51: Obesity and Sleep Apnea

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CHAPTER 51 Obesity and Sleep Apnea

Gillian E. Johnson, MBBChir, BSc

1 Define obesity

Obesity is defined using the body mass index (BMI) (Table 51-1).

TABLE 51-1 Definition of Obesity

BMI
18.5–25	Normal range
26–30	Overweight
31–35	Class I obesity
36–40	Class II obesity
41+	Morbid obesity

BMI, Body mass index.

2 Discuss the cardiovascular considerations in the obese patient

Systemic hypertension, pulmonary hypertension, left and right heart failure, and coronary artery disease are all more common in the obese patient. As body mass increases, so does oxygen consumption. Cardiac output, stroke volume, and circulating blood volume increase to meet increased demand. Systemic hypertension in time results in left ventricular hypertrophy. Coronary artery disease, combined with the increased likelihood of diabetes mellitus in the obese, increases the risk for silent myocardial infarction.

3 Review some pulmonary and respiratory considerations in the obese patient

Respiratory concerns include the potential for a difficult airway, increased incidence of asthma, sleep disordered breathing or obstructive sleep apnea, restrictive lung disease, chronic hypoxia with or without polycythemia, and pulmonary hypertension.

Obesity is typically associated with hypoxemia, the mechanisms of which include:

Increased work of breathing. Because of an increased chest wall mass, decreased chest wall compliance, and abdominal cavity adipose tissue decreasing diaphragmatic excursion, work of breathing is two to four times greater than normal.

Restrictive lung disease develops, areas of the lung become underventilated, ventilation/perfusion mismatch occurs, and the lungs compensate by selective vasoconstriction of poorly ventilated regions. Ultimately pulmonary hypertension develops, which leads to right heart failure.

The large tissue mass increases the total oxygen consumption and increases carbon dioxide production.

4 What are the gastrointestinal and hepatic changes seen in obese patients?

Obesity increases intra-abdominal and intragastric pressures. Hiatal hernias and gastric reflux are common. Despite an 8-hour fast, 85% to 90% of morbidly obese patients have gastric volumes >25 ml, thus increasing their risk of pulmonary aspiration. These patients typically have fatty infiltration of the liver and may have hepatic inflammation, focal necrosis, or cirrhosis. At present no causal relationship between fatty infiltration and cirrhotic changes is known. Hepatic enzymes are generally altered, especially after jejunoileal bypass operations. Aminotransferases and γ-glutamyltranspeptidase are increased in this patient population.

5 Discuss the pharmacokinetic changes found in the obese patient

Obesity causes unpredictable changes in drug metabolism. Worsening obesity increases the duration of effect and degree of biotransformation of lipid-soluble (lipophilic) anesthetics. Lipophilic intravenous agents, including opioids, benzodiazepines, and barbiturates, have an increased volume of distribution and decreased elimination half-life, resulting in lower serum drug concentrations and decreased clearance. However, fentanyl shows similar pharmacokinetics in the obese and nonobese patient. In obese patients hydrophilic, water-soluble drugs generally have volumes of distribution, elimination half-lives, and rates of clearance similar to those in the nonobese patient.

Pseudocholinesterase activity is increased with obesity; thus larger doses of succinylcholine are required. Nondepolarizing muscle relaxants show variability in dosing, duration, and recovery; thus redosing should be guided by peripheral nerve stimulator. There is no evidence to suggest that any one inhalational anesthetic is superior to another when caring for the obese patient.

Loading doses of most intravenous agents are based on volume of distribution; maintenance dosing is based on clearance. In obese patients the volume of distribution is usually increased, but clearance approaches normal or is increased compared to a lean patient (Table 51-2).

TABLE 51-2 Loading Dose Strategy for Intravenous Drugs

Drugs	Dosing Strategy
Fentanyl	Loading dose based on TBW; decrease maintenance
Sufentanil	Loading dose based on TBW; decrease maintenance
Remifentanil	Dose based on IBW
Succinylcholine	Dose based on TBW
Atracurium	Dose using TBW
Vecuronium	Dose using IBW
Rocuronium	Dose using IBW
Propofol	Loading dose and maintenance based on TBW
Thiopental	Reduce loading dose
Midazolam	Loading dose based on TBW; adjust maintenance to IBW

IBW, Ideal body weight; TBW, total body weight.

6 Discuss the appropriate preoperative assessment of this population

See Chapters 8 and 17 for important historical information and airway concerns. As far as laboratory testing is concerned:

An electrocardiogram should be obtained for all obese patients to evaluate for atrial or ventricular enlargement, arrhythmias, and ischemia. Ventricular arrhythmias are common. Further cardiac testing such as exercise stress tests or chemically induced stress testing may be required. Echocardiography can be used to assess cardiac function and presence of pulmonary hypertension.

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