Obesity and Nutrition Disorders

Key points

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Obesity is a common disease. The number of overweight, obese, and morbidly obese persons has increased alarmingly in the last three decades.
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Body mass index (BMI) is only one measure of obesity; some patients with high BMI are in good health.
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Distribution of fat is a better predictor of health risk than weight alone; gynecoid (gluteofemoral) fat distribution predicts better risk than android (abdominal) distribution. Fat tissue is an active endocrine organ.
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Hypertension, diabetes, cardiac disease, dyslipidemia, arthritis, and back pain are common problems in obese patients, who are also at increased risk for depression and cancer, as well as social discrimination.
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Obese women may have more menstrual irregularities, subfertility, stress incontinence, and hirsutism.
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Obstructive sleep apnea (OSA) is a common problem among obese persons, with many negative anesthetic implications and insufficient screening of surgical patients.
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Ventilation is more difficult in obese patients; OSA, BMI, and large neck circumference also can make intubation challenging. A consistent approach with head elevation and preoxygenation can increase apnea time significantly.
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Drug dosing in the obese patient must consider total, lean, and ideal body weight, along with lipophilic drug characteristics.
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Nutrition disorders can significantly impact surgical outcomes, especially postoperative infections.

Obesity

Obesity is a common disease, at least in most of the developed world; parts of the developing world, such as China and Egypt, also have increasing overweight and obesity rates. With one third of the U.S. adult population and one sixth of U.S. children classified as obese, obesity is now the second most common cause of preventable deaths in the United States, second only to smoking. In the 1970s, 15% of the U.S. population was classified as obese; this has steadily increased to 33% and for the first time has now leveled off in the past few years. Clinically, minority populations have a much higher incidence of obesity.

Alarmingly, the incidence of childhood obesity has tripled since 1980 and had reached 17% in 2008, with no signs of abating. Long-term health and social consequences for these children are substantial. They have a significantly greater chance of developing associated medical problems, such as diabetes, hypertension, and heart disease, and at a much younger age. As they grow into obese teenagers and young adults, they are the same children who are much more likely to suffer from depression and social isolation.

In economic terms, the estimated medical cost of obesity in the United States is a staggering $147 billion. Globally, the incidence of obesity has more than doubled since 1980, with more than 1 in 10 adults now classified as obese; obesity is now the fifth leading risk of death. According to the World Health Organization, “Once considered a high-income country problem, overweight and obesity are now on the rise in low- and middle-income countries, particularly in urban settings. Overweight and obesity are linked to more deaths worldwide than underweight.” Perhaps for the first time in human history, there are more overweight than underweight people in the world.

Pathophysiology

Obesity is a function of a person's weight being disproportionately greater than their height. There are different ways of classifying a person as overweight or obese and even morbidly obese. For example, a person who is 100 pounds above their ideal body weight is considered morbidly obese. A patient with a body mass index (BMI) of 30 kg/m ² or more is classified as being obese. BMI is calculated by using the person's weight in kilograms (kg) divided by the person's height in meters squared (m ² ). Table 6-1 lists other weight classifications based on BMI.

Table 6-1

Body Mass Index (BMI) and Weight Status

BMI	Weight Status
< 18.5	Underweight
18.5-24.9	Normal
25.0-29.9	Overweight
30.0-39.9	Obese
40.0-49.9	Morbidly obese
50.0-69.9	Super morbidly obese
> 70.0	Ultra-obese

Although one of many methods for estimating body fat, BMI does not correlate well with the distribution of fat (android or “apple” vs. gynecoid or “pear”). BMI also does not take into account the amount of pre-existing muscle mass often seen in some athletes or even weightlifters. Other methods used to estimate body fat and distribution include measurements of skin fold, waist circumference, waist-to-hip circumference ratios, or radiographic studies such as computed tomography (CT), ultrasound, and magnetic resonance imaging (MRI). For adult patients with a BMI of 25 to 34.9 kg/m ² , waist circumference should be used in addition to BMI to identify risk factors, with higher risks associated with males with a waist size greater than 40 inches and females with a waist size more than 35 inches .

Many factors contribute to overweight and obesity. On a basic level, an energy imbalance between the caloric intake and caloric expenditure—in which caloric intake exceeds calories consumed in basal metabolic demand, work, or exercise—leads to overweight and ultimately obesity. As excess calories are stored in the body as fat, obesity can result from overconsumption of food, insufficient physical activity, or other factors ( Table 6-2 ). A 2% differential in this balance can lead to a 5-pound (2.2-kg) increase in weight every year.

Table 6-2

Etiologies of Obesity

Etiologic Category	Examples
Familial/genetics	Bardet-Biedl syndrome Prader-Willi syndrome
Diseases	Hypothyroidism Cushing's disease Polycystic ovary syndrome Depression Eating disorders
Medications	Antidepressants Steroids
Societal factors	Poor access to healthy foods Larger food portions

Comorbidities

Obesity is associated with an increased incidence of several diseases. Higher morbidity caused by overweight and obesity has been observed for hypertension, type 2 diabetes, coronary heart disease, cerebrovascular accident (stroke), gallbladder disease, osteoarthritis, sleep apnea and respiratory problems, and endometrial, breast, prostate, and colon cancer. Obesity is also associated with complications of pregnancy, menstrual irregularities, hirsutism, stress incontinence, and psychological disorders such as depression.

Fat distribution also plays an important role in the type of associated disease. Visceral fat, typically seen in males with an android (truncal) fat distribution, represent a risk factor for the development of cardiovascular disease and type 2 diabetes. Visceral adipose tissue mass frequently correlates with the development of insulin resistance. This is not the case with total or subcutaneous adipose tissue mass. It is now known that the adipocytes of visceral fat tissue are more lipolytically active than subcutaneous adipocytes and contribute more to the plasma free fatty acid (FFA) level. Subcutaneous adipose tissue (SAT) is divided into superficial subcutaneous adipose tissue (sSAT) and deep subcutaneous adipose tissue (dSAT) by the layer of fascia superficialis. Deep SAT is strongly linked to insulin resistance, particularly in obese males. Interestingly, subcutaneous leg and hip adipose tissue have a protective role against diabetes and cardiovascular disease.

Obesity affects many organ systems ( Table 6-3 ). During the perioperative period, the anesthesiologist is primarily concerned with cardiovascular, respiratory, and gastrointestinal diseases.

Table 6-3

Comorbidities in Obese Patients

Disease Category	Select Comorbidities
Cardiovascular	Obesity cardiomyopathy Hypertension Ischemic heart disease Hyperlipidemia Sudden cardiac death
Respiratory	Obstructive sleep apnea (OSA) Obesity hypoventilation syndrome (OHS) Restrictive lung disease
Endocrine	Diabetes Cushing's disease Hypothyroidism Polycystic ovary syndrome (PCOS) Infertility
Gastrointestinal	Gastroesophageal reflux disease (GERD) Fatty liver Gallstones
Genitourinary	Menstrual abnormalities Female urinary incontinence Renal calculi
Malignancy	Breast, prostate, colorectal, cervical, renal, and endometrial cancer
Musculoskeletal	Osteoarthritis of weight-bearing joints Back pain

Obesity-hypoventilation syndrome

Between 10% and 20% of OSA patients eventually develop obesity-hypoventilation syndrome (OHS), also known as pickwickian syndrome. OHS is defined as a combination of obesity (BMI ≥ 30 kg/m ² ) and chronic hypercapnia (Pa co ₂ ≥ 45 mm Hg) accompanied by sleep-disordered breathing. Patients typically present with daytime hypersomnolence as well. Patients with severe OHS may develop polycythemia, pulmonary hypertension, and right-sided heart failure. Obstructive sleep apnea causes hypoventilation, leading to hypoxia and hypercapnia, with metabolic alkalosis to compensate for respiratory acidosis. Initially the acid-base disturbance is limited to nocturnal periods, with a return to homeostasis during the day. Over time, however, the central respiratory centers become desensitized to hypercapnia, and nocturnal episodes of apnea occur. Gradually, an increased reliance on hypoxic drive for ventilation develops (see Airway Considerations ).

Patients with OHS are more sensitive to the respiratory depressive effects of opioids and hypnotics. Because of the added complexity of arterial hypoxia, hypercapnia, pulmonary hypertension, and right-sided heart failure, invasive monitoring should be considered in obese patients along with baseline arterial blood gas (ABG) levels as a reference point for further management.

Respiratory Effects

Obese patients have an increased amount of chest wall adipose tissue, causing a mass effect on the thoracic cage and abdomen. This extra weight impedes the normal diaphragmatic motion, especially in a supine position, resulting in splinting of the diaphragm. This causes a decrease in functional residual capacity (FRC), expiratory reserve volume (ERV), and total lung capacity (TLC). The FRC may decrease to the point that small-airway closure occurs with resulting ventilation/perfusion mismatch, right-to-left shunting, and arterial hypoxemia. General anesthesia further decreases the FRC in an obese patient (~ 50%) compared with a nonobese patient (~ 20%), leading to a decrease tolerance of apnea. Preoxygenation with anesthesia induction helps prolong the apnea period, although arterial hypoxia is still quite common during direct laryngoscopy. The addition of continuous positive airway pressure (CPAP) helps improves FRC at the expense of cardiac output and oxygen (O ₂ ) delivery.

The extra weight around the chest wall also causes decreased lung compliance, resulting in rapid, shallow breathing patterns in obese patients. This increases the work of breathing, causing increased O ₂ consumption and increased carbon dioxide production. Therefore, obese patients have increased CO ₂ production and increased O ₂ consumption partly because of increased effort to mobilize and increased energy requirement for breathing in trying to move the chest wall, causing up to a 70% increase in the energy expenditure for breathing.

Cardiovascular Effects

The risk of comorbidities rises with increasing BMI. Even though exertional dyspnea and lower-extremity edema are common and nonspecific, even electrocardiography and physical examination can underestimate the degree of cardiac dysfunction in the obese patient group. The increased length of assisted ventilation, longer hospital stay, and increased risk of renal dysfunction are more often seen than increased mortality, at least in cardiac surgery patients.

Adipose tissue is highly vascular, with each kilogram of fat containing 3000 m of blood vessels. This causes cardiac output to increase 0.1 L/min for each kilogram of excess weight related to adipose tissue. The result is that 50% to 60% of obese patients also have hypertension from hypervolemia caused by excess extracellular fluid volume and increased cardiac output. Systemic hypertension could eventually lead to concentric left ventricular hypertrophy (LVH), ultimately leading to congestive heart failure (CHF). The right side of the heart is frequently affected because of CHF from the left ventricle or pulmonary hypertension from chronic arterial hypoxemia or increased pulmonary blood volume.

Obese patients often have poor exercise tolerance. Because of LVH and a stiffened left ventricle during exercise, cardiac output can only be increased by increasing heart rate, without a corresponding increase in stroke volume or ejection fraction.

In addition to these cardiac issues, obesity (especially central obesity) is also an independent risk factor for the development of ischemic heart disease. Acid-base and electrolyte disturbances, volume overload, and coronary heart disease also put obese patients at higher risk for arrhythmias, especially atrial fibrillation.

In evaluating risk of perioperative morbidity and mortality, the anesthesiologist focuses on age, gender, cardiac and respiratory fitness, electrolyte imbalances, and heart failure as predictors. The American Heart Association (AHA) provides recommendations for evaluation of obese surgical patients.

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