Anaesthesia and morbid obesity

Abstract

The prevalence of morbid obesity is increasing in the UK. Recent UK government statistics suggest that 20% of adults are obese and 1% morbidly obese. Anaesthesia and surgery may entail considerable risk for obese patients. Obesity is a multi-system disorder, particularly involving the respiratory and cardiovascular systems; therefore, a multidisciplinary approach is required. This article presents a broad overview of the pathophysiological and practical considerations for anaesthetizing such patients for major (non-bariatric) surgery. A body mass index (BMI) of .35 kg m with associated comorbidity, or .40 kg m without significant comorbidity, is considered to be morbidly obese; .55 kg m is considered super-morbidly obese (Table 1). However, morbidity and mortality increase sharply when BMI is .30 kg m, particularly in smokers, and risk is proportional to duration of obesity. For a given BMI, men are at higher risk of cardiovascular complications than women. Obesity is described classically as conforming to an android or gynaecoid fat distribution (‘apples and pears’). Both the actual BMI of a particular patient and the distribution of fat are important considerations. The gynaecoid fat distribution characteristically involves more fat distributed in peripheral, sites (arms, legs, and buttocks). An android fat distribution involves more central fat (intraperitoneal fat, including involvement of the liver and omentum). Specific definitions have been proposed based on the waist-to-hip ratio. A value .0.8 in women or 1.0 in men is typical of the android distribution. Although the android distribution predominates in males and is associated with a higher risk of morbidity, either distribution can occur in each gender. Weight loss reduces risk for both groups. Obese people have greater energy expenditure than lean individuals, and this is balanced by increased caloric intake. Basal metabolic rate is ‘normal’ in obese individuals when corrected for body surface area. However, with increasing weight, body surface area increases and hence absolute basal metabolic rate values are higher than in lean individuals. Consequently, there is a greater absolute oxygen consumption and carbon dioxide production.