Handbook of Clinical Anesthesia
Anesthesia and Obesity
Obesity, which is now at epidemic proportions worldwide, is a condition of excessive body fat with adverse health implications (Table 47-1) (Ogunnaike BO, Whitten CW: Anesthesia and obesity. In Clinical Anesthesia. Edited by Barash PG, Cullen BF, Stoelting RK, Cahalan MK, Stock MC. Philadelphia: Lippincott Williams & Wilkins, 2009, pp 1230–1246).
- Types of Obesity
In android (central) obesity, adipose tissue is located predominantly in the upper body (truncal distribution) and is associated with increased oxygen consumption and increased incidence of cardiovascular disease. In gynecoid (peripheral) obesity, adipose tissue is located predominantly in the hips, buttocks, and thighs (fat is less metabolically active, so it is less closely associated with cardiovascular disease). Ideal body weight (IBW) is the weight associated with the lowest mortality rate for a given height and gender. Lean body weight (LBW) is the total body weight minus the adipose tissue. In clinical practice, body mass index (body mass index [BMI] = weight in kg/height2 in meters) is used to estimate the degree of obesity (obesity is BMI >30 kg/m2, and morbid obesity is BMI >40 kg/m2). Waist circumference (>102 cm in men and >89 cm in women) strongly correlates with abdominal fat and is an independent risk predictor of disease.
- Pathophysiology of Obesity
- Respiratory System.Fat accumulation on the thorax and abdomen decreases chest wall and lung compliance. Polycythemia from chronic hypoxemia contributes to increased
total blood volume and may lead to pulmonary hypertension and cor pulmonale. Increased elastic resistance and decreased compliance of the chest wall further reduce total respiratory compliance while the individual is in a supine position, leading to shallow and rapid breathing.
Table 47-1 Medical Consequences of Obesity
- Decreased pulmonary compliance leads to decreased functional residual capacity (FRC), vital capacity (VC), and total lung capacity (TLC). Reduced FRC may result in lung volumes below closing capacity (CC) in the course of normal tidal ventilation, leading to small airway closure, ventilation/perfusion mismatch, right-to-left shunting, and arterial hypoxemia. (Anesthesia worsens this situation such that up to a 50%
reduction in FRC occurs in obese anesthetized patients compared with 20% in non-obese patients.)
- Expiratory reserve volume is the most sensitive indicator of the effect of obesity on pulmonary function.
- Obesity increases oxygen consumption and carbon dioxide production because of the metabolic activity of excess fat and the increased workload on supportive tissues. Arterial oxygen tension in morbidly obese patients breathing room air is lower than that predicted for similarly aged non-obese subjects in both sitting and supine positions.
- Obstructive Sleep Apnea (OSA)(Table 47-2). Up to 5% of obese patients have clinically significant OSA that is characterized by periodic, partial, or complete obstruction of the upper airway during sleep. In addition, these individuals may have frequent episodes of apnea or hypopnea during sleep and snoring and daytime symptoms that include sleepiness, impaired concentration, memory problems, and morning headaches.
- Apnea is defined as 10 seconds or more of total cessation of airflow five or more times per hour despite continuous respiratory effort against a closed glottis combined with a decrease in arterial oxygenation of greater than 4%.
- Physiologic abnormalities resulting from OSA include hypoxemia, hypercapnia, secondary polycythemia, and an increased risk of ischemic heart disease and cerebrovascular disease.
- Obesity Hypoventilation (OHS; Pickwickian Syndrome)(see Table 47-2). Long-term OSA may lead to OHS, which is seen in 5% to 10% of morbidly obese patients. These patients also have an increased sensitivity to the respiratory depressant effects of general anesthetics.
- Cardiovascular and Hematologic Systems.Total blood volume is increased in obese individuals, but on a volume-to-weight basis, it is less than in non-obese individuals (50 mL/kg compared with 70 mL/kg). Cardiac output increases with increasing weight, with resulting left ventricular hypertrophy, reduced compliance, and impairment of left ventricular filling (diastolic dysfunction and eventual biventricular failure).
- Obesity accelerates atherosclerosis, but symptoms such as angina and exertional dyspnea occur only occasionally because morbidly obese patients often have very limited mobility and may appear asymptomatic even when they have significant cardiovascular disease.
Table 47-2 Comparison of Obstructive Sleep Apnea and Obesity Hypoventilation Syndrome
- Intraoperative ventricular failure may occur from rapid intravenous (IV) fluid administration. Many obese patients have mild to moderate hypertension with a 3- to 4-mm Hg increase in systolic and a 2-mm Hg increase in diastolic arterial pressure for every 10 kg of weight gained.
- The renin–angiotensin system is important in the hypertension of obesity.
- Obese patients are prone to cardiovascular disease because adipose tissue releases bioactive mediators
(abnormal lipids, insulin resistance, inflammation, coagulopathies).
- Gastrointestinal System.Gastric volume and acidity may be increased, hepatic function altered (fatty liver infiltration, abnormal liver function tests), and drug metabolism adversely affected by obesity. Delayed gastric emptying may occur because of increased abdominal mass that causes antral distention, gastrin release, and a decrease in pH with parietal cell secretion. An increased incidence of hiatal hernia and gastroesophageal reflux may also increase the risk of aspiration.
- Gastric emptying has been said to be actually faster in obese individuals, especially with high energy content intake such as fat emulsions, but because of the larger gastric volume (≤75% larger) in obese patients, the residual volume is larger.
- Nonpremedicated, nondiabetic, fasting, obese surgical patients who are free from significant gastroesophageal pathology are unlikely to have high-volume, low pH gastric contents after routine preoperative fasting, so they should follow the same fasting guidelines as non-obese patients and be allowed to drink clear liquids until up to 2 hours before elective surgery.
- A positive correlation exists between obesity and frequent symptoms of gastroesophageal reflux disease (GERD).
- Morbidly obese patients who have undergone intestinal bypass surgery have a particularly high prevalence of hepatic dysfunction and cholelithiasis.
- The high prevalence of non-alcoholic fatty liver disease and cirrhosis necessitates careful assessment for pre-existing inpatients with liver disease who are scheduled for surgery.
- Renal and Endocrine Systems.Impaired glucose tolerance in morbidly obese individuals is reflected by a high prevalence of type II diabetes mellitus caused by resistance of peripheral fatty tissues to insulin. Exogenous insulin may be required perioperatively even in obese patients with type II diabetes mellitus to oppose the catabolic response to surgery.
- Subclinical hypothyroidism occurs in about 25% of all morbidly obese patients.
- With prolonged obesity, there may be a loss of nephron function, with further impairment of natriuresis and further increases in arterial pressure.
- Metabolic syndromeis a cluster of metabolic abnormalities (abdominal obesity, glucose intolerance, hypertension, dyslipidemia) that is associated with an increased risk of vascular events.
The volume of the central compartment in which drugs are first distributed remains unchanged in obese patients, but absolute body water content is decreased, and lean body and adipose tissue mass are increased, affecting lipophilic and polar drug distribution. Plasma albumin concentrations and plasma protein binding are not significantly changed by obesity. There is no significant difference in absorption and bioavailability of orally administered medication when comparing obese and normal weight subjects.
- Histologic abnormalities of the liver are common in obese people, with concomitant deranged liver function test results, but drug clearance is not usually affected.
- Renal clearance of drugs is increased in obesity because of increased renal blood flow and glomerular filtration rate.
- Highly lipophilic substances, such as barbiturates and benzodiazepines, show significant increases in volume of distribution (VD) for obese individuals. Less lipophilic compounds have little or no change in VD with obesity. Drugs with weak or moderate lipophilicity can be dosed on the basis of IBW or lean body mass (LBM).
- Adding 20% to the estimated IBW dose of hydrophilic medications (nondepolarizing muscle relaxants) is sufficient to include the extra lean mass.
- Specific Intravenous Agents
- Medical Therapy for Obesity
Medications used to treat obesity are formulated to reduce energy intake, increase energy utilization, or decrease absorption of nutrients.
- Sibutramineinhibits the reuptake of norepinephrine to increase satiety after the onset of eating rather than reduce appetite. Unlike fenfluramine and dexfenfluramine, sibutramine does not promote the release of serotonin, which may explain the absence of reports of sibutramine causing cardiac valvular lesions. Sibutramine also results
in transient dose-related increases in systolic and diastolic blood pressure (2–4 mm Hg) and a slight increase in heart rate (3–5 bpm).
Table 47-3 Determinants of Dosing for Intravenous Drugs in Obese Patients
- Orlistatblocks the absorption and digestion of dietary fat by binding lipases in the gastrointestinal tract. Chronic dosing of orlistat results in an increase in coumadin's anticoagulant effect because of decreased absorption of vitamin K. This leads to an abnormal prothrombin time (PT) with a normal partial thromboplastin time (PTT) because of deficiency of clotting factors II, VII, IX, and X. The resulting coagulopathy should be corrected 6 to 24 hours before elective surgery with a vitamin K analogue such as phytonadione and fresh-frozen plasma for emergency surgery or active bleeding.
- Rimonabantis a cannabinoid receptor antagonist that decreases appetite, leading to weight loss and a decreased incidence of the metabolic syndrome.
VI Bariatric Surgery
Bariatric surgery encompasses a variety of surgical procedures (classified as malabsorptive, restrictive, or combined) used to treat morbid obesity.
- Gastric restriction (gastroplasty) creates a small upper pouch (15–30 mL) in the stomach and is the most commonly performed bariatric procedure in the United States. Adjustable gastric banding is a restrictive gastric operation usually done by minimally invasive laparoscopic approach.
- Laparoscopic bariatric surgery is minimally invasive and is associated with less postoperative pain, lower morbidity, and faster recovery.
- Profound muscle relaxation is important during laparoscopic bariatric procedures to facilitate ventilation and maintain an adequate working space for visualization, safe manipulation of laparoscopic instruments, and extraction of excised tissues.
- Laparoscopic bariatric surgery requires maneuvering the operating table into various surgically favorable positions (a malleable “bean bag,” in addition to belts and straps, may help keep the patient secured). Anesthesia personnel may be asked to facilitate the proper placement of an intragastric balloon to help the surgeon size the gastric pouch and facilitate performance of leak tests with saline or methylene blue through a nasogastric (NG) tube. After the gastric pouch has been created, insertion of an NG tube should be aided by viewing the laparoscope monitor and carefully watching to avoid disruption of the anastomosis. Cephalad displacement of the diaphragm and carina from a pneumoperitoneum during laparoscopy may cause a firmly secured endotracheal tube to displace into a mainstem bronchus.
- Rhabdomyolysisis more common in morbidly obese patients undergoing laparoscopic procedures than in those undergoing an open procedure (Table 47-4).
- Gastric electrical stimulationby means of an implantable gastric stimulator (IGS) causes smooth muscles of the stomach to stop peristalsis so that the patient feels full. Possible lead dislodgment from violent stomach contractions during postoperative nausea and vomiting is a consideration for the anesthesiologist. The
stimulating pulses emitted by the IGS can be picked up on the electrocardiograph, which may lead to false readings.
Table 47-4 Management of Rhabdomyolysis
VII. Preoperative Considerations
- Preoperative Evaluation(Table 47-5)
- Concurrent, Preoperative, and Prophylactic Medications.Patients' usual medications should be continued until the time of surgery with the possible exception of insulin and oral hypoglycemics. Antibiotic prophylaxis is important because of an increased incidence of wound infections in obese patients. Anxiolysis and prophylaxis against both aspiration pneumonitis and deep vein thrombosis (DVT) should be addressed at premedication.
- Morbid obesity is a major independent risk factor for sudden death from acute postoperative pulmonary embolism. Subcutaneous heparin 5000 IU administered before surgery and repeated every 8 to 12 hours until the patient is fully mobile reduces the risk of DVT. Other risk factors include venous stasis disease, BMI of 60 kg/m2or above, truncal obesity, and obesity hypoventilation syndrome (OHS) or sleep apnea syndrome (SAS), suggesting that enoxaparin dosing in obese patients should be varied with age and LBM.
Table 47-5 Preoperative Considerations in Obese Patients
- A combination of short duration of surgery, lower extremity pneumatic compression, and routine early ambulation may preclude mandatory heparin anticoagulation except in patients with a history of DVT or a known hypercoagulable state.
VIII. Airway (Table 47-6)
- Neck circumference has been identified as the single biggest predictor of problematic intubation in morbidly obese patients. Intubation is problematic in
approximately 5% of patients with a 40-cm neck circumference compared with a 35% probability of those with a 60-cm neck circumference.
Table 47-6 Anatomic Changes Associated with Obesity that Contribute to a Potentially Difficult Airway
- The magnitude of BMI does not seem to have much influence on the difficulty of laryngoscopy.
- Ambulatory Anesthesia
- There is no evidence to suggest increased morbidity in morbidly obese patients with stable concomitant diseases. Patients should not be excluded from day-case surgery based solely on absolute weight or BMI.
- Airway surgery (uvulopalatopharyngoplasty, tonsillectomy) should not be performed on an outpatient basis.
- Use of regional blocks (which are technically more difficult), rapidly dissipating anesthetic agents, and appropriately sized equipment and procedures for positioning and monitoring should be combined with the availability of prolonged observation and overnight admission for optimum safety.
- Intraoperative Considerations
- Specially designed tables or two regular operating tables may be required for safe anesthesia and surgery in obese patients. Regular operating tables have a maximum weight limit of approximately 205 kg.
- Brachial plexus injury, ulnar neuropathy, and lower extremity nerve injuries are frequent. Carpal tunnel syndrome is the most common mononeuropathy after bariatric surgery.
- Supine positioning causes ventilatory impairment and inferior vena cava and aortic compression in obese patients. FRC and oxygenation are further decreased with supine positioning.
- Trendelenburg positioning (which should be avoided, if possible), as may be required during bariatric procedures, further worsens FRC.
- The head-up reverse Trendelenburg position provides the longest safe apnea period during induction of anesthesia.
- Lateral decubitus positioning allows for better diaphragmatic excursion and should be favored over prone positioning whenever the surgical procedure permits.
- Monitoring.Invasive arterial pressure monitoring may be indicated for morbidly obese patients, patients with cardiopulmonary disease, and patients in whom the noninvasive blood pressure cuff may not fit properly. Blood pressure measurements can be falsely elevated if a cuff is too small.
- Induction, Intubation, and Maintenance
- Adequate preoxygenation is vital in obese patients because of rapid desaturation after loss of consciousness caused by increased oxygen consumption and decreased FRC.
- Larger doses of induction agents may be required by obese patients because blood volume, muscle mass, and cardiac output increase linearly with the degree of obesity.
- If a difficult intubation is anticipated, awake intubation using topical or regional anesthesia is a prudent approach. Sedation with dexmedetomidine during awake intubation provides adequate anxiolysis and analgesia without respiratory depression.
- Towels or folded blankets under the shoulders and head can compensate for the exaggerated flexed position of posterior cervical fat (head-elevated laryngoscopy position [HELP]).
- Continuous infusion of a short-acting IV agent, such as propofol, or any of the inhalational agents or a combination may be used to maintain anesthesia.
- Rapid elimination and analgesic properties make nitrous oxide an attractive choice for anesthesia in obese patients, but high oxygen demand in this patient population limits its use.
- Nitrous oxide does not cause noticeable bowel distention during short-duration laparoscopic bariatric procedures.
- Dexmedetomidine has no clinically significant adverse effects on respiration and is useful for sedation and analgesia in obese patients.
- Positive end-expiratory pressure (PEEP) is the only ventilatory parameter that has consistently been shown to improve respiratory function in obese subjects. PEEP may, however, decrease venous return and cardiac output.
- Regional anesthesiais a useful alternative to general anesthesia in morbidly obese patients because it may help avoid potential intubation difficulties. It can, however, be technically difficult because of an inability to identify the usual bony landmarks.
- Epidural vascular engorgement and fatty infiltration reduce the volume of the space, making dose requirements of local anesthetics for epidural anesthesia 20% to 25% lower in obese patients.
- Subarachnoid blocks are not as technically difficult as epidural blocks, but the height of a subarachnoid block in obese patients can be unpredictable.
- Postoperative Considerations
- Prompt extubation reduces the likelihood that a morbidly obese patient who may have underlying cardiopulmonary disease will become ventilator dependent.
- Supplemental oxygen should be administrated after extubation. There is an increased incidence of atelectasis in morbidly obese patients after general anesthesia that persists into the postoperative period.
- Postoperative continuous positive airway pressure may improve oxygenation but does not facilitate carbon dioxide elimination.
- Postoperative Analgesia.Perioperative use of regional anesthesia and analgesia reduces the incidence of postoperative respiratory complications. Potential advantages of epidural analgesia in obese patients include prevention of DVT, improved analgesia, and earlier recovery of intestinal motility. Delayed respiratory depression is one of the known complications of central neuraxial opioids.
- The possible need for cardiopulmonary resuscitation should be considered during anesthesia for morbidly obese patients.
- Chest compressions may not be effective, and mechanical compression devices may be required. The maximum 400 joules of energy on regular defibrillators is sufficient for morbidly obese patients because their chest walls are usually not much thicker, but the higher transthoracic impedance from the fat may obligate several attempts.
- The laryngeal mask airway and the esophageal tracheal Combitube are temporary supraglottic airway devices that are useful during resuscitation of obese patients. Tracheostomy and percutaneous cricothy-rotomy are time-consuming and technically difficult procedures in emergency situations and should be reserved as final options to be performed by experienced practitioners.
Editors: Barash, Paul G.; Cullen, Bruce F.; Stoelting, Robert K.; Cahalan, Michael K.; Stock, M. Christine
Title: Handbook of Clinical Anesthesia, 6th Edition
Copyright ©2009 Lippincott Williams & Wilkins
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