The Obese Patient

Summary of Key Points

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Obesity and the diseases associated with it are preventable.
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An estimated 1.4 billion people worldwide are overweight.
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About 500 million people worldwide are obese.
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Obese patients require careful planning and thorough medical evaluation prior to spine surgery.

Obesity is a global issue. An estimated 500 million people throughout the world are obese, defined as having a body mass index (BMI) of 30 kg/m ²or more, and 1.4 billion are overweight (BMI ≥ 25 kg/m ²). From 2011 to 2012, the prevalence of obesity in the United States was 33.7% among men and 36.5% among women. Healthy People 2020 set a goal, in 2010, of lowering the percentage of people in the United States with obesity to 30.5%. In 2013, 43 states had a prevalence of obesity 25% or higher, with the prevalence in 20 of those 43 states being 30% or greater. The implications of obesity are far reaching, with total estimated annual medical costs estimated to be $147 billion in 2008. 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 United Kingdom and United States of America project that by 2030 they will have a combined increase of 6 million to 8.5 million cases of diabetes and 5.7 million to 7.3 million cases of heart disease and stroke, with the combined associated medical costs incurred from treatment for these preventable diseases estimated to increase by $48 billion to $66 billion per year in the United States and 1.9 billion to 2 billion pounds per year in the UK. 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/m ²). A BMI over 25 kg/m ²is defined as overweight, and a BMI greater than 30 kg/m ²is defined as obese. Globally rising obesity rates is attributed to societal changes that have led to increased consumption of energy-dense foods combined with reduced physical activity, creating an energy imbalance. These changes have dramatically affected children—worldwide it is estimated that 42 million children under age 5 and 170 million children age 18 years or younger are overweight. 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, are major factors in childhood obesity throughout the world. Additionally, childhood obesity is associated with breathing difficulties, increased fracture risk, hypertension, insulin resistance, and psychological sequela. As obese children mature, the higher risks of premature death, disability, and continued obesity as an adult are not insignificant potential health problems causing personal and societal burdens.

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. 174-1 ).

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.

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.

Hangai and colleagues 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. 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.

A 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 and colleagues, including seeking care for low back pain and chronic low back pain in persons who are overweight or obese. 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. 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. Shiri and colleagues investigated the relationship of overweight/obese individuals with lumbar radicular pain or sciatica in a meta-analysis of 26 studies. Overweight and obese persons were at a higher risk of developing lumbar radicular pain, with a higher prevalence in women, who also experienced a slower recovery. Men were noted to have a higher incidence of hospitalization and surgery.

Twenty-five bariatric surgery (BMI 47 ± 7; weight 125 kg ± 12 kg) and 20 control patients (BMI 42 ± 6; 115 kg ± 22 kg) were prospectively assessed at baseline and at 3 months for knee and low back pain, mobility, gait and walking speed, and Short Form-36 (SF-36) outcomes. At 3 months, the bariatric surgery group weighed 19.4 kg ± 7.7 kg less than preoperatively. Low back pain severity decreased by 54%, with no improvement in the control group. Walking speed increased by 15% in the surgery group with no increase in the control group. The authors concluded that bariatric surgery may provide morbidly obese patients with accelerated decrease in back and joint pain, allowing for further increase in physical activity, weight loss, and improved quality of life.

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.

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.

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. However, most of the extra volume is distributed to the adipose tissue, whereas renal and cerebral blood flow are normal.

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.

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.

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 non-recumbent position, such as in a reclining chair. Electrocardiograms may be low voltage and may underestimate right and left ventricular hypertrophy. Careful preoperative assessment is necessary.

Diabetes

Guidelines presented by the American Heart Association, American College of Cardiology, and The Obesity Society (the 2013 AHA/ACC/TOS Guideline for the Management of Overweight and Obesity in Adults) found that in persons who were at risk for type 2 diabetes, an average weight loss of 2.5 to 5.5 kg at 2 years utilizing lifestyle interventions reduced participant risk of developing type 2 diabetes by 30% to 60%. 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.

Olson and colleagues found diabetes to be the highest independent risk of spine surgical site infection, and an elevated preoperative or postoperative serum glucose level was independently associated with an increased risk of surgical site infection.

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 glucose ( Fig. 174-2 ).

Obesity is a well-known risk factor for postoperative surgical site infection. Watters and associates 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.

In addition to antibiotic prophylaxis to prevent surgical site infection, thorough preoperative skin cleaning with antibacterial soap in the home preoperatively and adequate scrub prior to incision are of the utmost importance.

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.

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.

Obstructive sleep apnea, difficult intubation, risk of aspiration, and positioning during induction can present unique anesthetic challenges. Anatomic alterations associated with obesity include increased adipose tissue around the upper airway, and upper airway collapsibility is higher. Central obesity has been associated with reduced lung volumes. Structural alterations are further compounded by neuromuscular control disturbances, which also appear to play a role in sleep apnea.

Obstructive sleep apnea occurs because the pharyngeal muscles relax, leading to upper airway collapse during inspiration during sleep. It is characterized by frequent episodes of apnea during sleep and recurrent episodes of hypoxemia and hypercapnia. Obstructive sleep apnea is more likely to lead to total obstruction if the pharyngeal muscle tone is reduced by drugs, as with surgery.

Because of the risks involved with intubation of the obese patient, awake fiberoptic intubation should be considered. Care must be taken to ensure that sufficient ventilation is being provided. During the postoperative period, effects of anesthetics, sedatives, and opioids can diminish ventilatory drive and upper airway control. Nursing staff must be educated on the potential of carbon dioxide retention.

Pharmacokinetics

Factors that affect drug dosing in obese patients include increased adipose tissue, renal blood flow, glomerular filtration rate, and total blood volume; elevated cardiac output; altered protein binding; and decreased total body water. Both fat and lean masses are increased as compared with nonobese counterparts. The increase in lean body mass is 20% to 40% of total excess weight. However, the fat mass of total body weight increases more than lean mass, resulting in a decreased percentage of lean body mass and water as compared with otherwise similar nonobese patients, with the volume of drug distribution larger in obese individuals.

Lipophilic drugs such as benzodiazepines, barbiturates, and propofol require larger loading doses in obese patients due to their elevated volume of distribution. However, maintenance dosing should be based on drug clearance. If the drug clearance is similar or reduced in the obese patient, the maintenance dose generally is based on ideal body weight. If drug clearance is increased in the obese person, then the dose is calculated on total body weight. Thus, midazolam used as a loading dose should be calculated on total body weight, whereas maintenance dosing should be calculated on ideal body weight.

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