Key Point

Higher risk of cardiac and venous thromboembolic complications, difficult intubation, and altered pharmacokinetics are among the challenges facing bariatric surgery patients treated in the ICU.

HONOLULU—Between 1996 and 2002 the population-adjusted rate of bariatric surgery in the United States grew more than sevenfold, from 3.5 per 100,000 persons to 24.0 per 100,000 persons. This dramatic increase of surgery in obese patients presents unique challenges in both the postanesthesia care unit and the ICU. Among the speakers who addressed these concerns at the Society of Critical Care Medicine’s 37th Critical Care Congress were Philip S. Barie, MD, MBA, of Weill Cornell Medical College and Louis Brusco Jr, MD, of the Department of Anesthesiology at St. Luke’s-Roosevelt Hospital Center, both in New York City.

OBESITY AS A CRITICAL ILLNESS

“In many ways obesity mimics a critical illness,” said Dr. Barie. Patients with a BMI of more than 40 often have associated disorders that require specialized care after surgery. Morbidly obese patients are hypercoagulable, with increased fibrinogen concentrations, increased concentrations of plasminogen activator 1, and decreased concentrations of antithrombin III and levels of fibrinolytic activity.

Consequently, when these patients undergo surgery or enter the ICU they “are at exceedingly high risk of venous thromboembolic complications. At a minimum they should probably get 5,000 units of subcutaneous unfractionated heparin every eight hours,” Dr. Barie said. He noted that the risk of hypercoagulability is greatest within the 30 minutes immediately following induction of general anesthesia, and thus they should receive heparin before the start of surgery.

Morbidly obese patients experience other complications as well. Many are hyperlipidemic, and many have hyperinsulinemia and insulin resistance. They have decreased lipolysis and preferentially metabolize protein. The accelerated proteolysis associated with morbid obesity results in very rapid deconditioning, particularly in bed-bound patients, such as those in the ICU. The switch from lipolysis to proteolysis increases the respiratory quotient in these patients, resulting in hypercapnia and making it more difficult to wean them from mechanical ventilation.

The goal in providing nutritional support to these patients is to supply enough glucose to spare protein.

“In a patient of normal weight, it probably requires as little as 200 g of glucose a day to provide a protein-sparing effect. It requires more glucose than that to spare protein in a morbidly obese patient, and you probably have to give them the bulk of their nutrition as carbohydrates rather than fats,” Dr. Barie emphasized, adding that it is not necessary to give a lot of lipids to prevent essential fatty-acid deficiency. Recent data suggest that as with the general ICU population, it may be advisable to underfeed these patients—providing them with as little as 20 to 25 kcal/kg per day.

CARDIOVASCULAR AND GASTROINTESTINAL DYSFUNCTION

The physiologic derangements that occur with obesity affect nearly every major organ system. Coronary artery disease develops in obese patients regardless of whether they have diabetes mellitus. Dr. Barie noted that as many as 60% of these patients have hypertension.

“In order to perfuse adipose tissue, obese patients have a large blood volume requirement—3 mL for every 100 g of fat,” he said. The expanded blood volume increases cardiac preload, stroke volume, cardiac output, and ultimately, myocardial workload, as well as cardiac afterload—putting patients at greater risk for ischemia. Catecholamine concentrations are elevated, and the renin-angiotensin-aldosterone system is up-regulated. Increased mineralocorticoid secretion causes salt and water retention.

The increased myocardial workload and increased afterload eventually cause ventricular hypertrophy and decreased ventricular contractility, and these patients often have diastolic dysfunction. As a consequence, although they may be able to regulate their body fluids adequately when they are euvolemic, large-volume fluid resuscitation needed to treat illnesses such as severe sepsis is not well tolerated. As obese patients commonly have poor diastolic function, pulmonary edema can develop rapidly. Although data suggest that β-adrenergic blockade is protective during the perioperative period, particularly for older overweight patients, this must be approached carefully because of the diastolic dysfunction, Dr. Barie cautioned.

Among obese patients, there is an increased prevalence of hiatal hernia, which could possibly increase the risk of aspiration. Research has established that obese patients have higher gastric secretion volumes (and thus may have problems with gastric emptying) with a tendency toward low pH levels, Dr. Barie noted. As a result, if patients do aspirate secretions, they are more likely to develop severe aspiration pneumonitis or pneumonia.

PULMONARY EFFECTS

The increased blood volume can lead to pulmonary hypertension, and the physiology of respiration is altered by the large chest wall mass, which makes it more difficult to maintain lung volumes and increases substantially the work of breathing. Abnormal diaphragm position, increased upper airway resistance, and the increased CO₂ production that results from catabolism and proteolysis all conspire to make breathing harder. Consequently, these patients should be secured in an upright, seated position as much as possible, Dr. Barie noted.

The results of pulmonary function tests (PFTs) are globally deranged in morbidly obese patients because of decreased functional residual capacity, decreased vital capacity, decreased total lung capacity, and decreased expiratory reserve volume. These patients also can develop an obstructive airflow pattern superimposed on their resistive airways disease, resulting in an increased FEV₁/FVC ratio. Morbidly obese patients have a high prevalence of sleep apnea syndrome.

“The incidence has been estimated to be as high as 70% of all obese patients,” Dr. Barie noted. He suggested that obtaining PFTs and arterial blood gases is advisable before going into surgery.

Sleep apnea is characterized by limited periods of severe hypoxia. Patients snore and tend to have both systemic and pulmonary hypertension. They may have nocturnal angina and cardiac dysrhythmias. They have a higher incidence of gastrointestinal reflux disease, polycythemia, insomnia, and daytime somnolence.

“There is an increased incidence of postoperative pulmonary complications,” Dr. Barie said. These patients may be more difficult to intubate, and they are more prone to atelectasis.

Typically, obese patients are mechanically ventilated for longer periods, take longer to wean off the ventilator, and require a higher inspired oxygen concentration than other ICU patients. Managing the ventilation of these patients with their multiple complications can be a challenge. However, to avoid overinflation of the lungs, initial tidal volume should be based on ideal weight rather than actual body weight. Sitting upright can improve lung mechanics and oxygenation. Tracheostomy is controversial and can be hazardous. Depending on the size of the patient, longer tubes may be necessary. Because of possible complications, Dr. Barie recommended that obese patients receive a tracheostomy in the operating room rather than at bedside. “The overall complication rate of tracheostomy may be 40% or more,” he noted.

ALTERED PHARMACOKINETICS

Pharmacokinetics are altered significantly in obese patients. For example, because obese patients with normal renal function have increased glomerular filtration rates, renally excreted drugs are cleared more rapidly than in patients of normal weight. However, many of these patients have both diabetes mellitus and hypertension and thus may have renal dysfunction, so this must be considered as well.

Creatinine clearance should be measured, not calculated, as calculations are not accurate in the morbidly obese population. Volume distribution of lipophilic agents is altered by obesity, and these drugs accumulate and therefore should be dosed according to actual body weight. In contrast, hydrophilic drugs do not accumulate and should be dosed by ideal body weight. Because underdosing is a risk with the hydrophilic agents, where possible, serum concentrations of the drug should be measured.

PREOPERATIVE EVALUATION AND INTRAOPERATIVE MANAGEMENT

Bariatric surgery has a reported cardiac risk of 1% to 5%, which would put it into the intermediate cardiac risk category, noted Dr. Louis Brusco. Under the 2004 American Heart Association/American College of Cardiology guidelines, stress tests were recommended for patients with diabetes and patients with poor exercise tolerance who plan to undergo an intermediate-risk procedure. However, morbidly obese patients have difficulty performing stress tests, and even with negative stress tests, perioperative myocardial infarctions may occur.

“The 2007 guidelines call for less testing. If patients have angina at rest or some unstable cardiac condition, then we get a stress test,” Dr. Brusco remarked. In some patients at higher risk of cardiac complications, loss of 10% of expected body weight loss before surgery will reduce their overall risk.

“For patients with a positive stress test, I will give them a β-blocker and demand that they do some weight loss before they undergo surgery,” he said. Otherwise, for patients with a cardiac history and risk factors, β-blockade alone is sufficient.

Perioperative mortality rates may be higher in patients with obesity hypoventilation syndrome or sleep apnea. Although there are no objective data to support it, some experts feel that treating these patients with continuous positive airway pressure (CPAP) or bi-level positive airway pressure (biPAP) for a period of time preoperatively improves outcomes.

PFTs rarely change management, and if a patient with no pulmonary evaluation presents with oxygen saturation of less than 85%, Dr. Brusco said that he probably would cancel the procedure and perform sleep studies. The patient would be given preoperative biPAP/CPAP, mostly to get them used to wearing the mask. The main concern is that postoperative pain medication may further worsen the sleep apnea and hypoventilation. Despite increasing use of laparoscopic procedures, pain is still an important issue with bariatric surgery, and narcotic medication is a necessity.

“If you do not give them adequate analgesia, they may retain CO₂ and get into this vicious spiral of CO₂ narcosis,” he said.

The ideal agent would cause analgesia without respiratory depression and allow for quick recovery from anesthesia. Dexmedetomidine, a centrally acting α-adrenergic agonist, has been used successfully in bariatric surgery patients. Dr. Brusco noted that it has a number of advantages in this population, including its tendency to induce bradycardia.

“These patients always have a resting tachycardia. This makes the case much smoother from a hemodynamic standpoint,” he said. Dexmedetomidine does not adversely affect patients with asthma or reactive airways. It induces sleep, but patients are arousable, and there is no amnesia associated with it. Although dexmedetomidine cannot be used as monotherapy for sedation or anesthesia, and it is relatively expensive to use, the lack of adverse respiratory effects are a substantial advantage.

“It really helps when you have a difficult patient. I have used this on some of my bigger patients, and they don’t have the respiratory depression,” Dr. Brusco stated.

—Laurel McKee Ranger

Suggested Reading
Pieracci FM, Barie PS, Pomp A. Critical care of the bariatric patient. Crit Care Med. 2006;34(6):1796-1804.

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