Introduction

The obesity epidemic affects a very significant proportion of the adult population in the United States and throughout developed nations. The body mass index (BMI) is the most widely accepted classification used to assess weight status. The BMI is defined as the individual’s weight, measured in kilograms, divided by the square of the individual’s height, measured in meters. With this system, patients are considered overweight with a BMI between 25 and 29.9 kg/m ² and obese with a BMI between 30 and 49.9 kg/m ² . Obese classification is further subdivided into Class 1 (BMI range, 30-34.9 kg/m ² ), Class 2 (35-39.9 kg/m ² ), and Class 3 (40-49.9 kg/m ² ), based on increasing risk of developing health problems. Patients with a BMI of 50 kg/m ² or greater are considered superobese and have an extreme risk of developing health problems.

Over 100,000,000 residents of the United States, or 65% of the country’s adult population, are overweight or obese. Obesity is often accompanied by multiple comorbid states, including insulin resistance, type 2 diabetes mellitus, obstructive sleep apnea, hypoventilation, cardiovascular disease, hypertension, certain malignancies, and osteoarthritis. The clustering of a group of defined metabolic and physical abnormalities is known as the “metabolic syndrome.” Patients having metabolic syndrome are subject to abdominal obesity, reduced levels of high-density lipoprotein (HDL), hyperinsulinemia, glucose intolerance, hypertension, and additional characteristic features ( Box 36-1 ). Clinical criteria for diagnosing metabolic syndrome require that at least three of the five specific diagnostic criteria appearing in Table 36-1 be present. In the United States, some 50 million people have metabolic syndrome; thus its age-adjusted prevalence is nearly 24%, and more than 40% of the population is affected by the age of 60 years. Patients with metabolic syndrome are at increased risk of cardiovascular disease events and are at increased risk for all-cause mortality. Metabolic syndrome is also associated with many other health abnormalities including polycystic ovary syndrome, nonalcoholic fatty liver disease, gallstones, sleep disturbances, sexual impotence, and various cancers, giving it significant overlap with obesity for comorbid states.

Obesity is associated with early death. The rapid rate of increase in the prevalence of both morbid obesity and superobesity, taken together with the increased risk of early demise within the obese population and complicated by the presence of metabolic syndrome, has significantly increased the number of bariatric surgical procedures performed annually to enable patients to undergo weight loss. It is estimated that more than 200,000 bariatric surgeries were performed in 2010 and probable that more than 250,000 will be performed in 2014 and beyond. Care of obese patients is not limited to obesity surgery, however, because these patients are seen for all types of operations.

Obese patients present special challenges for the anesthesiologist in airway management, maintenance of lung volume, positioning, monitoring, choice of anesthetic technique and anesthetic agents, pain control, and postoperative care. The most significant and best studied of these are in the areas of endotracheal intubation after careful patient positioning and pulmonary physiology and maintenance of oxygenation and lung volume. Evidence continues to accumulate that specific interventions, techniques, and approaches used in caring for obese patients alter outcomes.

Patient Positioning and Airway Management

Laryngoscopy and endotracheal intubation have historically been considered more difficult to perform in obese patients than in those with a normal BMI. This is usually thought to result from the obese patient having a short and thick neck, a large tongue, and significant redundant pharyngeal soft tissue. The correlation between morbid obesity and difficulty with laryngoscopy and intubation is not, however, the universally observed clinical experience. In fact, it is also frequently reported that there is no difference between laryngoscopy and intubation in thin and obese individuals. This may be the result of simple but important differences in clinical practice. Careful attention to patient positioning before induction of general anesthesia plays an important role in providing optimal conditions for successful placement of the endotracheal tube under direct vision.

Pulmonary Physiology and Maintenance of Oxygenation and Lung Volume

Obese patients have multiple pulmonary abnormalities, including decreased vital capacity, inspiratory capacity, expiratory reserve volume, and functional residual capacity. Closing capacity in obese individuals is close to, or may fall within, tidal breathing, particularly with patients in a supine or recumbent position. The obese patient therefore is likely to undergo rapid oxygen desaturation, particularly during periods of apnea such as those that occur during induction of general anesthesia. Derecruitment of gas exchange units may occur throughout the anesthetic course. A variety of maneuvers have been studied as measures to preserve oxygenation and maintain lung volume, specifically in the obese population.

Evidence

Many studies have been conducted to determine the incidence of difficult laryngoscopy or intubation in the obese population. Although many of these studies have demonstrated a significant increase in the incidence of difficult laryngoscopy or intubation in comparison with the general population, several studies have shown no difference whatsoever. One study attempting to associate oropharyngeal Mallampati classification along with BMI as predictors of difficult laryngoscopy found a significantly higher positive predictive value of difficult laryngoscopy using both indices (BMI and Mallampati classification). During laryngoscopy, patients’ heads were maintained in optimum sniffing position, regardless of BMI. In a study conducted exclusively with obese patients, BMI was not found to be associated with intubation difficulties. A high Mallampati score was identified as a predictor of “potential intubation problems,” but intubation by direct laryngoscopy was successful in 99 of 100 patients studied. All patients were positioned with pillows or towels under their shoulders, with the head elevated and neck extended. Another group studied both lean and obese patients and found a Mallampati score of III or IV to be the only independent risk factor for difficult intubation in the obese study group. The authors determined the Mallampati score to have low specificity and positive predictive values (62% and 29%, respectively) for difficult intubation. They concluded that intubation was more difficult in the obese patients. During intubation, patients in this study were placed in a semirecumbent position (30 degrees) with the head in the sniffing position. Another group of authors used ultrasound to quantify the amount of soft tissue between the skin and the anterior aspect of the trachea at the level of vocal cords. They also used classic assessment of difficult intubation including measurement of thyromental distance, mouth opening, degree of neck mobility, Mallampati score, neck circumference, and presence of sleep apnea. Only the abundance of pretracheal soft tissue measured ultrasonically and neck circumference were positive predictors of difficult intubation. Laryngoscopy was carried out with patients in the sniffing position. A meta-analysis of 35 studies, including the four studies just described, was conducted to determine the diagnostic accuracy of preinduction tests for predicting difficult intubation in patients having no airway pathology. A major finding was that the incidence of difficult intubation in obese patients was three times the incidence determined in the nonobese population. This may have resulted from suboptimal patient positioning, which was not clearly described in any of the preceding studies to include ramped positioning or elevation of the upper body and head of morbidly obese patients to align the ear with the sternum horizontally, as has been shown to improve laryngoscopic views. In that study of morbidly obese patients, patients were assigned to be in either sniffing position or ramped position for the laryngoscopy and intubation. The study results showed a statistically significant difference in laryngeal view, in that the ramped position provided the superior view.

Research has also been conducted to examine the rate of development of hypoxemia in patients during apnea. In one study patients received 100% oxygen by face mask for denitrogenation before induction of general anesthesia. Apnea was permitted until the SpO ₂ fell to 90%. Obese patients reached the endpoint in less than 3 minutes, whereas it took 6 minutes in patients having a normal BMI. Efforts to prevent atelectasis formation and desaturation during induction of general anesthesia in the obese population have included application of continuous positive airway pressure (CPAP) during preoxygenation, along with the addition of positive end-expiratory pressure (PEEP) and mechanical ventilation by mask after induction. Application of 10 cm H ₂ O CPAP during preoxygenation in the supine position resulted in a higher PaO ₂ after intubation and decreased the amount of atelectasis that developed. The combination of CPAP during preoxygenation and PEEP/mechanical ventilation after induction significantly prolonged the nonhypoxemic apnea duration to 3 minutes from 2 minutes found in control subjects not receiving CPAP or PEEP. The use of 7.5 cm H ₂ O CPAP during 3 minutes of preoxygenation while supine, however, did not alter the time required for obese patients to show desaturation to an SpO ₂ of 90%. Preoxygenation using 25 degrees head-up (i.e., back inclined), as opposed to supine, positioning without positive airway pressure did prolong the time required for anesthetized, apneic, obese individuals to show desaturation to an SpO ₂ of 92%. The patients in head-up position had a significantly higher PaO ₂ after preoxygenation, just before induction. The obesity-associated gas exchange defect was shown to depend on the waist-to-hip ratio, an index of the distribution of adipose tissue surrounding the thorax. This study also demonstrated that morbidly obese men are more likely to have poorer pulmonary gas exchange than morbidly obese women. In another study conducted to assess effects of patient positioning on development of hypoxemia in superobese patients during apnea after anesthetic induction and intubation, patients received ventilation with 50% oxygen/50% air mixture for 5 minutes before the ventilator circuit was disconnected. Apnea persisted until the SpO ₂ fell to 92% before ventilation resumed. Patients in the supine position reached the endpoint in 2 minutes, whereas it took 30 seconds longer for those in a supine position with the back elevated 30 degrees, and 1 minute longer for patients in a 30-degree reverse Trendelenberg position. The use of 30-degree reverse Trendelenberg position in obese patients undergoing bariatric surgery was also shown to reduce the alveolar-to-arterial oxygen difference, as well as increase total ventilatory compliance and reduce peak and plateau airway pressures when compared with the supine position. Vital capacity has also been shown to decrease to a greater extent under general anesthesia in obese patients compared with normal-weight patients.

Perioperative maneuvers to maintain lung volume and oxygenation have also been studied. Increasing tidal volume incrementally from 13 to 22 mL/kg in obese patients receiving ventilation under general anesthesia did not improve the gas exchange defect but did increase airway pressures. The use of 10 cm H ₂ O PEEP in obese patients compared with normal-weight subjects has been demonstrated to have a greater effect on improving ventilatory mechanics, increasing PaO ₂ , and decreasing the alveolar-to-arterial oxygen difference during general anesthesia with neuromuscular blockade. It is especially important to consider obese patients undergoing laparoscopic procedures because pneumoperitoneum negatively effects pulmonary mechanics by increasing pulmonary resistance and decreasing dynamic lung compliance. During pneumoperitoneum, alterations in body position, tidal volume, and respiratory rate did not alter the alveolar-to-arterial oxygen difference in obese patients. During pneumoperitoneum for laparoscopic bariatric surgery, alveolar recruitment by repeated sustained lung inflation to 50 cm H ₂ O followed by mechanical ventilation with 12 cm H ₂ O PEEP was shown to increase PaO ₂ intraoperatively while causing hypotension that required vasopressor use. An attempt to optimize PEEP in obese patients undergoing laparoscopic gastric bypass surgery showed that a normal functional residual capacity was maintained with 15 ± 1 cm H ₂ O PEEP, but intravascular volume expanders had to be infused to prevent PEEP-induced hemodynamic embarrassment. Regarding postextubation care, in a study of morbidly obese patients proven to have obstructive sleep apnea who underwent laparoscopic bariatric surgery, patients received either CPAP via the face mask–oxygen tank Boussignac system or supplemental oxygen by face mask immediately after extubation. All patients then received CPAP by traditional noninvasive ventilation during subsequent recovery and postoperative care. Spirometric lung function was significantly better preserved 24 hours after surgery in those patients who immediately received CPAP than in those who received supplemental oxygen before CPAP was applied in the postanesthesia care unit.