Obesity and Anesthesia
WHEC Practice Bulletin and Clinical Management Guidelines for healthcare providers. Educational grant provided by Women's Health and Education Center (WHEC).
The worldwide prevalence of obesity has increased substantially over the past few decades. Economic, technologic, and life style changes have created an abundance of cheap, high-calorie food coupled with decreased required physical activity. Data from Pregnancy Risk Assessment Monitoring System (PRAMS) has shown that the prevalence of pre-pregnancy obesity increased by 69% over a 10-year period, from 13% in 1993-1994 to 22% in 2002-2003 (1). In this report, maternal obesity increased across all categories of age; race; education; smoking status; Special Supplemental Nutrition Program for Women, Infants, and Children enrollment; and parity. The Institute of Medicine and the National Heart, Lung, and Blood Institute of the National Institutes of Health established guidelines for healthy ranges of weight gain in pregnancy. PRAMS data showed that only 1 out of 3 women had weight gain consistent with the recommendations. Obesity is a condition of excessive body fat with adverse health implications, including increased risk for hypertension, coronary artery disease, hyperlipidemia, diabetes mellitus, gall bladder disease, degenerative joint disease, obstructive sleep apnea (OSA), and psychological and socioeconomic impairment. Daughters of obese women may themselves be vulnerable to becoming obese and more likely to have offspring who share this vulnerability.
The purpose of this document is to review pathophysiology of obesity and challenges it poses for obstetrical anesthesia. The incidence of maternal obesity and its attendant comorbid conditions (diabetes, cardiovascular disease) continues to increase at an alarming rate, with major public health implications. Obese patients should be counseled before labor, advising them what intrapartum complications to anticipate. This also includes an anesthesia consult, especially to evaluate the airway.
Definitions of Obesity:
The most commonly used measurement for defining obesity is body mass index (BMI), which refers to an individual's weight in kilograms divided by the square of his or her height in meters. Obesity is defined as abnormally high percentage of body weight as fat, and as a BMI greater than or equal to 30 kg/m2. Extreme Obesity is defined as a BMI greater or equal to 40 kg/m2. Super morbid obesity is when BMI is greater than 55 kg/m2. Overweight means as increased body weight above a standard related to height, when BMI is between 25 and 30 kg/m2. Ideal body weight (IBW) is the weight associated with the lowest mortality rate for a given height and gender and can be estimated using Broca's index: IBW (kg) = height (cm) -- x; where x is 100 for adult males and 105 for adult females. It is important to note, that BMI can be misleading. For example, weight lifters and professional athletes tend to have high BMI because they have a high muscle mass, not excess fat. These individuals are not at risk for metabolic health problems because the health consequences of obesity come from excess adipose tissue, not the size of one's body. Despite its limitation, BMI continues to be used today because it is easily calculated and is the best tool available from a broad-based health policy perspective (2).
Body circumference indices such as waist circumference, waist-to-height ratio, and waist-to-hip ratio can identify pattern of obesity (eg, android obesity) and correlate strongly with mortality and the risk for developing obesity-related diseases. 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. This fat is less metabolically active so it is less loosely associated with cardiovascular disease. Intra-abdominal fat is particularly associated with cardiovascular risk and left ventricular dysfunction. Waist circumference strongly correlates with abdominal fat and is an independent risk predictor of disease. A waist circumference exceeding 102 cm (40 inches) in men and 89 cm (35 inches) in women indicates increased risk in overweight and obese individuals. A waist-to hip ratio >0.9 in women and >1.0 in men is associated with a higher risk of morbidity and mortality than a more peripheral distribution of body fat (<0.75 in women and <0.85 in men). Weight loss reduces risk associated with obesity in men are at a higher risk than women for a given level of obesity. Immediate pre-operative weight loss has not been shown to reduce overall perioperative morbidity and mortality.
Pathophysiology of Obesity:
Respiratory System: Fat accumulation on the thorax and abdomen decreases chest wall and lung compliance. Decreased lung compliance is partially explained by increased pulmonary blood volume because of an overall increase in blood volume as more volume is required to perfuse the additional body fat. Polycythemia from chronic hypoxemia contributes to increased total blood volume. Increased elastic resistance and decreased compliance of the chest wall further reduces total respiratory compliance while supine, leading to shallow and rapid breathing, increased work of breathing, and limited maximum ventilatory capacity. Respiratory muscle efficiency is below normal in obese individuals. Decreased pulmonary compliance leads to decreased functional residual capacity (FRC), vital capacity (VC), and total lung capacity (TLC). Reduction in FRC is primarily a result of reduced expiratory reserve volume (ERV) but the relationship between FRC and closing capacity (CC), the volume at which small airways begin to close, is adversely affected. Residual volume and CC are unchanged. Reduced FRC can result in lung volumes below 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 the obese anesthetized patient compared with 20% in the non-obese (3). Forced expiratory volume in one second (FEV1) and forced vital capacity (FVC) are usually within normal limits.
Cardiovascular System: Total blood volume is increased in the obese, but on a volume-to-weight basis, it is less than in non-obese individuals (50 mL/kg compared with 70 mL/kg). Most of this extra volume is distributed to the fat organ. Renal and splanchnic blood flows are increased. Cardiac output increases with increasing weight by as much as 20 to 30 mL/kg of excess body fat because of ventricular dilatation and increases in stroke volume. The resulting increased left ventricular wall stress leads to hypertrophy, reduced compliance, and impairment of left ventricular filling (diastolic dysfunction) with elevated left ventricular and diastolic pressure and pulmonary edema, but when left ventricular wall thickening fails to keep pace with dilatation, systolic dysfunction (obesity cardiomyopathy) results with eventual biventricular failure (4). 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 blood pressure for every 10 kg of weight gained. Normotensive obese patients have reduced systemic vascular resistance (SVR), which rises with the onset of hypertension. Their expanded blood volume causes an increased cardiac output with a lower calculated SVR for the same level of arterial blood pressure.
Gastrointestinal System: Gastric volume and acidity are increased, hepatic function altered, and drug metabolism adversely affected by obesity. Most fasted morbidly obese patients presenting for elective surgery have gastric volumes in excess of 25 mL and gastric fluid pH less than 2.5 (the generally accepted volume and pH indicative of high risk for pneumonitis should regurgitation and aspiration occur). Peculiar morphologic and biochemical abnormalities of the liver that are associated with obesity include fatty infiltration (high prevalence of non-alcoholic fatty liver disease), inflammation, focal necrosis, and cirrhosis. Fatty infiltration reflects the duration rather the degree of obesity. Abnormal liver function tests are seen in up to one-third of obese patients who have no evidence of concomitant liver disease, of which increased alanine aminotransferase (ALT) is most frequently seen. Weight loss results in sustained improvement in liver enzymes in direct proportion to the extent of weight reduction (5).
Renal and Endocrine System: Impaired glucose tolerance in the morbidly obese is reflected by a high prevalence of type II diabetes mellitus as a result of resistance of peripheral fatty tissues to insulin. Obesity is associated with glomerular hyper-filtration as evidenced by increased renal plasma flow (RPF) and increased glomerular filtration rate (GFR). Excessive weight gain increases renal tubular resorption and impairs natriuresis through activation of the sympathetic and rennin-angiotensin system as well as physical compression of the kidney. With prolonged obesity, there may be a loss of nephron function, with further impairment of natriuresis and further increases in arterial pressure. Obesity-related glomerular hyper-filtration decreases after weight loss, which decreases the incidence of overt glomerulopathy (6).
Airway: Anatomic changes of obesity that affect the airway include limitation of movement of the atlantoaxial joint and cervical spine by upper thoracic and low cervical fat pads; excessive tissue folds in the mouth and pharynx; short thick neck; suprasternal, presternal, and posterior cervical fat; and a very thick submental fat pad. All these factors contribute to potential difficult airway management. Even with the presence of these anatomic and pathologic changes, the magnitude of BMI does not seem to have much influence on the difficulty of laryngoscopy. Such difficulty correlates better with increased age, male sex, temperomandibular joint (TMJ) pathology, Mallampati classes 3 and 4, history of obstructive sleep apnea (OSA), and abnormal upper teeth (7).
Determinants of Dosing for Intravenous Drugs in the Obese Patients:
The volume of the central compartment in which drugs are first distributed remains unchanged in obese patients, but absolute body water content and lean body and adipose tissue mass are increased, affecting lipophilic and polar drug distribution. The volume of distribution (VD) in obese patients is affected by reduced total body water, increased total body fat, increased lean body mass (LBM), altered protein binding, increased blood volume, increased cardiac output, increased blood concentrations of free fatty acids, triglycerides, cholesterol, and alpha-1-acid glycoprotein, lipophilicity of the drug, and organomegaly. Increased distribution of a drug prolongs its elimination half-life even when clearance is unchanged or increased.
Specific Intravenous Agents (8):
|Systemic clearance and VD at steady state correlates well with TBW. High affinity for excess fat and other well-perfused organs. High hepatic extraction and conjugation relates to TBW.|
|Thiopental||TBW||Increased VD. Increased blood volume, cardiac output, and muscle mass. Increased absolute dose. Prolonged duration of action.|
|Midazolam||TBW||Central VD increases in line with body weight. Increased absolute dose. Prolonged sedation because higher loading doses needed to achieve adequate serum concentrations.|
|Succinylcholine||TBW||Plasma cholinesterase activity increases in proportion to body weight. Increased absolute dose.|
|Vecuronium||IBW||Recovery may be delayed if given according to TBW because of increased volume of distribution and impaired hepatic clearance.|
|Rocuronium||IBW||Faster onset and longer duration of action. Pharmacokinetics and pharmacodynamics not altered in obese subjects.|
|TBW||Absolute clearance, VD, and elimination half-life do not change. Unchanged dose per unit body weight without prolongation of recovery because of organ independent elimination.|
|Increased VD and elimination half-time which correlates positively with the degree of obesity. Distributes as extensively in excess body mass as in lean tissues. Dose should account for total body mass. Fentanyl dosing based on a derived pharmacokinetic mass correlates better with clearance.|
|Remifentanil||IBW||Systemic clearance and VD corrected per kg of TBW -- significantly smaller in the obese. Pharmacokinetics are similar in obese and non-obese patients. Age and lean body mass should be considered for dosing.|
Ideal Body Weight (IBW); Total Body Weight (TBW); Volume of Distribution (VD)
Attention should focus on issues peculiar to the obese patient including evaluation of the cardio-respiratory systems and the airways. Previous anesthetic experiences as detailed by the patient and previous anesthetic records are useful sources on information. Obese patients should be evaluated for systemic hypertension, pulmonary hypertension, signs of right and/or left ventricular failure, and ischemic heart disease. Signs of cardiac failure such as elevated jugular venous pressure (JVP), added heart sounds, pulmonary crackles, hepatomegaly, and peripheral edema may all be difficult to detect because of masking by excess adiposity. An EKG may demonstrate signs of right ventricular hypertrophy such as tall precordial R waves, right axis deviation, and right ventricular strain. The higher the pulmonary artery pressure the more sensitive the EKG. Patients with bariatric surgery may have long term metabolic and nutritional abnormalities. Common deficiencies include vitamin B12, iron, calcium, and folate. Vitamin and nutritional deficiencies can lead to a collective form of postoperative polyneuropathy, known as acute postgastric reduction surgery (APGARS) neuropathy, a poly-nutritional multisystem disorder characterized by protracted postoperative vomiting, hyporeflexia, and muscular weakness. APGARS neuropathy should cause anesthesiologists to pay close attention to dosing and monitoring neuromuscular blocking agents. Electrolyte and coagulation indices should be checked before surgery, particularly in poorly compliant or acutely ill patients because chronic vitamin K deficiency can lead to coagulation abnormalities. Administration of vitamin K analog or fresh plasma may be needed.
Counsel obese patients before labor, advising them what intrapartum complications to anticipate. This also includes an anesthesia consult, especially to evaluate the airway and preoperative medications. Consider having additional available blood products, a large operating table, extra personnel, and surgical back-up if necessary. Insufficient data exist to determine the best surgical incision for cesarean in the obese gravid, as this may depend upon the category and type (eg, central) of obesity. If the incision is placed above the panniculus adiposus, which at times means above umbilicus, in a very obese woman, investigators recently assessed the Mobious retractor during cesarean delivery of obese gravidas in a randomized trial, which revealed the retractor was associated with no difference in operating time but improved visualization and surgeon satisfaction (9).
Preoperative and Prophylactic Medications:
Patient's 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 the obese. Anxiolysis and prophylaxis against both aspiration pneumonitis and deep vein thrombosis (DVT) should be addressed at pre-operative assessment. Oral benzodiazepines are reliable for anxiolysis and sedation. Intravenous midazolam can also be titrated in small doses for anxiolysis during the immediate preoperative period. Pharmacologic intervention with H2-receptor antagonists, non-particulate antacids, or proton pump inhibitors will reduce gastric volume, acidity, or both, thereby reducing the risk and severity of aspiration pneumonitis. Morbid obesity is a major independent risk factor for sudden death from acute postoperative pulmonary embolism (PE). Subcutaneous heparin 5,000 IU administered before surgery and repeated every 8 to 12 hours until the patient is fully mobile reduces the risk of DVT. Neck circumference has been identified as the single biggest predictor of problematic intubation in morbidly obese patients. The probability of a problematic intubation is approximately 5% with a 40-cm neck circumference compared with a 35% probability at 60-cm neck circumference. A larger neck circumference is associated with the male sex, a higher Mallampati score, grade 3 views at laryngoscopy, and OSA.
Positioning: 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, but operating tables capable of holding up to 455 kg, with a little extra width to accommodate the extra girth, are available. Electrically operated or motorized tables facilitate maneuvering into various surgically favorable positions. Strapping obese patients to the operating table in combination with a malleable bean bag helps keep them from falling off the operating table. Particular care should be paid to protecting pressure areas because pressure sores and neural injuries are common in this group. Brachial plexus and lower extremity nerve injuries are frequent. Supine positioning causes ventilatory impairment and inferior vena cava and aortic compression in obese patients. FRC and oxygenation are decreased further with supine positioning. Trendelenburg positioning, worsens FRC and should be avoided if possible. Simply changing the obese patient from a sitting to supine position can cause a significant increase in oxygen consumption, cardiac output, and pulmonary artery pressure. The head-up reverse Trendelenburg position provides the longest safe apnea period during induction of anesthesia.
Monitoring: Invasive arterial pressure monitoring may be indicated for the super morbidly obese patient, for those patients with cardiopulmonary disease, and for those patients where the noninvasive blood pressure cuff may not fit properly. Blood pressure measurements can be falsely elevated if a cuff is too small. Cuffs with bladders that encircle a minimum of 75% of the upper arm circumference or, preferably, the entire arm, should be used. Forearm measurements with a standard cuff overestimate both systolic and diastolic blood pressures in obese patients. Central venous and pulmonary artery catheters can be used in patients with significant cardiopulmonary disease or in patients undergoing extensive surgery where significant fluid shifts are anticipated. Central venous catheterization may also be required for intravenous access, which can be problematic in this patient population, although insertion of peripheral lines is almost always successful (10).
Induction, Intubation, and Maintenance: Adequate pre-oxygenation is vital in obese patients because of rapid desaturation after loss of consciousness because of increased oxygen consumption and decreased FRC. Application of positive pressure ventilation during pre-oxygenation decreases atelectasis formation and improves oxygenation in morbidly obese patients. Four vital capacity breaths with 100% oxygen within 30 seconds have been suggested as superior to the usually recommended 3 minutes of 100% pre-oxygenation in obese patients. 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. An increased dose of succinylcholine is necessary because of an increase in activity of pseudocholinestrase. If a difficult intubation is anticipated, awake intubation utilizing topical or regional anesthesia is a prudent approach. During awake intubation, sedative-hypnotic medications should be reduced to a minimum. The head-elevated laryngoscopy position is a step beyond stacking. It significantly elevates the obese patient's head, upper body, and shoulders above the chest to the extent that an imaginary horizontal line connects the sterna notch with the external auditory meatus to better improve laryngoscopy and intubation. Continuous infusion of a short-acting intravenous 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. Short-acting opioids, at the lowest possible dose, combined with a low solubility inhalation anesthetic, facilitate a more rapid emergence without increasing opioid-related side effects. Excess adipose tissue may mask peripheral perfusion, making fluid balance difficult to assess. Blood loss is usually greater in the obese than in the non-obese for the same type of surgery.
A regional technique is a useful alternative to general anesthesia in the morbidly obese patient as it may help avoid potential intubation difficulties. It can, however, be technically difficult because of inability to identify usual bony landmarks. A peripheral nerve stimulator with an insulated needle may be of use. Central neuraxial block is easier in the lumbar region because the midline in this area has a thinner layer of fat than other areas of the spinal column. Longer needles and the sitting position are other useful tools that facilitate induction of central neuraxial anesthesia. Ultrasound and fluoroscopy can be used to guide the needle or continuous infusion catheter into the epidural space. Subarachnoid blocks are not technically as difficult as epidural blocks but the height of a subarachnoid block in obese patients can be unpredictable because it may spread considerably upward within a short time, causing cardiovascular and respiratory embarrassment. A continuous catheter in sub-arachnoid block therefore seems an attractive choice that allows careful titration of the local anesthetic to desired effect and level. Combined epidural and balanced general anesthesia allows for better titration of anesthetic drugs, use of larger oxygen concentration, and optimal muscle relaxation. It also allows for continuation of postoperative analgesia through the same catheter used to provide surgical anesthesia, thereby facilitating early post-operative mobilization.
Obese women who have history of cesarean delivery need to be counseled about the higher risk of failed vaginal birth after cesarean (VABC) and likely need for repeat cesarean delivery. They also need to be told about the elevated risk of infection in the event that VABC fails. Morbidly obese women (BMI >35) have approximately a 50% greater chance of requiring cesarean delivery than do women of normal weight (12). The need for cesarean delivery rise with increasing BMI, probably because of a greater degree of maternal pelvic soft tissue, which increases the risk of both dystocia and cephalopelvic disproportion. It may be advisable to place an early epidural catheter in laboring obese patients because they are more likely to require multiple placement attempts and cesarean delivery.
Prompt extubation reduces the likelihood that the morbidly obese patient, who may have underlying cardiopulmonary disease, will become ventilator dependent. The patient should be preferably extubated in the semi-recumbent position, which has less adverse effect on respiration. There is an increased incidence of atelectasis in morbidly obese patients after general anesthesia, which persists into the postoperative period (11, 12). Consequently, initiation of continuous positive airway pressure (CPAP) or bi-level positive airway pressure (Bi-PAP) has been advocated to combat airway obstruction. Postoperative CPAP does not increase the incidence of major anastomotic leakage after gastric bypass despite a theoretical risk. Pulse oximetry and arterial blood gases should be monitored appropriately. Perioperative use of regional analgesia and anesthesia reduces incidence of postoperative respiratory complications. Epidural analgesia with local anesthetics, opioid is also a viable effective form of analgesia. Intrathecal opioid is also a viable option. The possible need for cardiopulmonary resuscitation should be entertained during anesthesia for the morbidly obese. Of concern are the equipment and technical aspects of resuscitation. 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 the morbidly obese, because their chest wall is usually not much thicker, but the higher transthoracic impedance from the fat may obligate several attempts. Tracheostomy and percutaneous cricothyrotomy are time-consuming and technically difficult options in such emergency situations. Fiberoptic or retrograde wire intubation may be quicker alternatives in the absence of, or inadequate ventilation with, supraglottic devices.
Search of Excellence in Anesthesiology:
We owe it to our patients, our colleagues, and ourselves to strive for excellence in all that we do, safe, high-quality patient care, good working relationships, and our own professional pride and fulfillment are all at stake. Perhaps the main hallmark of anesthetic expertise is the way that different types and streams of knowledge are integrated and reconciled. Anesthesiologist use many different types of knowledge in their conceptualization of the anesthetized patient; for instance, anesthesiologists make use of social, clinical, electronic (by means of monitoring devices) and textual knowledge (through the patient's case-notes). Most importantly, and in contrast to models of knowledge used elsewhere in medicine, it is observed anesthesiologists interpreting medicine knowledge from electronic monitoring and constantly balancing this with other sources. How should research into, and the practical improvement in, anesthesiology go forward? The first thing is that we must recognize excellence as a dynamic and fluid concept. As practice standards are constantly improving, it must change over time. Fortunately, there are many models to inspire and sustain us in developing excellence in practice (13). Future inquiry could usefully focus on a number of areas. Clearly, research into new drugs and techniques must continue because they are vital to the advancement of anesthesia practice. Finally, the impetus to define, achieve, and maintain excellence in anesthesiology can only come from within ourselves.
Many morbidly obese patients have clinically significant obstructive sleep apnea (OSA), which in the long term can result in the obesity hypoventilation syndrome (OHS). OSA predisposes to airway difficulties during anesthesia. Angina or exertional dyspnea may rarely present because morbidly obese patients often have limited mobility and may appear asymptomatic even when they have significant cardiovascular disease. Non-alcoholic fatty liver disease and elevated liver function tests (mostly elevated ALT) are seen in a significant number of obese patients. Despite these histologic and enzymatic changes, no clear correlation exists between abnormalities of routine liver function tests and the capacity of the liver to metabolize drugs. Rhabdomyolysis is sometimes seen in morbidly obese patients undergoing prolonged operative procedures. Unexplained elevations in serum creatinine and creatinine phosphokinase (CPK) levels or complaints of buttock, hip, or shoulder pain in the postoperative period may indicate that rhabdomyolysis has occurred. Patients scheduled for surgery after bariatric surgery may have long-term vitamin and nutritional abnormalities, which can lead to acute postgastric reduction surgery (APGARS) neuropathy, a poly-nutritional multisystem disorder characterized by protracted postoperative vomiting, hyporeflexia, and muscular weakness.
Morbid obesity is a major independent risk factor for deep venous thrombosis (DVT) and sudden death from acute postoperative pulmonary embolism. Subcutaneous heparin reduces the risk of DVT; however, low molecular weight heparins are currently popular for thromboembolism prophylaxis because of their bioavailability when injected subcutaneously. Neck circumference has been identified as the single best predictor of problematic intubation in morbidly obese patients. A larger neck circumference is associated with the male sex, a higher Mallampati score, grade 3 views at laryngoscopy, and obstructive sleep apnea. Forearm blood pressure is a fairly good predictor of upper arm blood pressure in most patients, but in obese patients, forearm measurements with a standard cuff may overestimate both systolic and diastolic blood pressure. The head-elevated laryngoscopy position elevates the obese patient's head, upper body and shoulders above the chest to the extent that an imaginary horizontal line connects the sternal notch with the external auditory meatus to better improve laryngoscopy and intubation.
Women's Health and Education Center (WHEC) thanks Dr. Petra Kriz, Department of Anesthesiology, Mercy Medical Center, Springfield, MA (USA) for the assistance with the Practice Bulletin. Thanks for the useful comments towards improving the review.
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