Obesity is associated with many chronic medical problems (Table 10–1). A patient who is moderately overweight probably carries no excess health risks, especially while still young. However, morbidity and mortality rise sharply with increasing age and BMI. Medical comorbidities must be recognized, and when possible optimized before elective bariatric surgery. Any patient who has had previous bariatric surgery should be evaluated for metabolic changes that can include protein, vitamin, iron, and calcium deficiencies.
Table 10–1. Medical Conditions Associated with Obesity
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Table 10–1. Medical Conditions Associated with Obesity
restrictive lung disease, asthma, obstructive sleep apnea (OSA) syndrome, obesity hypoventilation syndrome, Pickwickian syndrome
hypertension, cardiomegaly, congestive heart failure, coronary artery disease, peripheral vascular disease, pulmonary hypertension, thromboembolism, sudden death
type 2 diabetes mellitus, Cushing's syndrome, hypothyroidism, hyperlipidemia, vitamin deficiency
hiatal hernia, inguinal and ventral hernia, fatty liver (NASH), gallstones
osteoarthritis on weight bearing joints, low back pain
breast, prostate, cervix, uterine, colorectal
depression, low self-esteem
A list of all current medications must be available to the anesthesiologist, including nonprescription appetite suppressors and diet drugs. Many of these drugs can have important side effects. For example, the combination of phentermine and fenfluramine (phen–fen), which is no longer prescribed in the United States, is associated with serious heart and lung problems. Another weight loss medication, sibutramine, works in the brain by inhibiting the reuptake of norepinephrine, serotonin, and dopamine producing a feeling of “anorexia” which limits food intake. Sibutramine has been implicated as a cause of dysrhythmias and hypertension. Orlistat blocks digestion and absorption of dietary fat by binding lipases in the gastrointestinal tract and can cause deficiencies in fat-soluble vitamins (A, D, E, K). A reduction in vitamin K levels can increase the anticoagulation effects of coumadin.
During the physical examination the anesthesiologist's attention is directed to the cardiac and pulmonary systems, and to head and neck anatomy in order to evaluate the airway for tracheal intubation.
Cardiac output rises proportionally with increased weight. Stroke volume also increases since a greater total blood volume is needed to perfuse the added body fat. Increased cardiac output combined with normal peripheral vascular resistance leads to systemic hypertension. Mild to moderate hypertension is seen in most morbidly obese patients. The increased left ventricular wall stress caused by increased stroke volume and the resultant ventricular dilation leads to cardiac hypertrophy.2
Left ventricular dysfunction is often present in young, asymptomatic patients. Even normotensive patients have increased preload and after-load, increased mean pulmonary artery pressure (PAP), and elevated right and left ventricular stroke work. Since these patients are often not physically active, they may appear to be asymptomatic even in the presence of significant cardiovascular disease. Signs of pulmonary hypertension (exertional dyspnea, fatigue, syncope) should be sought and trans-esophageal echocardiography (TEE) obtained in symptomatic patients. Right heart failure is common in older patients. A medical consultation with a cardiologist may be indicated before bariatric surgery.
The ECG may show increased rate, changes in QRS voltage, left QRS axis shift, slowed conduction, and evidence of ischemia or previous myocardial infarction. The ECG even in normotensive morbidly obese patients may reveal left ventricular hypertrophy, cardiac chamber enlargement, ventricular ectopy, and other arrhythmias. Cardiac dysrhythmias are precipitated by chronic hypoxia (especially in patients with obstructive sleep apnea, OSA), hypercapnia, increased circulating levels of catecholamines, electrolyte disturbances caused by diuretic therapy, fatty infiltration of the conduction system, and ischemic heart disease.3 Polycythemia suggests chronic hypoxemia.
Adipose tissue is metabolically active. Oxygen consumption and CO2 production rise with increasing weight due to increased metabolic demands. The work of breathing is increased since more energy must be expended to carry the additional body mass, while respiratory muscle performance is impaired. The fatty chest and abdominal walls plus the increased pulmonary blood volume contribute to reduced pulmonary compliance. Mass loading of the thoracic and abdominal chest walls causes abnormalities in both lung volumes and gas exchange, especially when the patient is supine. The increased total respiratory resistance and decreased compliance associated with extreme obesity results in shallow, rapid breathing. Functional residual capacity (FRC) is significantly reduced due to a decrease in expiratory reserve volume, therefore total lung capacity is reduced. Airways close during normal ventilation. Continued perfusion of nonventilated alveoli will result in an oxygen tension (PaO2 that is lower than predicted for similar-aged nonobese patients. These changes increase in direct proportion with increasing BMI.4 General anesthesia further reduces FRC.
Preoperative pulmonary function tests show a restrictive breathing pattern. For symptomatic patients, an arterial blood gas obtained while the patient breathes room air is useful to establish a baseline. Younger obese patients have an increased ventilatory response to hypoxia and a relatively decreased response to hypercapnia. Their arterial blood sample often demonstrates alveolar hyperventilation (PaCO2 30–35 mm Hg) and relative hypoxemia (PaO2 70–90 mm Hg) while breathing air.5 With increasing age, sensitivity to CO2 decreases so PaCO2 rises and PaO2 falls further.
A careful thorough preoperative assessment of patient's face, neck, and upper airway is always required since mask ventilation and tracheal intubation can be a challenge in some obese patients. A review of the patient's previous anesthetic records will reveal whether airway problems had been encountered during previous surgical procedures.
Obstructive Sleep Apnea Syndrome
Many obese patients maintain a normal PaCO2 during the day but have CO2 retention, sleep disturbances, intermittent airway obstruction with hypoxemia, pulmonary hypertension, and cardiac arrhythmias at night. OSA syndrome is characterized by frequent episodes of apnea (>10-s cessation of airflow despite continuous respiratory effort against a closed airway) and hypopnea (50% reduction in airflow or reduction associated with a decrease of SpO2 > 4%). OSA is frequently undiagnosed in patients scheduled for bariatric surgery.
Obesity is an important risk factor for OSA, but not every obese patient suffers from OSA.6 OSA occurs more often in patients with large fat necks and high Mallampati (III and IV) scores. The patients may not be aware of symptoms, so it is important to interview their spouse. If OSA is present, they will describe loud snoring followed by silence as airflow ceases with obstruction, then gasping or choking as the patient awakes and airflow restarts. A definitive diagnosis of OSA can only be confirmed by polysomnography in a sleep laboratory. Because of fragmented sleep patterns, OSA patients may complain of daytime sleepiness and headaches. Chronic sleep apnea leads to secondary polycythemia, hypoxemia, and hypercapnia, all of which increase the risk of cardiac and cerebral vascular disease.
Patients with a history of snoring or a definitive diagnosis of OSA are often difficult to ventilate by mask, and their tracheas may be more difficult to intubate than similar weight patients without OSA. And OSA patients who use nasal continuous positive airway pressure (CPAP) devices at home should be instructed to bring them to the hospital to use following surgery.
If the patient is known or even suspected of having OSA they should be continuously monitored by pulse oximetry in the postoperative period, even following a completely uneventful operation.
Obesity Hypoventilation Syndrome
A small number of patients have the “obesity hypoventilation syndrome” (OHS), which is characterized by somnolence, cardiac enlargement, polycythemia, hypoxemia, and hypercapnia. OHS patients tend to be older, super obese (BMI > 50 kg/m2, and have more restricted pulmonary function than other patients with OSA.
Hypoventilation is central and independent of intrinsic lung disease, and is probably due to a progressive desensitization of the respiratory center to hypercapnia from nocturnal sleep disturbances. In its most severe form, the “Pickwickian Syndrome,” there is hypersomnolence, hypoxia, hypercapnia, pulmonary hypertension, right ventricular enlargement, and hypervolemia. These patients rely on a hypoxic ventilatory drive and may hypoventilate or even become apneic following emergence from general anesthesia after being given 100% O2 to breathe.
Gastrointestinal and Urinary Systems
It is widely believed that morbidly obese patients are at greater risk for acid aspiration during induction of general anesthesia. Risk factors include increased intra-abdominal pressure, high incidence of gastroesophageal reflux disease (GERD) and hiatus hernia, increased gastric volume (usually >25 mL), and decreased gastric fluid pH (usually <2.5).7 Recently this belief has been challenged. One study reported that fasting obese patients actually had a lower incidence of high-volume, low-pH gastric fluid than lean patients,8 while another found no differences in gastric volume or pH between lean and moderately obese surgical patients.9 Obese patients without symptoms of GERD have relatively normal gastroesophageal sphincter tone. Obese patients at special risk for gastric acid aspiration may be those with diabetes and gastroparesis.
Nonalcoholic steatohepatitis (NASH, “fatty hepatitis”), with or without liver dysfunction, is extremely common. Histologic abnormalities are present in the livers of as many as 90% of morbidly obese patients.10 Preoperative liver function tests should be obtained, but they often do not reflect the actual severity of liver dysfunction. Alanine aminotransferase is the most frequently elevated liver enzyme. Surprisingly, liver clearance of many anesthetic agents is usually not altered with NASH.
There is increased renal blood flow and an increased glomerular filtration rate (GFR) associated with obesity. Renal clearance of drugs may be greater compared to the normal-weight patient. The most common renal abnormality seen is proteinuria.