Obese patients present many significant medical and technical challenges to neuroanesthesiologists. These challenges can be characterized as coexisting medical risks, pharmacological alterations, intraoperative problems and extubation/ventilation problems. Cognizance of these issues by all members of the operative neurosurgical team is essential for optimal outcomes in these patients.
A number of medical problems are associated with obesity. Of particular importance are the hypoxia-hypoventilation syndrome and obstructive sleep apnea. The hypoxia-hypoventilation syndrome is seen in up to 10 percent of morbidly obese patients. When this syndrome is present, blood gas analysis increases in importance because pulse oximetry detects oxyhemoglobin desaturation without indentifying the potential contribution from hypercapnia. Failure to recognize hypercapnia can result in inappropriate treatment with supplemental oxygen that does not reverse, and typically exacerbates, hypoventilation. An elevated bicarbonate level is consistent with chronic hypercarbia. Reductions in functional residual capacity, FRC, and expiratory reserve volume are the most common aberrations in pulmonary function in obese patients. Importantly, low FRC renders the obese patient vulnerable to the effects of even minimal periods of apnea; hence, rapid desaturation is typically encountered after induction of anesthesia, despite adequate preoxygenation. Furthermore, both chest wall and lung compliance are reduced owing to fat accumulation on the thorax and abdomen.
Obstructive sleep apnea, OSA, clearly plays a significant role in contributing to the troubling morbidity and mortality rates encountered in obese patients. OSA is defined as cessation of airflow for more than 10 seconds despite continuing ventilatory effort, five or more times per hour of sleep, and it usually is associated with a decrease in arterial oxygen saturation of greater than 4 percent. The severity of OSA correlates better with large neck circumference (greater than 17 inches) than with the degree of general obesity. Not surprisingly, weight loss impressively reduces the severity of OSA. Centrally acting anesthetic drugs (benzodiazepines, opioids, induction agents and inhalation agents) depress the pharyngeal dilator muscles, causing serious pharyngeal collapse in obese patients with OSA.
Differences in tissue distribution, hemodynamics, and blood flow to adipose, splanchnic, and other tissues as well as plasma composition and hepatorenal function all affect pharmacokinetics in obese patients. The influence of obesity on pharmacokinetic parameters depends on lipid solubility and diffusion through body compartments and tissues. Drug dosing, therefore, should take into consideration the volume of distribution, Vd, for the loading dosage and the clearance when calculating the maintenance dose. Both loading and maintenance dosage for a drug that is mainly distributed to lean tissues should be calculated based on ideal body weight, whereas dose for a drug that is equally distributed between lean and adipose tissues should be calculated based on total body weight. Changes in Vd correlate well with drug lipophilicity. Thus, lipophilic compounds generally are affected by obesity to a greater extent than hydrophilic compounds. Repeated injections accumulate in fat, resulting in a prolonged effect owing to subsequent release from the large fat deposit.
Intraoperatively, positioning can be extremely challenging with conflicts between optimal surgical and medical considerations. In addition, venous access may be difficult to establish in these patients, and noninvasive blood pressure monitoring may be hampered by an improperly fitting cuff or one that takes too long to inflate. Invasive arterial pressure monitoring is prudent for morbidly obese patients with severe cardiopulmonary disease, for patients undergoing extensive surgery, and for those with poor fit of the noninvasive blood pressure cuff. Obese patients desaturate quickly after loss of consciousness owing to reduced FRC and increased oxygen consumption and require adequate preoxygenation. Mask ventilation may be extremely troublesome because of redundant soft tissue in the upper airway, and endotracheal intubation may be challenging.
Problems continue even after surgery. Tracheal extubation should be considered only when there is complete reversal of neuromuscular blockade and the effects of anesthetic agents have abated. Indeed, patients with OSA should be extubated only when fully awake and able to follow commands. Risk of life-threatening airway obstruction or respiratory depression is very real in patients with OSA. Preferably, the patient should be extubated in the semi-sitting position because of more favorable respiratory parameters. Although patients who were on a continuous positive airway pressure machine preoperatively should be placed on the CPAP postoperatively, it is best to not use the device immediately postoperatively lest it interfere with the ability to suction vomit or impair the patient’s ability to communicate. Lastly, should cardiopulmonary resuscitation be required, mechanical compression devices may be necessary to achieve adequate perfusion. Although the maximum 400 joules of energy on most defibrillators usually is adequate for the morbidly obese patient, the higher transthoracic impedance characteristic of these patients may necessitate several attempts at defibrillation.
In conclusion, the roles played by effective communication, monitoring, vigilance and prudent judgment with regard to safe airway management, as well as appropriate drug selection and dosing and contingency planning, cannot be overemphasized.
Medical Problems Associated With Obesity
- Arrhythmia
- Deep Vein Thrombosis / Pulmonary Embolism
- Delayed Gastric Emptying
- Diabetes Mellitus
- Hypertension
- Hypoxia
- Hypoxia-Hypoventilation Syndrome
- Increased Aspiration Risk
- Ischemic Heart Disease
- Left Ventricular Failure
- Liver Disease
- Obstructive Sleep Apnea
- Renal Disease
- Restrictive Ventilatory Dysfunction
Kathryn E. McGoldrick, MD, is professor and chair of anesthesiology, New York Medical College, and director of anesthesiology at Westchester Medical Center, Valhalla, N.Y. Deborah L. Benzil, MD, is associate professor at New York Medical College and a neurosurgeon at Westchester Spine and Brain Surgery PLLC, Hartsdale, N.Y. The authors reported no conflicts for disclosure.