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Obese patients make anesthesiologists’ work more arduous. Obese patients, especially morbidly obese and super obese patients, are at increased risk when they need surgery. Perhaps you’re overweight and you wish you weren’t. Your anesthesiologist wishes the same thing. Let’s look at the reasons why.
Two hundred million Americans, or 65% of the U.S. adult population, are overweight or obese. Obesity as a disease is second only to smoking as a preventable cause of death.
The body mass index (BMI) has become the most widely applied classification tool used to assess individual weight status. BMI is defined as the patient’s weight, measured in kilograms, divided by the square of the patient’s height, measured in meters.
A normal BMI is between 18.5 and 24.9. Patients are considered to be overweight with a BMI between 25 and 29.9 obese with a BMI between 30 and 39.9, morbidly obese between 40 and 49.9, and super obese at greater than 50.
Morbid obesity is associated with far more serious health consequences than moderate obesity, and creates additional challenges for health care providers. Between 2000 and 2010, the prevalence of morbid obesity in the U.S. increased by 70%, whereas the prevalence of super obesity increased even faster. It’s estimated that in 2010, 15.5 million adult Americans, or 6.6% of the population, had an actual BMI >40, and carried the diagnosis of morbid obesity.
MEDICAL PROBLEMS ASSOCIATED WITH OBESITY
Obesity is an independent risk factor for heart disease, hypertension, stroke, hyperlipidemia, osteoarthritis, diabetes mellitus, cancer, and obstructive sleep apnea (OSA). A neck circumference > 17 inches in men or > 16 inches cm in women is associated with obstructive sleep apnea. As a result of these concomitant conditions, obesity is also associated with early death.
There is a clustering of metabolic and physical abnormalities referred to as the “metabolic syndrome.” To be diagnosed with metabolic syndrome, you must have at least three of the following: abdominal obesity, elevated fasting blood sugar, hypertension, low HDL levels, or hypertriglyceridemia. In the United States, nearly 50 million people have metabolic syndrome, for an age-adjusted prevalence of almost 24%. Of people with metabolic syndrome, more than 83% meet the criterion of obesity. Patients with metabolic syndrome have a higher risk for cardiovascular disease and are at increased risk for all-cause mortality.
Obstructive sleep apnea (OSA) is a condition characterized by recurrent episodes of upper airway obstruction occurring during sleep. Obesity is the greatest risk factor for OSA, and about 70% of patients (up to 80% of males and 50% of females) with OSA are obese. OSA is defined as complete blockage of airflow during breathing lasting 10 seconds or longer, despite maintenance of neuromuscular ventilatory effort, and occurring five or more times per hour of sleep (Apnea Hypopnea Index, or AHI, greater than or equal to five), and accompanied by a decrease of at least 4% in arterial oxygen saturation. This diagnosis can be made only in patients who undergo a sleep study. Obstructive sleep apnea is classified as mild, moderate, or severe, as follows:
- Mild OSA =A HI of 5 to 15 events per hour
- Moderate OSA = AHI of 15 to 30 events per hour
- Severe OSA = AHI of more than 30 events per hour
Treatment is recommended for patients with moderate or severe disease, and initial treatment is the wearing of a continuous positive airway pressure (CPAP) device during sleep.
ANESTHETIC CHALLENGES
Every anesthesia task can be more difficult to perform in an obese patient. Excess adipose tissue (fat) on the upper extremities makes it harder to place an IV catheter. Excess fat surrounding the mouth, throat, and neck can make it more difficult to place an airway tube. Excess fat can make it more difficult to place a needle in the proper position for a spinal anesthetic, an epidural anesthetic, or a regional block of a specific peripheral nerve. On thick, cone-shaped upper arms, it can be difficult for a blood pressure cuff to detect the blood pressure accurately.
During surgery, an anesthesiologist’s job is to maintain the patient’s A-B-C’s of Airway, Breathing, and Circulation, in that order. All three tasks are more difficult in obese patients.
Airway procedures are often much more difficult to perform in obese patients than in patients with normal BMIs. Every general anesthetic begins with the anesthesiologist injecting intravenous medications that induce sleep. Next the anesthesiologist controls the breathing by using a mask over the patient’s face, and then he or she places an airway tube through the patient’s mouth into the windpipe.
The airway anatomy of obese patients, with or without OSA, may show a short, thick neck, large tongue, and significantly increased amounts of soft tissue surrounding the uvula, tonsils, tongue, and lateral aspects of their throats. This can contribute to the development of airway obstruction and also increase the probability that it will be more difficult to keep the airway open during mask ventilation. This can also contribute to difficulty placing an anesthesia airway tube into the windpipe at the beginning of general anesthesia.
What about breathing difficulties? The chief reason that obese patients have difficulty with breathing during anesthesia is that they have abnormally low lung volumes for their size. When lying flat on their back, a patient’s increased abdominal bulk pushes up on their lungs, and prevents the lungs from inflating fully. Once the patient is anesthetized, this mechanical situation is worsened, because breathing is impaired by the anesthetic drugs and muscle relaxation allows the abdomen to sink further into the chest. The essence of the problem is that the abdomen squashes the lungs and makes them less efficient both as a reservoir and as an exchange organ for oxygen. Because of this, the obese patient is at risk for running out of oxygen and turning blue more quickly than a lean patient.
In one study, patients undergoing general anesthesia received 100% oxygen by facemask before induction of general anesthesia. After the induction of general anesthesia, the patients were left without ventilation until their oxygen saturation fell from 100% to 90%. Patients with normal BMIs took 6 minutes for their oxygen level to fall to 90%. Obese patients reached that end point in less than 3 minutes.
What about circulation? Maintaining stable circulatory status can be difficult because obese patients have a higher prevalence of cardiovascular disease, including hypertension, arrhythmias, stroke, heart failure, and coronary artery disease. During anesthesia and surgery, unexpected high or low blood pressure events are more common in obese patients than in those with normal BMIs. Morbidly obese patients have a higher rate of heart attack postoperatively than patients with normal BMIs.
Regional anesthesia, especially epidural and spinal anesthesia, is often a safer technique than general anesthesia in obese patients. However, regional anesthesia can be technically more difficult because of the physical challenge of the anatomy being obscured by excess fat.
Operative times are often longer in obese patients, owing to technical challenges for the surgeon regarding anatomy distorted or hidden behind excessive fat. Longer surgery means a longer time under general anesthesia, which is a cause of delayed awakening from anesthesia. At the conclusion of surgery, obese patients wake more slowly than lean patients. Anesthetic drug and gas concentrations drop more slowly post-surgery, because traces of the chemicals linger in the reservoirs of excessive adipose tissue.
Common serious postoperative complications in obese patients include blood clots in the legs (deep venous thrombosis) and wound infections at the surgical incision line.
(Reference for this section: Miller’s Anesthesia, 7th Edition, 2009, Chapter 64).
DATA ON THE RISKS OF OBESITY AND SURGERY
In one landmark study, researchers analyzed postoperative complications in 6,773 patients treated between 2001 and 2005 at the University of Michigan. Of the patients who had complications, 33% were obese and 15% were morbidly obese. Obese patients had much higher rates of postoperative complications than nonobese patients, as follows: 5 times more heart attacks, 4 times more peripheral nerve injuries, 1.7 times more wound infections, and 1.5 times more urinary tract infections. The overall death rate was no different for obese and nonobese patients, but the death rate was nearly twice as high among morbidly obese patients as compared with nonobese patients (2.2% vs. 1.2%).
CONCLUSIONS
Experienced anesthesiologists respect the risks and difficulties presented by obese, morbidly obese, and super obese patients. The ranks of overweight Americans are growing, and every week we anesthetize thousands of them for surgery. As an obese American, are you safe in the operating room? You probably are, because anesthesia professionals are well-educated in the risks of taking care of you. But you must realize that you are at higher risk for a complication than those with a normal BMI.
What can you do about all this? If you are morbidly obese and your surgery is optional, you may consider not having surgery at all. If you have time before surgery, you can try to lose weight. Before any surgery, you should consult your primary care physician to make sure that any obesity-related medical problems have been addressed. You may be placed on medication for hypertension, hyperlipidemia, or diabetes. You may have undiagnosed OSA, and may benefit from a nightly CPAP treatment for that disorder.
Bariatric surgery (e.g., gastric banding, gastric bypass) is a well-accepted and effective treatment for weight loss in super obese and morbidly obese patients. Bariatric surgery refers to surgical alteration of the small intestine or stomach with the aim of producing weight loss. More than 175,000 bariatric surgeries were performed in 2006, and more than 200,000 were performed in 2008 (Miller’s Anesthesia, 7th Edition, 2009, Chapter 64). Weight loss after bariatric surgery is often dramatic. On the average, patients lose 60% of their extra weight. For example, a 350-pound person who is 200 pounds overweight could lose about 120 pounds. All the anesthetic considerations and risks discussed above would still apply to any patient coming to the operating room for weight loss surgery.
Obesity was considered a rarity until the middle of the 20th century. Now more than 300,000 deaths per year in the United States and more than $100 billion in annual health care spending are attributable to obesity. Obesity most frequently develops when food calorie intake exceeds energy expenditure over a long period of time.
If you’re obese, this doctor recommends you eat less, and exercise more. Stay lean if you can. Your anesthesiologist will thank you.
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