Anesthesia for Obesity

Obesity

Overweight and obesity are classified using the body mass index (BMI). Overweight is defined as a BMI of 24 kg/m² or higher, obesity as a BMI of 30 or higher, and extreme obesity (formerly termed “morbid obesity”) as a BMI of more than 40. BMI is calculated by dividing the weight (in kilo-grams) by the height (in meters) squared. Health risks increase with the degree of obesity and with increased abdominal distribution of weight. Men with a waist measurement of 40 in. or more and women with a waist measurement of 35 in. or more are at increased health risk. For a patient 1.8 m tall and weighing 70 kg, the BMI would be as shown in the following formula:

Clinical Manifestations

Obesity is associated with many diseases, includ-ing type 2 diabetes mellitus, hypertension, coronary artery disease, obstructive sleep apnea, degenera-tive joint disease (osteoarthritis), and cholelithiasis. Even in the absence of obvious coexisting disease, however, extreme obesity has profound physiologi-cal consequences. Oxygen demand, CO₂ production, and alveolar ventilation are elevated because meta-bolic rate is proportional to body weight. Excessive adipose tissue over the thorax decreases chest wall compliance even though lung compliance may remain normal. Increased abdominal mass forces the diaphragm cephalad, yielding lung volumes suggestive of restrictive lung disease. Reductions in lung volumes are accentuated by the supine and Trendelenburg positions. In particular, functional residual capacity may fall below closing capacity. If this occurs, some alveoli will close during normal tidal volume ventilation, causing a ventilation/per-fusion mismatch.

Whereas obese patients are often hypoxemic, only a few are hypercapnic, which should be a warning of impending complications. Obesity-hypoventilation syndrome, or obstructive sleep apnea (OSA), is a complication of extreme obesity characterized by hypercapnia, cyanosis-induced polycythemia, right-sided heart failure, and som-nolence. These patients appear to have blunted respiratory drive and often suffer from loud snoring and upper-airway obstruction during sleep. OSA patients often report dry mouths and daytime som-nolence; bed partners frequently describe apneic pauses. OSA has also been associated with increased perioperative complications including hyperten-sion, hypoxia, arrhythmias, myocardial infarction, pulmonary edema, stroke, and death. The potential for difficult mask ventilation and difficult intuba-tion, followed by upper airway obstruction during recovery, should be anticipated.

OSA patients are vulnerable during the postop-erative period, particularly when sedatives or opi-oids have been given. When OSA patients are placed supine, the upper airway is even more prone to obstruction. For patients with known or suspected OSA, postoperative continuous positive airway pres-sure (CPAP) should be considered until the anesthe-siologist can be sure that the patient can protect his or her airway and maintain spontaneous ventilation without evidence of obstruction. Both the American Society of Anesthesiologists and the Society of Ambulatory Anesthesia offer guidelines on periop-erative management of the patient with OSA.

An OSA patient’s heart has an increased work-load, as cardiac output and blood volume rise to perfuse additional fat stores. The elevation in cardiac output (0.1 L/min/kg of adipose tissue) is achieved through an increase in stroke volume—as opposed to heart rate. Arterial hypertension leads to left ven-tricular hypertrophy. Elevations in pulmonary blood flow and pulmonary artery vasoconstriction from persistent hypoxia can lead to pulmonary hyperten-sion and cor pulmonale.

Obesity is also associated with gastrointestinal pathophysiology, including hiatal hernia, gastro-esophageal reflux disease, delayed gastric empty-ing, and hyperacidic gastric fluid, as well as with an increased risk of gastric cancer. Fatty infiltration of the liver also occurs and may be associated with abnormal liver tests, but the extent of infiltration does not correlate well with the degree of liver test abnormality.

Anesthetic Considerations

A. Preoperative

For the reasons outlined above, obese patients are at an increased risk for developing aspiration pneumo-nia. Pretreatment with H ₂ antagonists and metoclo-pramide should be considered. Premedication with respiratory depressant drugs must be avoided in patients with OSA.

Preoperative evaluation of extremely obese patients undergoing major surgery should attempt to assess cardiopulmonary reserve. Preoperative testing may include such items as chest radiograph, ECG, and arterial blood gas analysis. Physical signs of cardiac failure (eg, sacral edema) may be difficult to identify. Blood pressures must be taken with a cuff of the appropriate size. Potential sites for intrave-nous and intraarterial access should be checked in anticipation of technical difficulties. Obscured land-marks, difficult positioning, and extensive layers of adipose tissue may make regional

anesthe-sia difficult with standard equipment and techniques. Obese patients may be difficult to intubate as a result of limited mobility of thetemporomandibular and atlantooccipital joints, a narrowed upper airway, and a shortened distance between the mandible and sternal fat pads.

B. Intraoperative

Because of the risks of aspiration and hypoventilation, morbidly obese patients are usually intubated for all but short general anesthetics. If intubation appears likely to be difficult, the use of a fiberoptic bron-choscope or video laryngoscopy is recommended. Positioning the patient on an intubating ramp is help-ful. Auscultation of breath sounds may prove difficult. Even controlled ventilation may require relatively increased inspired oxygen concentrations to prevent hypoxia, particularly in the lithotomy, Trendelenburg, or prone positions. Subdiaphragmatic abdominal laparotomy packs can cause further deterioration of pulmonary function and a reduction of arterial blood pressure by increasing the resistance to venous return. Volatile anesthetics may be metabolized more exten-sively in obese patients. Increased metabolism may explain the increased incidence of halothane hepatitis observed in obese patients. Obesity has little clinical effect on the rate of decline of alveolar anesthetic con-centrations and wake-up time, even following long surgical procedures.

Theoretically, greater fat stores would increase the volume of distribution for lipid-soluble drugs (eg, benzodiazepines, opioids) relative to a lean per-son of the same body weight. However, the volume of distribution of, for example, fentanyl or sufentanil is so large that obesity has minimal influence. Water-soluble drugs (eg, NMBs) have a much smaller vol-ume of distribution, which is minimally increased by body fat. Nonetheless, the dosing of water-soluble drugs should be based on ideal body weight to avoid overdosing. In reality, of course, clinical practice does not always validate these expectations.

Although dosage requirements for epidural and spinal anesthesia are difficult to predict, obese patients typically require 20–25% less local anes-thetic per blocked segment because of epidural fat and distended epidural veins. Continuous epi-dural anesthesia has the advantage of providing pain relief and the potential for decreasing respi-ratory complications in the postoperative period. Regional nerve blocks, when appropriate for the surgery, have the additional advantages of not inter-fering with the postoperative deep vein thrombosis prophylaxis, rarely producing hypotension, and of reducing the need for opioids.

C. Postoperative

Respiratory failure is a major postoperative problem of morbidly obese patients. The risk of postopera-tive hypoxia is increased in patients with preopera-tive hypoxia, following surgery involving the thorax or upper abdomen (particularly vertical incisions). Extubation should be delayed until the effects of NMBs are completely reversed and the patient is awake. An obese patient should remain intubated until there is no doubt that an adequate airway and tidal volume will be maintained. This does not mean that all obese patients need be ventilated overnight in an intensive care unit. If the patient is extubated in the operating room, supplemental oxygen should be provided during transportation to the postan-esthesia care unit. A 45° modified sitting position will improve ventilation and oxygenation. The risk of hypoxia extends for several days into the postop-erative period, and providing supplemental oxygen or CPAP, or both, should be routinely considered. Other common postoperative complications in obese patients include wound infection, deep venous thrombosis, and pulmonary embolism. Morbidly obese and OSA patients may be candidates for out-patient surgery provided they are adequately moni-tored and assessed postoperatively before discharge to home, and provided the surgical procedure will not require large doses of opioids for postoperative pain control.