Obesity and Anaesthesia Managing the risks

The anatomical and physiological changes associated with obesity present specific risks. Careful risk management starts with an adequate pre-operative assessment.

One of the first considerations regards cardio-respiratory function, which initially may appear asymptomatic due to limited mobility, but following further assessment may present significant dysfunction.¹ For this reason, signs and symptoms of respiratory failure and cardiac failure must be assessed. Specific exams and investigations should be tailored to the individual patient, depending on comorbidity and the type and urgency of surgery.

Obesity is also associated with a 30% greater chance of difficult/failed intubation⁵ and therefore, careful airway assessment during the pre-operative phase is crucial. Mouth opening, Mallampati score, neck extension, and circumference (collar size >17.5 in) should be noted as it helps predict a potentially difficult airway.¹ Following the assessment, a robust airway strategy must be planned and discussed, to prevent quick desaturation and increased morbidity.

Intra-operative management of patients with obesity presents particular anaesthetic considerations, starting with the need for experienced anaesthetic and surgical staff. Several authors recommend considering regional anaesthesia as it reduces the risks associated with difficult intubation and acid aspiration and also provides safer and more effective postoperative analgesia.^2,5

When regional anaesthesia is not possible, due to surgical or patient limitations, specific care must be taken when administering general anaesthesia. Many patients may not tolerate the usual supine position for general anaesthesia induction and intubation due to oxygen desaturation, airway obstruction or respiratory compromise. Awake intubation in a sitting position is often better tolerated.¹

Choosing easily reversible drugs, with fast onset and offset for general anaesthesia is recommended because the pharmacokinetics of most general anaesthetic drugs are affected by the mass of adipose tissue, producing a prolonged, less predictable effect.^1,5 Drug dosing should be based upon lean body weight and titrated to effect and caution is required with the use of long-acting opioids and sedatives. Using local anaesthetics is recommended as a way of minimising opioid use.

Adequate monitoring throughout anaesthesia is crucial. Invasive arterial pressure monitoring has been advocated for all but the most minor procedures in the morbidly obese. Pulse oximetry, electrocardiography, capnography and monitoring of neuromuscular block are all mandatory.² Use of depth of anaesthesia monitoring techniques to limit anaesthetic load, particularly when neuromuscular blocking drugs and/or a total intravenous anaesthetic technique are utilised.⁵

Use of central venous and pulmonary artery flotation catheters should be considered in patients undergoing extensive surgery or those with serious cardiorespiratory disease.²

When general anaesthesia has been used in obese patients, lung volumes are significantly reduced in the postoperative period. Where possible, extubation should be performed with patients wide-awake in the sitting position and then transferred to an appropriate postoperative environment. Maintaining the head–up position throughout recovery and monitoring oxygen saturations until mobile is recommended.⁵

Many anaesthetists consider perioperative epidural anaesthesia an important part of a multimodal approach to improving patient outcome and analgesia rather than relying solely on systemic opioid administration for pain management.¹

Appropriate prophylaxis against venous thromboembolism and early mobilisation are recommended taking into consideration the increased incidence of venous thromboembolism in obese patients.⁵

Managing the risks of obesity and anaesthesia requires trained and experienced professionals, adequate patient assessment, preparedness of environment, careful choice of drugs, and intra and post-operative monitoring until full recovery has been achieved.

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