What is the difference in anaesthetic risk for patients with a body mass index (BMI) of 35-40 versus those with a BMI above 40?

From the Guidelines

Patients with a BMI above 40 face significantly higher anaesthetic risks compared to those with a BMI of 35-40, with increased risks of difficult airway management, respiratory complications, and cardiovascular risks. The risk increases proportionally with BMI, with those above 40 (Class III obesity) experiencing more challenging airway management, greater respiratory complications, and higher cardiovascular risks 1. Specifically, patients with BMI >40 have approximately 30-50% higher rates of difficult intubation, more frequent desaturation events, and increased risk of postoperative atelectasis and pneumonia.

Key Considerations

• Cardiovascular complications such as arrhythmias and myocardial ischemia are also more common in patients with BMI >40 1.
• These patients typically require lower doses of propofol (1-1.5 mg/kg based on ideal body weight rather than total weight) and may need higher doses of neuromuscular blocking agents like rocuronium.
• Anesthesiologists should anticipate the need for specialized equipment including larger blood pressure cuffs, reinforced laryngoscopes, and possibly video laryngoscopy for patients with BMI >40.
• The increased risk stems from anatomical changes including excess adipose tissue in the pharynx and chest wall, reduced functional residual capacity, increased oxygen consumption, and altered drug pharmacokinetics due to increased adipose tissue and cardiac output 1.

Preoperative Optimization

Preoperative optimization with careful airway assessment, cardiopulmonary evaluation, and consideration of sleep apnea is particularly important for patients with BMI >40. This includes identifying patients with central obesity and metabolic syndrome as risk factors, as well as considering sleep-disordered breathing and its consequences 1. A robust airway strategy must be planned and discussed, as desaturation occurs quickly in the obese patient and airway management can be difficult. The use of the ramped or sitting position is recommended as an aid to induction and recovery, and drug dosing should generally be based upon lean body weight and titrated to effect, rather than dosed to total body weight 1.

From the Research

Anaesthetic Risk Comparison

The difference in anaesthetic risk for patients with a body mass index (BMI) of 35-40 versus those with a BMI above 40 can be understood through various studies that have investigated the relationship between BMI and anaesthetic complications.

• Difficult Intubation: A study published in the Journal of Clinical Anesthesia 2 found that each 5 kg/m2 increase in BMI was associated with a marginal increase in difficult intubation, with an odds ratio of 1.06. However, the study noted that difficult intubations with double lumen tubes remain common, but BMI is a weak predictor thereof.
• Pharmacokinetic Considerations: Research on anesthesia in obese patients 3 highlights the importance of considering physiological changes and pharmacokinetic implications of obesity for the anesthesiologist. The study suggests that benzodiazepine loading doses should be adjusted on actual weight, and maintenance doses should be adjusted on ideal body weight.
• Optimal Dose of Propofol: A study published in the Annals of Palliative Medicine 4 investigated the optimal dose of propofol in patients with morbid obesity and found that the appropriate dosage of propofol was 2.310-3.567 mg/kg when the anesthetic induction dosage for patients with morbid obesity was calculated based on lean body weight.
• Anaesthetic Management: Another study 5 discussed aspects of anaesthetic management in morbidly obese patients and compared them with previous similar reports. The study found that patients who had thoracic epidural analgesia required less volatile anaesthesia than the group who had general anaesthesia and narcotic analgesics.
• Thromboprophylactic Dosing: A study published in the journal Surgery for Obesity and Related Diseases 6 evaluated the adequacy of prophylactic dosing of enoxaparin in patients with severe obesity and found that BMI-based thromboprophylactic dosing of enoxaparin after bariatric surgery could be suboptimal in 15% of patients with obesity.

Key Findings

• The relationship between BMI and anaesthetic risk is complex and influenced by various factors, including difficult intubation, pharmacokinetic considerations, and optimal dosing of anaesthetic agents.
• Studies suggest that patients with a BMI above 40 may be at higher risk for anaesthetic complications, such as difficult intubation and thromboprophylactic dosing issues.
• However, the evidence also highlights the importance of individualized anaesthetic management and careful consideration of pharmacokinetic implications of obesity to minimize risks and ensure optimal outcomes.