Chapter 129 The Obese Patient
Obesity is a global issue. An estimated 300 million people throughout the world are obese, defined as having a body mass index (BMI) of 30 kg/m2 or more and 1.5 billion are overweight (BMI ≥25 kg/m2).1 From 2007 to 2008, the prevalence of obesity in the United States was 32.2% among men and 35.5% among women.2 Healthy People 2010 set a goal, in 2001, of lowering the percentage of people in the United States with obesity to 15%. In 2010, Colorado was the only state with an obesity rate below 22%. Thirty-six states had obesity rates of 25% or higher, with the rate in 12 of those 36 states being 30% or above.3 The implications of obesity are far reaching. Medical costs related to obesity accounted for 9.1% of total U.S. medical expenditures in 1998, with total direct and indirect costs estimated to be $147 billion in 2000.4 If the prevalence of obesity currently were the same as it was in 1987, health care spending in the United States would be 10% lower per person, or about $200 billion less each year.5
Obesity is a major risk factor for many diseases and conditions, including type 2 diabetes, hypertension, cardiovascular disease, pulmonary dysfunction, kidney disease, metabolic syndrome, and certain types of cancer. The prevalence of 11 chronic conditions, including those previously mentioned, associated with obesity grew 180% from 1997 to 2005, an increase equal to approximately 29 million additional cases of chronic conditions.5 The implications for spine surgery are multifaceted, including poor wound healing, infection risk, caregiver safety, anesthesia considerations, intraoperative challenges, equipment needs, and biomechanical considerations.
Obesity and Overweight
BMI is defined as the weight in kilograms divided by the square of the height in meters (kg/m2). A BMI over 25 kg/m2 is defined as overweight, and a BMI greater than 30 kg/m2 is defined as obese. Obesity rates, rising threefold or more since 1980 in some areas of North America, the United Kingdom, Eastern Europe, the Middle East, the Pacific Islands, Australasia, and China, is attributed to societal changes that have led to increased consumption of nutrient-poor and energy-dense foods combined with reduced physical activity.1 These changes have dramatically affected children—worldwide it is estimated that 22 million children under age 5 and 18 million children between the ages of 6 to 13 years are overweight or obese. Approximately 30% of Australian, 25.5% of American, and 28.7% of Greek children are overweight or obese.6 The ability to be outdoors is the strongest correlate with physical activity in children. Decreased physical activity and, consequently, decreased energy expenditure, coupled with increased caloric intake, is a major factor in childhood obesity throughout the world.6
Lumbar Spine Disease
Increasing weight causes increased axial loading on the spine. People with increased abdominal girth experience a ventral shift of the center of gravity, leading to loss of neutral position and sagittal alignment. As a result, the thorax is ventral to the pelvis, dramatically increasing the forces experienced by the spine. The repetitive, usual movements associated with activities of daily living are cumulative, subjecting the spine to excessive loads (Fig. 129-1).
The cumulative effect was demonstrated in a radiographic comparison of range of motion in obese and nonobese patients with chronic back pain. Reduced range of motion in obese participants was found to be due to reduced mobility at the pelvic and thoracic levels as well as an increased ventral pelvic tilt. Obesity in this study also was associated with increased lumbar lordosis.7
Repetitive axial loading and loss of sagittal balance in the obese population is known to lead to degenerative changes in the spine. Degenerative characteristics in the lumbar spine were evaluated in a sample of 187 individuals randomly selected from an ancillary project to the Framingham Study. There was a significantly higher prevalence of facet joint disease in obese subjects.8
Hangai et al. investigated factors associated with lumber intervertebral disc degeneration in the elderly. Aging, high BMI, high levels of low-density lipoprotein cholesterol, occupational lifting, and sport activities were all associated with degenerative disc disease in this group of 51- to 86-year-old subjects.9 High BMI was associated with level 4 degenerative disc disease. Aortic calcification/atherosclerosis and lumbar artery stenosis/occlusion also have been associated with lumbar disc degeneration and low back pain.10
A recent meta-analysis of 33 studies evaluated the association between obesity and low back pain. A statistically significant association between BMI and low back pain was noted by Shiri et al., including seeking care for low back pain and chronic low back pain in persons who are overweight or obese.11 The association between excess weight and a higher prevalence of low back pain was stronger in women than men. Increased BMI was a risk factor for low back pain and low back pain–related disability in women.12 A relationship has been reported between low back pain and obesity in women but not men. Waist circumference has been found to be a more significant factor than BMI.13
Thirty-eight patients (30 women, 8 men) were prospectively assessed for their axial low back pain before and after bariatric surgery. Study participants decreased their BMI from 52.25 ± 12.61 kg/m2 to 38.32 ± 9.66 kg/m2 (P < .0001). Twelve months after surgery, participants’ VAS scale scores had decreased from 5.2 ± 3.35 to 2.9 ± 3.1 postoperatively (P = .006). Patients experienced increases in mean physical health and mental health components on the 36-Item Short Form Health Survey (SF-36). Low back pain disability demonstrated significant improvement postoperatively as noted on participants’ Oswestry Disability Index scores.14
Obesity and Associated Comorbidities
Cardiovascular
The cardiovascular implications of obesity include altered metabolism as well as changes in cardiac structure and function. Risk factors associated with obesity include dyslipidemia, hypertension, glucose intolerance, elevated inflammatory markers, and sleep apnea. Adipose tissue is surrounded by an extensive capillary network and functions as an endocrine organ, synthesizing and releasing a variety of compounds. Approximately 30% of the total circulating concentrations of interleukin-6 (IL-6), which modulates C-reactive protein production, originates from adipose tissue.15
Hypervolemia from increased extracellular volume and increased cardiac output are evident in the hypertensive obese patient. Hypertension leads to left ventricular hypertrophy. As hypertension continues, the left ventricle becomes progressively noncompliant, increasing the risk of heart failure.16
Hemodynamically, obese individuals have an increased total blood volume and cardiac output caused by the metabolic demand of increased BMI. Left ventricular hypertrophy and diastolic dysfunction as well as cardiomyopathy are correlated with obesity, and may be predisposing factors to heart failure.15 However, most of the extra volume is distributed to the adipose tissue, whereas renal and cerebral blood flow are normal.16
Diagnostic electrocardiogram interpretation also is influenced by obesity. Changes that may occur include increased heart rate, increased P-R interval, increased QRS interval, increased or decreased QRS voltage, increased Q-T interval, ST-T abnormalities, left axis deviation, flattening of the T wave in inferolateral leads, left atrial abnormalities, and false-positive criteria for inferior myocardial infarction.15
Hypertension is about six times more common in obese individuals. Cardiovascular dysmetabolic syndrome, also known as metabolic syndrome, associates hypertension with an increase in visceral fat. Additionally, a chronic inflammatory state, as evidenced by elevated C-reactive protein and IL-6 levels, may play a role in elevated blood pressure.15
Obese patients may be quite sedentary, with limited mobility, and may not have subjective complaints of cardiac impairment. Early symptoms usually are exertional dyspnea and orthopnea; however, severely obese individuals often do not sleep in bed, and sleep in a nonrecumbent position, such as in a reclining chair. Electrocardiograms may be low voltage and may underestimate right and left ventricular hypertrophy.16 Careful preoperative assessment is necessary.
Diabetes
A recent meta-analysis found type II diabetes associated with overweight and obese individuals.17 Diabetic patients who underwent cervical fusion secondary to myelopathy were found to be male, older, and have more levels fused. They also were more likely to have respiratory, cardiac, and peripheral vascular complications; hematoma or bleeding; transfusion; and dysphagia, with longer lengths of stay and more nonroutine discharges.18
Olsen et al. found diabetes to be the highest independent risk of spinal surgical site infection, and an elevated preoperative or postoperative serum glucose level was independently associated with increased risk of surgical site infection.19
Infection Risk
Obesity is associated with a number of changes in skin physiology, including larger skin folds and increased amount of sweating, which increase moisture and skin friction. Lymphedema can occur because of altered lymphatic flow. Lymphedema is associated with reduced tissue oxygenation and a chronic inflammatory state. Subcutaneous fat, made up almost entirely of white adipose tissue, plays a role in endocrine functions and metabolism of lipids and glucose20 (Fig. 129-2).
Obesity is a well-known risk factor for postoperative surgical site infection.21–24 Watters et al. completed an evidence-based clinical guideline for the use of prophylactic antibiotics in spine surgery. Their conclusions were that while the obese patient population is at a higher risk for postoperative infection, the literature did not yield evidence that would support modification of standard therapy.25
Pulmonary
The respiratory consequences of obesity include alterations in lung volumes, lung mechanics, gas exchange, and respiratory control. In short, reduced expiratory reserve volume, reduced functional residual capacity, reduced total lung capacity, and increased residual volume have significant implications for the anesthetized patient. Careful attention is needed during induction, because obese patients rapidly desaturate despite preoxygenation as a result of a smaller functional residual capacity and an increase in oxygen consumption.16
Mechanically, there is reduced lung and chest wall compliance, increased airway resistance and work of breathing, as well as respiratory muscle inefficiency. Total compliance can fall to 30% of predicted normal.16
