The Obesity Epidemic

The most widely used classification of obesity is the body mass index (BMI).¹ Between 1986 and 2000, the number of individuals with BMIs greater than 30, 40, and 50 kg/m² were reported to have doubled, quadrupled, and quintupled, respectively, in the United States.²

The Obesity Paradox

Obesity has long been considered to be a risk factor for poor outcomes from a variety of surgical procedures, yet recent studies of critically and chronically ill patients suggested that overweight and obese patients may paradoxically have better outcomes than “normal”-weight patients.³ Mullen et al.³ demonstrated, in a prospective multi-institutional risk-adjusted study of 118,707 patients undergoing nonbariatric surgery, that the highest rates of death occurred in the underweight and morbidly obese, and the lowest rates were found in the overweight and moderately obese patients. This study revealed that there was a progressive increase in the likelihood of a complication with increasing BMI that was almost entirely due to increasing rates of infection. They hypothesized that metabolic regulation and immune response are highly integrated. Malnourished patients have protein calorie malnutrition, which impairs immunologic response mechanisms; obese patients are known to have a low-grade inflammatory response, which primes their immune system.³

Despite emerging evidence of the “obesity paradox,” there are recommendations for the preoperative evaluation and preparation of obese patients. The extent of the preoperative evaluation depends on the assessment of their surgical risk and the degree of surgery-specific risk.

Major surgery is accompanied by an increased demand for oxygen consumption. This places increased demands on the cardiorespiratory system.⁴ If patients are unable to increase their oxygen delivery to meet these requirements, they have increased mortality.⁵

Patient Preoperative Surgical Risk

Preoperative risk assessment always starts with an in-depth history and comprehensive physical examination. Will the patient be able to tolerate the physiological stresses of the planned surgery? The American College of Cardiology and American Heart Association (ACC/AHA) have established clinical predictors of cardiac risk. Patients can be categorized as having minor, intermediate, and major risks. However, a more important predictor of risk is the patient’s functional capacity. This assessment helps us understand how combined cardiopulmonary function will tolerate the stress of surgery. This can be readily ascertained using a simple set of questions adopted from the Duke Activity Status Index. This concept measures a patient’s physiologic response by determining the metabolic equivalent tasks (METs). One MET is 3.5 mL/min/kg average resting oxygen consumption in a 70-kg, 40-year-old man.⁴ The ACC/AHA guidelines state the patients with exercise tolerance of greater than 4 METs may proceed to major surgery without further investigation.⁴ Patients with poor exercise tolerance (<4 METs) have significantly greater cardiovascular and neurological complication (20.4% vs. 10.4%, p < 0.001).⁶

Surgical Procedure Risk

In addition to risk assessing the patient, one must risk adjust the surgical procedure. The ACC/AHA has classified procedures into high, intermediate, and low.⁴ In this algorithm, risk is defined as the combined risk of cardiac death and nonfatal myocardial infection. Perhaps the most utilized predictor of postoperative death is the American Society of Anesthesiologists (ASA) scoring system.^4,7

Preoperative Investigations

Ramaswamy et al.⁸ investigated the efficiency of extensive preoperative testing in morbidly obese patients undergoing gastric bypass. They analyzed 193 patients who routinely had chest x-rays, arterial blood gas (ABG) tests, spirometry, electrocardiograms (ECGs), stress echoes, basic metabolic panel (BMP), complete blood cell counts (CBCs), coagulation profiles, thyroid function tests, and B₁₂ and serum iron studies. Only 4% of chest x-rays showed abnormalities, none of which required preoperative intervention. Of the ECGs, 15% were abnormal, but none required preoperative intervention. Spirometry evaluations found 21% of patients had abnormalities. Preexisting asthma was predictive of obstructive physiology. BMI was predictive of restrictive physiology. ABGs identified 1 case of severe hypoxemia requiring intervention. Echo cardiography showed 2% abnormalities, and previous history of cardiac disease was the only risk factor. Routine CBCs did not identify 84% and 50% of the patients with iron and vitamin B₁₂ deficiencies.

The authors⁸ concluded that routine preoperative testing in these morbidly obese patients should include CBC, electrolyte, ECG, and anemia studies. Coagulation profiles, chest x-rays, cardiac stress tests, and pulmonary function tests should be performed based on patient history of bleeding tendencies and cardiopulmonary disease. Patients with major clinical predictors of risk (e.g., poor functional capacity and high surgical-specific risk) should be considered for more in depth preoperative testing.

Echo Cardiography

Echo cardiography provides information concerning wall motility, valvular disease, and systolic and diastolic function. The ACC/AHA, however, has found that there is a poor correlation between ECG and a patient’s functional capacity. Thus, it is not felt to be a consistent predictor of perioperative ischemic events.⁹

Exercise Electrocardiography

Exercise electrocardiography has been shown to have a sensitivity of 81% for multivessel coronary disease but a specificity of only 66%.¹⁰ Patients with an estimated 7 METS or a heart rate greater than 130 without demonstrable ischemia are at low risk.¹¹

Cardiopulmonary Exercise Testing (the Gold Standard)

Presently, most preoperative surgical patients have evaluation of either their cardiac or their pulmonary system in isolation. Inherently, it would seem that concurrent exercise testing of both systems would provide a more accurate assessment of the patient’s ability to withstand the stress of surgery. Exercise testing requires that oxygen consumption and carbon dioxide production be measured while the patient exercises on a bicycle. The anaerobic threshold is determined, which is the point at which oxygen delivery is insufficient and anaerobic metabolism begins. A 12-lead ECG is simultaneously obtained to evaluate for ischemia and arrhythmia.⁴ In major abdominal surgery, patients with aerobic thresholds less than 11 mL/kg/min have significantly higher mortality rates than those with higher aerobic thresholds (18% vs. 0.8%).¹²

Obstructive sleep apnea (OSA), obesity hypoventilation syndrome (OHS), and pulmonary hypertension (PH) are gaining increasing recognition as pulmonary risk factors for patients undergoing noncardiac surgery.¹³

Obstructive Sleep Apnea

Young et al. estimated the prevalence of OSA with an apnea-hypopnea index (AHI). The apnea-hypopnea index is defined as the number of apneic and hypopnea events that occur per hour of sleep. The prevalence of OSA with an AHI of 15 or higher in patients aged 30–69 with a BMI greater than 40 is noted to be 42%–55% for men and 16%–24% for women.¹⁴

Screening for OSA should start with questions about daytime sleepiness, heavy snoring and sudden awakening with the need to catch a breath, and apnea witnessed by a partner.¹³ Hypertension; short, thick neck; BMI greater than 30; narrow oropharynx; and retrognathia may be found by physical exam.¹³ Patients with a high suspicion for OSA should have polysomnography (PSG) to confirm or rule out the diagnosis.

Gupta et al., using PSG and pulse oximetry data for OSA diagnosis in 101 patients undergoing orthopedic surgery, found a statistically significant higher incidence of postoperative serious complications which include intensive care unit [ICU] days, reintubations, and cardiac events (24 vs. 9 complications, p = .004) and hospital length of stay (6.8 vs. 5.1 days, p < .007).¹⁵ Other studies involving patients undergoing noncardiac surgery have confirmed that patients with OSA have a higher incidence of postoperative hypoxemia (p = .009), overall complications (p = .003), unplanned ICU transfer (p = .069), and longer hospital length of stay (p = .049) compared to controls.¹³

When using general anesthesia, the possibility of difficult intubation and induction should be considered. Use of ASA guidelines for management of the difficult airway may be necessary.¹⁶ Extubation should be considered only after full reversal of neuromuscular blockade. Because patients with OSA are more prone to perioperative oxygen desaturation, opioids should be minimized. Intravenous acetaminophen, tramadol, pregabalin, and cyclooxygenase 2 (COX-2) inhibitors have been useful in postoperative opioid-sparing protocols. The patients’ continuous positive airway pressure (CPAP) apparatus and settings should be instituted as soon as possible postoperatively to avoid airway obstruction and desaturation. A 16% absolute risk reduction in the rate of respiratory failure was reported in a bariatric surgery population with the use of noninvasive ventilation during the first 48 hours after extubation.¹⁷

Obesity Hypoventilation Syndrome

Obesity hypoventilation syndrome is characterized by the triad of chronic daytime hypercapnia (PaCO₂ > 45 mm Hg), sleep disordered breathing, and obesity with a BMI greater than 30 kg/m².¹⁸ To compensate for chronic respiratory acidosis, patients with OHS have high serum bicarbonate levels. Patients with known OSA and high serum bicarbonate levels should be considered for the diagnosis of OHS.

Mortality as high as 23% has been reported in untreated patients with OHS compared to 9% in matched obese cohorts.¹⁹ Because of the chronic hypercapnia, these patients have blunting of their respiratory drive and have a high risk of respiratory failure after elective surgery (44.4% vs. 2.6% in controls).²⁰

The perioperative management of these patients is similar to patients with OSA. Preparation for a possible difficult intubation, full reversal of neuromuscular blockage, opioid sparing, and early use of CPAP should be considered. In patients for whom the positive airway pressure settings are not known, an empiric inspiratory positive airway pressure of 16–18 cm H₂O and expiratory airway pressure of 9–10 cm H₂O can be initiated.²¹

Pulmonary Hypertension

Symptoms and signs of PH evolve slowly over time. Initially, patients experience subtle exertional dyspnea and fatigue. Eventually, exertional chest pain, syncope, peripheral edema, ascites, and pleural effusion may be present.²²

Chest x-ray may reveal enlargement of central pulmonary arteries with attenuation of peripheral vessels. The ECG may have evidence of right ventricular hypertrophy, an R wave/S wave ratio that is greater than 1 in lead V, and right bundle branch block. Echocardiography can estimate the pulmonary artery pressure and assess right ventricular size, thickness, and function. However, the definitive diagnosis of PH requires right heart catheterization. PH is confirmed when the mean pulmonary artery pressure is greater than 25 mm Hg at rest or greater than 30 mm Hg during exercise.¹⁴ PH is classified by the World Health Organization into groups based on etiologies of left heart disease, chronic lung disease, chronic thromboembolic disease, and unclear multifactorial mechanisms.²²

Kaw et al. confirmed that patients with PH undergoing elective noncardiac surgery were more likely than non-PH patients to develop congestive heart failure (#p< .001), hemodynamic instability (p < .002), sepsis (p < .005), and respiratory failure (p < .004).²³ These patients also had longer ICU stays (p < .04), a higher 30-day readmission rate (p < .008), and longer mechanical ventilation (p < .002).

Risk Reduction Strategies in Patients with Pulmonary Hypertension

Perioperative pulmonary artery catheter monitoring allows for accurate measurement of pulmonary artery pressure, mixed venous oxygen saturation, cardiac output, central venous pressure (CVP), and pulmonary capillary wedge pressure. This information can be used to guide administration of fluids and vasopressors.¹³ General anesthesia is most commonly used in patients with PH. Spinal anesthesia is usually avoided due to its profound sympatholytic effect.¹³ Key factors in the intraoperative management of these patients is to avoid hypertension, hypothermia, and acidosis.¹³ Refraction hypotension can be managed by prompt intra-aortic balloon counterpulsation, left ventricular assist device, or extracorporeal membrane oxygenation.²⁴