obesity creates a prothrombotic state requiring extended postoperative VTE prophylaxis beyond two weeks depending on surgery type and BMI 1
• Poor glycemic control increases morbidity, requiring careful perioperative glucose management 1

Risk Stratification

Patients with central obesity and metabolic syndrome carry the highest perioperative risk, not those with isolated extreme obesity 1

The Obesity Surgery Mortality Risk Stratification (OS-MRS) score identifies high-risk patients:

• Class A (0-1 points): 0.2-0.3% mortality
• Class B (2-3 points): 1.1-1.5% mortality
• Class C (4-5 points): 2.4-3.0% mortality 1

Patients scoring 4-5 require closer postoperative monitoring, potentially in an intensive care setting 1

Essential Management Requirements

Pre-operative Mandates

• Screen all patients for sleep-disordered breathing with clear pathways for specialist sleep studies 1
• Record height, weight, BMI, lean body weight, and adjusted body weight on the operating list 1, 4
• Experienced anaesthetic and surgical staff must manage these patients - this is not a case for trainees 1
• Arterial PCO₂ >6 kPa indicates respiratory failure and markedly increased anaesthetic risk 1

Intraoperative Essentials

• Neuromuscular monitoring must always be used when neuromuscular blocking drugs are administered 1
• Consider depth of anaesthesia monitoring, especially with total intravenous anaesthesia plus neuromuscular blockade 1
• Use pressure-controlled ventilation with tidal volumes of 5-7 ml/kg ideal body weight 2
• Add sufficient PEEP and recruitment maneuvers to reduce atelectasis 2
• Caution with long-acting opioids and sedatives due to prolonged respiratory depression risk 1

Postoperative Care

• Extended PACU monitoring for at least one hour unstimulated, observing for hypoventilation, apnea, or desaturation 5
• Multimodal opioid-sparing analgesia using local anesthetics and regional techniques 5
• Early mobilization, physical therapy, and aggressive VTE prophylaxis 6
• ICU admission is determined by co-morbidities and surgery type rather than obesity alone, though inadequate pulmonary gas exchange is the main reason for admission 1, 3

Common Pitfalls to Avoid

• Never dose induction agents to total body weight - this causes severe hypotension while paradoxically increasing awareness risk due to faster redistribution 1
• Never delay initiation of maintenance anesthesia after induction - this creates the awareness window identified in NAP5 1
• Failure to recognize positioning-related airway compromise severity can lead to catastrophic outcomes 2
• Attempting prone positioning without adequate chest and pelvic support restricts abdominal movement and causes severe respiratory compromise 2
• Underestimating the speed of desaturation during apnea - pre-oxygenation is critical 5, 3