Organ system
Clinical manifestations
Neurologic
Cerebrovascular accident (stroke), idiopathic intracranial hypertension, dementia, depression
Cardiovascular
Coronary artery disease, chronic inflammation, obesity cardiomyopathy, congestive heart failure, hypertension, cor pulmonale, arrhythmias
Respiratory
Obstructive sleep apnea, obesity hypoventilation syndrome, atelectasis, ventilation/perfusion mismatch
Gastrointestinal
GERD, hiatal hernia
Metabolic
Diabetes mellitus, metabolic syndrome, dyslipidemia, gout, degenerative joint disease
Hepatic
Fatty liver (nonalcoholic), cholelithiasis
Renal
Chronic kidney disease, glomerulosclerosis, incontinence
Reproductive
Menstrual disorders, erectile dysfunction, hypogonadism, birth defects
Classification of Obesity
As a physician, it is important to identify high risk patients. Two variables that help define obesity are metabolic syndrome and body mass index (BMI). The American Heart Association (AHA) and the National Heart, Lung, and Blood Institute (NHLBI) categorize patients as having metabolic syndrome if three or more of the following are met:
Blood pressure ≥ 130/85
Fasting blood glucose ≥ 100 mg/dL
Waist circumference for men ≥ 40 inches, and for women ≥ 35 inches
HDL cholesterol for men < 40 mg/dL and for women < 50 mg/dL
Triglycerides ≥ 150 mg/dL
It is important to note that these criteria specifically point out waist circumference because central obesity, as compared to a more peripheral distribution, is associated with a higher risk of morbidity and mortality primarily related to cardiovascular disease. BMI is a calculated value used to determine amount of body fat and is commonly used to classify patients who are underweight, overweight, obese, and morbidly obese (Table 42.2).
Table 42.2
Classification of patients as per their BMI
Category | BMI |
|---|---|
Underweight | 16–18.5 |
Normal | 18.5–25 |
Overweight | 25–30 |
Obese Class I | 30–35 |
Obese Class II | 35–40 |
Obese Class III/morbid obesity | Over 40 |
BMI = weight (kg)/height2 (meters)
Pathophysiology of Obesity
Cardiovascular System
Numerous cardiovascular complications are associated with obesity. Clinically, all of these changes represent an increase in myocardial oxygen demand and carbon dioxide production as well as increased diastolic filling pressures and ventricular dysfunction.
First, there is an increase in total blood volume, metabolic demand, and cardiac output which are secondary to an increase in complex capillary network in adipose tissue as well as increase in body mass.
In the nonhypertensive patient, there is a decrease in total peripheral resistance, which is accompanied by an increased stroke volume. Over time this leads to an increase in ventricular wall stress, dilation and eccentric left ventricular hypertrophy.
However, concomitant hypertension is often encountered with an elevated systemic resistance, which causes an increase in left ventricular afterload and concentric hypertrophy.
Obese patients are also at risk of developing biventricular failure due to a high incidence of pulmonary hypertension and right ventricular dysfunction. Pulmonary hypertension most often occurs secondary to high left-sided pressures or underlying OSA.
Obesity is an independent risk factor for the development of coronary artery disease, which occurs at an accelerated rate with a higher risk of myocardial infarction.
A less known condition, cardiomyopathy of obesity, or Adipositas Cordis, has also been described. These patients demonstrate abnormal fatty infiltration of the myocardium as well as excessive epicardial fat deposition that leads to myocyte dysfunction and cardiac conduction defects.
Respiratory System
Obese patients experience several significant changes in pulmonary physiology.
Decreased chest wall and lung compliance is secondary to increase in adipose tissue in the thorax and abdomen. This results in a significantly decreased vital capacity (VC), functional residual capacity (FRC), and expiratory reserve volume (ERV).
The reduction in ERV is greatest in the supine position, at which point the FRC approximates the residual volume (RV), placing these patients at high risk of gas trapping, atelectasis, ventilation/perfusion (V/Q) mismatch, and rapid oxygen desaturation.
OSA is another condition commonly diagnosed in obese patients. OSA refers to a periodic, partial, or complete obstruction of the upper airway due to respiratory effort against a closed glottis when asleep. The cessation of airflow is for at least for 10 s or more. The airway is usually floppy, and profound muscle relaxation during anesthesia or sleep worsens the syndrome. A simple screening questionnaire includes questions about snoring, apneic events or arousals when sleeping, and daytime somnolence. These questions should be directed at both the patient and additional household members. The importance of diagnosing OSA is evident when considering the severe physiologic abnormalities that can result from interrupted breathing, including hypoxemia, hypercapnia, secondary polycythemia, pulmonary hypertension, and eventually cor pulmonale. A formal sleep study accurately diagnoses the disorder, at which time the patient can receive trials for continuous positive airway pressure (CPAP) therapy.
Obesity hypoventilation syndrome (OHS) is defined by extreme obesity and alveolar hypoventilation during wakefulness. Patients have hypersomnolence, dyspnea, and hypoxemia, with resulting cyanosis and polycythemia. These can result in pulmonary hypertension, leading to right ventricular failure and peripheral edema.
Gastrointestinal System
Obesity increases the risk for gastrointestinal disorders, including gastroesophageal reflux disorder (GERD), hiatal hernia, and delayed gastric emptying, which are largely due to increased abdominal mass compressing the stomach and possibly causing herniation of the gastroesophageal junction into the thorax. There is also an increased incidence of erosive gastritis that is related to the greater acidity and volume of gastric contents. When combined, these factors increase the risk of pulmonary aspiration. In addition, obese patients are at a higher risk of liver dysfunction due to fatty infiltration and gallbladder disease.
Endocrine System
Research has clearly shown that obesity is associated with an increased risk of impaired glucose tolerance and type II diabetes mellitus. While the mechanism is not completely understood, there appear to be genetic factors, which predispose patients to develop insulin resistance in the peripheral fatty tissue and pancreatic B-cell dysfunction. When combined with an increased caloric intake and sedentary lifestyle, patients develop a persistently elevated glucose. Perioperatively, exogenous insulin may be needed even for non-insulin-dependent diabetic patients.
Renal System
The initial effect of obesity on the kidneys is seen as an increase in renal blood flow, glomerular filtration rate, and renal hypertrophy. When combined with conditions that often accompany obesity such as hypertension, diabetes mellitus, hyperlipidemia, and atherosclerosis, the result is a thickening of the basement membrane, podocyte dysfunction, focal and segmental glomerulosclerosis, renal vascular injury, and proteinuria. If left untreated, these patients are at a high risk of developing chronic kidney disease.
Preoperative Assessment
Obesity has been shown to cause multiorgan dysfunction, which has a direct impact on anesthetic care. If time permits, each patient, especially the morbid obese, should be evaluated preoperatively in a multidisciplinary clinic that offers access to various subspecialties including cardiology and pulmonology, as well as imaging and laboratory services. Due to a sedentary lifestyle and limited mobility, obese patients often report being asymptomatic despite having significant underlying cardiopulmonary disease. For those patients who are not seen in a preoperative anesthesiology clinic, a thorough work up should be completed if time permits and the surgery is not deemed an emergency. All obese patients should be made aware of the potential for a difficult intubation, postoperative mechanical ventilation, or extubation to CPAP to ensure adequate ventilation and oxygenation. Goals of preoperative evaluation include
Obtain pertinent data regarding the patient’s medical or surgical history
Obtain detailed list of medications, including any weight-reducing medications and herbal medications
Obtain necessary laboratory data, including chest X-ray, pulmonary function tests, EKG, echocardiogram, and coagulation profile
Optimize current physiologic functioning
Determine an appropriate anesthetic care plan
Evaluation of the Cardiovascular System
A thorough review of the cardiovascular system is important. Of particular concern are a history of hypertension, congestive heart failure, pulmonary hypertension, and ischemic heart disease, which may be present but not clearly defined. Detecting signs of compromised cardiac function may be challenging due to patient morphology; however, it is important to examine the patient for pulmonary congestion, elevated jugular venous pressure, hepatomegaly, and peripheral edema.
An EKG may reveal signs of ischemia, left or right ventricular hypertrophy, left atrial enlargement, and atrial fibrillation. A chest X-ray can show signs of pulmonary congestion and prominent pulmonary arteries as well as underlying lung disease. A transthoracic echocardiogram is a simple, noninvasive test, which demonstrates both systolic and diastolic function, the degree of ventricular hypertrophy and the presence of any valvular disease. Based on this information, a stress test may be considered. Lab tests should include electrolytes, blood glucose, and renal function studies, which may be compromised secondary to coexisting heart failure or diabetes mellitus.
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