Obesity and sleep apnea

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= 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.

BMI, Body mass index.

Height squared (m2)

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 g-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.

  • Chest radiographs may not be generally useful unless the history suggests additional information may be gleaned.
  • Laboratory analysis should include a complete blood count, electrolytes, blood sugar, blood urea nitrogen, and creatinine. Elevated bicarbonate suggests carbon dioxide retention. Hypokalemia may occur with chronic use of diuretics. A room air arterial blood gas analysis will identify hypoxemia, hypercarbia, and metabolic compensation.
  • If chronic lung disease is suspected, pulmonary function testing may also be necessary to characterize the extent of disease and amenability to preoperative optimization.

7. What are the advantages or disadvantages of offering regional anesthesia to the obese patient?

Advantages

  • Minimal airway intervention, avoiding difficult airway and obstructive sleep apnea (OSA) concerns
  • Decreased cardiopulmonary depression
  • Good postoperative analgesia with decreased need for narcotics
  • Less postoperative nausea and vomiting, shorter postanesthesia care unit stay Disadvantages
  • Technical difficulties arise in performing the blocks because of body habitus.
  • Intraoperative failed block may require intubation under suboptimal conditions.
  • The patient may not be able to tolerate positioning for the surgical procedure.

For example, the patient may not be able to tolerate prolonged periods in the supine position.

8. Review the challenges in monitoring these patients.

ASA standard monitoring devices should be used in all cases. Blood pressure cuffs should span a minimum of 75% of the patient's upper arm circumference. The ankle or wrist may be used as alternate sites for taking blood pressure. Invasive arterial monitoring should be used for severely obese patients and those with significant cardiopulmonary disease or when noninvasive blood pressure monitoring devices are unreliable.

Establishing peripheral intravenous access can be difficult in obese patients. Central venous catheters should be used in cases in which peripheral access may be difficult either intraoperatively or after surgery.

9. Discuss positioning the obese patient.

Care should be taken to ensure that an appropriately sized operating room bed, built to accommodate additional body weight and girth, is available. In addition, all pressure points should be adequately padded, as postoperative neuropathies and pressure necrosis are more common in the obese and anesthetized population. There have been reports of rhabdomyolysis of the gluteal muscles, leading to renal failure. Sequential compression devices for the lower extremities are recommended. These are especially useful in the setting of laparoscopic surgery, where increased intra-abdominal pressure is likely to decrease venous return. It is also preferable to have assisted lifting devices to help move the patient from table to bed at the end of the case.

When planning airway management, it should be noted that many of these patients do not tolerate a flat, supine position. Placing the patient head up in a reclining position improves airway mechanics by dropping the panniculus down and unloading the diaphragm, increasing the functional residual volume.

Obese patients have an increased risk of atelectasis, which persists for a greater number of days than in the nonobese patient. In addition, they have increased risk of hypoxia for 4 to 7 days after surgery. Postoperative pulmonary physiotherapy is recommended.

10. What extubation criteria would you use for the obese patient?

The patient must be hemodynamically stable. Muscle relaxants should be adequately reversed and verified by peripheral nerve stimulator findings (sustained tetanus and with no post-tetanic facilitation of twitches). The patient should be awake, alert, and able to sustain a head lift for 5 seconds (Table 51-3).

TABLE 51-3. RESPIRATORY VALUES

NEEDED FOR EXTUBATION

Respiratory rate

<30 Breaths/min

Maximal inspiratory force

-25 to -30 cm H2O

Vital capacity

10-15 ml/kg

Tidal volume

5 ml/kg (lean body weight)

11. Review special concerns for pregnant women and children with obesity.

Management of the morbidly obese parturient can be hazardous. In addition to the considerations discussed previously, there is also the desire for labor analgesia and potential for emergent cesarean section. The risk of difficult or failed intubation in these women is high. Placement of an epidural catheter early in the laboring process may overcome the need for general anesthesia. However, because of the technical difficulty of catheter placement and maintenance, regular assessment of the block with a lowthreshold for replacement is advised. Some authors recommend a continuous spinal technique, thus providing predictability and reliability of the block and rapid conversion to appropriate anesthesia for cesarean section should the need arise.

Recent studies suggest that, unlike their adult counterparts, obese children have a relatively small increase in anesthetic risk. A slight increase in minor postoperative complications is seen, predominantly respiratory. A higher rate of oxygen desaturation requiring supplemental oxygen and unplanned hospital admission is seen. This may reflect the lower prevalence of physiologic changes associated with long-standing obesity.

12. What is obstructive sleep apnea?

OSAis a syndrome characterized by periodic, partial, or complete obstruction of the upper airway during sleep. The sufferer (or his or her bed partner) reports snoring and periods of apnea. He or she has daytime somnolence and an increased incidence of hypoxia, dysrhythmias, hypertension, myocardial ischemia, and pulmonary hypertension. A number of case reports have documented an increase in the incidence of postoperative complications and deaths among patients suspected of having OSA. These patients have been shown to be particularly sensitive to the respiratory depressant effects of opioids, likely because of an up-regulation of the mu receptors, and postoperative respiratory arrest is reported more frequently in this group. Untreated OSA patients are known to have a higher incidence of difficult intubation, postoperative complications, increased intensive care unit admissions, and greater duration of hospital stay.

13. Is obstructive sleep apnea common?

The prevalence of adult sleep apnea is approximately 2% in women and 4% in men. The prevalence of childhood sleep apnea is up to 10%. These children are commonly seen on the otolaryngology surgical schedule for tonsillectomy and adenoidectomy; this is curative in 75% to 100% of cases, even in the obese population. However, it is estimated that nearly 80% of men and 93% of women with moderate to severe sleep apnea are undiagnosed. Undiagnosed OSA may pose a variety of problems for anesthesiologists.

14. Which techniques can be used to identify patients with obstructive sleep apnea?

Identifying patients with OSA is the first step in preventing postoperative complications caused by OSA. Questionnaires that have been designed to screen for the presence of OSA include the Berlin questionnaire, the STOP questionnaire, and the American Society of

Anesthesiologists' (ASA) checklist. Each of these has been validated in surgical patients as a screening tool for OSA and has demonstrated a moderately high level of sensitivity. All three of the tests evaluate snoring, daytime tiredness, presence of hypertension, and observed apnea. Because of the easy-to-use format and self-administered testing, the STOP questionnaire might be easier for patients to complete and more suitable in the busy preoperative clinics.

15. What procedures are performed to aid in weight loss?

Bariatric surgery broadly describes several surgical weight loss procedures used in the treatment of morbid obesity. The most common procedures include gastric banding, adjustable gastric banding, and Roux-en-Y gastric bypass. The surgeries cause weight loss by limiting both the volume of food a person can ingest and the absorption of calories, vitamins, and minerals. Bariatric surgery has become increasingly common, and the surgical candidate population is widening.

These procedures are often performed laparoscopically. The increased intraabdominal pressure of morbid obesity combined with a pneumoperitoneum results in venous stasis, reduced intraoperative portal venous blood flow, decreased urinary output, lower respiratory compliance, increased airway pressure, and impairment of cardiac function. In addition, there is less efficient elimination of carbon dioxide and increased PaCO2 when compared with nonobese patients. As a consequence, pneumoperitoneum is not recommended in morbidly obese patients with severe cardiac, pulmonary, hepatic, or pulmonary dysfunction.

Intraoperative management to minimize the adverse changes include appropriate ventilatory adjustments to avoid hypercapnia and acidosis, the use of sequential compression devices to minimize venous stasis, and optimized intravascular volume to minimize the effects of increased intraabdominal pressure on renal and cardiac function.

16. Is bariatric surgery used in the pediatric population?

Childhood and adolescent obesity is associated with the onset of adult cardiovascular and endocrine problems at earlier ages. Dietary modifications alone have not been successful in slowing the rising incidence of obesity. Bariatric surgery in adolescents may provide the start to long-term improvements in the quality of life, psychosocial status, and physical well-being for these patients. Although obese patients have been traditionally believed to be at very high risk during the perioperative period, recent investigations in adults and children are challenging this view, and bariatric surgery is now seen more commonly in the pediatric population.

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