Abstract
Obesity seems to be the modern concern to society. An increasing number of obese patients present annually to surgical wards to undergo surgical procedures. As morbid obesity affects most of the vital organs, the anesthesiologist must be prepared to deal with several challenges. These include the preoperative evaluation of the consequences of obesity, particularly on cardiac, respiratory, and metabolic systems; airway management; different pharmacokinetic and pharmacodynamic drug regimen; and perioperative management (i.e., hemodynamic, respiratory, and hyperglycemic). This paper reviews and assesses the most important anesthetic issues in managing obese patients.
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Introduction and epidemiology
Obesity is now considered the modern epidemic and is associated with serious problems faced by public health and clinical physicians. The incidence is increasing, and several nations report their statistics: In 2003–2004, 17.1 % of US children and adolescents were overweight and 32.2 % of adults were obese. According to the National Health and Nutrition Examination Survey data for 2007–2008, 68 % of American adults were obese, predominantly men [1]. The incidence reported in the UK and Australia is approximately 25 %, whereas in Japan, China, and in The Netherlands, 1:20 women and 1:10 men are classified as obese [2–5]. The World Health Organization (WHO) estimates that by 2015, 2.3 billion people worldwide will be overweight and 700 million will be obese.
Definition of obesity and overall preoperative morbidity risk
The definition of obesity and its clinical significance is well determined and accepted around the world. Obesity is defined as a body mass index (BMI) >30 kg/m2. Table 1 summarized the values of BMI and nutritional status and obesity.
Pathophysiological changes range from airway management difficulties and changes in disturbances in respiratory physiology to many diseases as diabetes, hypertension, and heart disease. Morbidity rate increases substantially with increase in body weight, with subsequent increase in risks of hypertension, diabetes mellitus, coronary artery disease, dyslipidemia, gall bladder disease, osteoarthritis, respiratory problems, and cancers of various organs [6]. Related risks are surgical and medical. There are differences in the reported risk of surgery in the obese. Dindo et al. [7] reported that obesity did not pose a particular risk of morbidity or mortality for general elective surgery, but the mortality risk of gastric bypass surgery is high. Courcoulas et al. [8] reported a mortality rate 0.6 % and was as high as 5 % for surgeons doing less than ten procedures per year and 0.3 % if the surgeon did ten or more. Therefore, the more surgeries performed by an individual surgeon, the lower the morbidity risk. Morbidity and mortality risk is proportional to obesity grade, duration, and form [9, 10]. Depending on fat distribution, there are two forms of obesity: an android form and a gynecoid form. The android form occurs when fat distributes centrally (liver, omentum, intraperitoneal), whereas the gynecoid form involves fat distribution in buttocks, arms, and legs. The android form is associated with a higher morbidity and mortality risk. Risk factors are summarized in Table 2. An interesting study [11] concluded that increasing BMI is associated with increased hospital mortality, intensive care unit (ICU) admissions, mechanical ventilation (MV), hospital stay, and costs. Table 3 presents results of the aforementioned studies.
Obesity-induced pathophysiological changes and anesthetic implications
Perioperative management of obese patients undergoing surgery for bariatric or other surgeries is difficult. Working as a team (anesthesiologist, surgeon, endocrinologist, nutritional physician, nurse) can guarantee a higher success rate and fewer complications.
Metabolism and fat distribution effects on anesthesia conduction
Obesity and fat distribution have several effects on metabolism and systemic organs. The fact that obesity increases the risk for cardiovascular, respiratory, and liver disease and has several metabolic consequences is well known. Obesity is often associated with diabetes, dyslipidemia, increased basal metabolic rate, greater oxygen consumption, and carbon dioxide production making the obese patient prone of rapid desaturation [12, 13].
Obesity-induced cardiac diseases and anesthesia risks
Obesity is associated with several cardiovascular effects. Obese patients tend to have increased blood volumes and increased risk of hypertension and ischemic heart disease. There is a strong correlation between BMI and blood pressure (BP) elevation, which in turn can be explained by different mechanisms (hormonal, renal, and hemodynamic). Approximately 60 % of obese patients may have mild to moderate systemic hypertension [14]. The reasons for hypertension include hypervolemia, increased cardiac output, increased peripheral vascular resistance, and hyperinsulinemia (causing sympathetic activation and sodium retention) [15, 16].
Progressive increases in BMI values may impair cardiac contractility, therefore decreasing the stroke volume and ejection fraction. This impaired cardiac contractility is usually due to hypertrophy and/or dilatation of the left ventricle [17]. These changes can be detected on electrocardiography (ECG) examination, as leftward shift of QRS axis, prolonged PR, QRS, and QTc [18]. Hypertension, increased heart rate, ventricular hypertrophy, diabetes, and—frequently—hypercholesterolemia can explain the ischemic heart disease often encountered in obese individuals. Obesity is now considered an independent risk factor for coronary artery disease. The incidence of right and left ventricle failure [19, 20] tends to be proportional with BMI. Heart failure happens when the stroke volume, cardiac output, and BP decrease due to impaired ventricular function. Obesity increases blood volume, stroke volume, and cardiac output. As a result, left ventricle enlargement may occur, increasing wall stress. The increased wall stress induces hypertrophy and then dilatation of left ventricle, impairing its diastolic and systolic functions. Due to obstructive sleep apnea (OSA), several changes can occur in the right ventricle. As a consequence of hypoxia and hypercapnia, pulmonary hypertension is often present, inducing right ventricle hypertrophy and then dilatation. These pathophysiological changes can explain the right ventricle syndrome and right ventricle failure. It is well known that ischemic heart disease can also impair both ventricular functions.
Airway management and respiratory changes during anesthesia in the obese
Several respiratory changes are present in obese patients [21–23]. Obesity reduces both pulmonary and total chest compliances. Decreased pulmonary compliance reduces functional residual capacity, which cannot overpass the closing capacity. As a consequence, obese patients are prone to increased intrapulmonary shunt V/Q mismatching. Thus, the obese patient may be hypoxic, with increased alveolar–arterial oxygen partial pressure PAaO2, increasing the risk for postoperative atelectasis. General anesthesia and postoperative pain can precipitate these changes.
OSA is usually an associated disorder of obesity [24, 25] that results from the increased adipose tissue in the pharyngeal walls, increasing pharyngeal wall compliance. Studies [26, 27] suggest that between 40 % and 90 % of obese patients have obstructive sleep apnea. OSA is defined as a minimum of 10 s of total respiratory cessation that occurs more than 30 times a night. This explains the tendency of the pharyngeal wall to collapse during negative pressure on inspiration. These patients often complain of sleepiness, depression, and morning headache (hypercapnia) and often experience pulmonary hypertension, right ventricular failure, stroke, and hypertension. Nocturnal polysomnography (sleep study) can confirm the diagnosis. Decreased leptin [28] blood level (a frequent finding in obese individuals) makes the obese patient insensitive to carbon dioxide retention, producing the so called “obesity hypoventilation syndrome.” This syndrome can deteriorate due to general anesthetics and sedative drugs and predisposes the patient to resting hypoxemia.
Airway management may be problematic. Small oral cavity, small mouth opening, large amount of adipose tissue, impaired joint movements (diabetes, adipose tissue), thick neck, impaired neck and head movements, and possible short sternomental or thyromental distances, are suggestive for difficult laryngoscopy and tracheal intubation. However, obesity does not correlate with difficult intubation but with difficult mask ventilation and postoperative respiratory failure. Several studies [29–31] report a weak relationship between obesity and difficult intubation. Lundstron et al. [32] analyzed 91,332 consecutive patients undergoing general surgery and tracheal intubation. They reported that BMI is a weak predictor for difficult intubation. Another study (The Australian Incident Monitoring Group) analyzed 2,000 patients and confirmed that BMI alone is not predictive of difficult intubation [33].
Obesity-induced gastrointestinal changes and diseases
Obesity can often be associated with hypercholesterolemia. Dyslipidemia can induce reversible fatty liver but occasionally progresses to steatohepatitis and cirrhosis [34]. The obese patient is also at risk of developing biliary tract disease, reflecting cholesterol metabolism disorder. A great incidence of gastroesophageal reflux disease (GERD) and hiatus hernia is reported. Increased intra-abdominal pressure may aggravate the reflux, increasing the risk of aspiration of gastric content. The mechanisms include an increased prevalence of esophageal motor disorders, diminished lower esophageal sphincter pressure, and increased intragastric pressure [35]. Diabetes is also an important disease associated with obesity as a result of insulin resistance in peripheral adipose tissue.
Perioperative thromboembolic events and stroke risks
Polycythemia, deep venous stasis, and increased intra-abdominal pressure double the risk for deep venous thrombosis (DVT) in obese individuals. Obesity has also been associated with an increased risk of pulmonary embolism, especially in women [36–38]. Each 1-U increase in BMI was associated with a multiple-adjusted increase of 4 % in the risk of ischemic stroke and 6 % for hemorrhagic stroke. Ischemic stroke severity seemed not to be associated with BMI [39].
Practical anesthesia conduction
General preoperative evaluation
Information on previous surgeries is useful to prepare the anesthetic plan and to foresee possible complications. It should focus on previous surgery and anesthetic problems, such as difficulties in airway management or intravenous access, ICU admission, possible MV, drug hypersensibility, etc. Preoperative evaluation should include consideration of cardiovascular and respiratory system (hypertension, heart failure, arrhythmias, obesity-related hypoventilation syndrome), and metabolic disorders (diabetes).
Cardiovascular evaluation [40] should include medical history (clinical signs suggestive for cardiac pathologies such as angina, dyspnea, headache, palpitations, activity capacity), clinical examination (cardiac tones, jugular vein, peripheral edema), and instrumental examination [electrocardiography (EEG), ECG, and coronary angiography if indicated). Respiratory system evaluation [41, 42] includes clinical (e.g., dyspnea) and imagining assessment. Chest X-ray examines heart size and pulmonary vasculature (for evidence of pulmonary hypertension). Blood gas analyses are useful to determine respiratory function and plan airway management. Functional respiratory test is useful to predict possible postoperative respiratory problems and determine the respiratory regimen during MV. Sleep study [41, 42] (nocturnal oximetry and polysomnography) is important in this context. Apnea Hypopnea Index (AHI) score >30 is a sign for rapid desaturation at induction. Recommended laboratory evaluations include blood glucose, lipid profile, serum chemistries (to evaluate renal and hepatic function and ferritin), and complete blood count. Liver-test abnormalities are common findings, being a determining factor for perioperative risk. Cirrhotic liver disease with portal hypertension is often considered a relative contraindication to bariatric surgery [41]. Dyspepsia indicates the presence of Helicobacter pylori and heart burn is significant to GERD, which requires preoperative medical treatment.
Monitoring and vascular access difficulties
BP [43] must be monitored using correct cuff size (40–45 cm) in order to give accurate values. Peripheral intravenous access may be difficult because of adipose tissue. Central venous catheter insertion is generally difficult due to hidden anatomical landmarks, but the procedure can be facilitated using ultrasound examination [44, 45].
Premedication and gastric aspiration risk prevention
Because of respiratory changes, it is advisable to avoid opioids, propofol, benzodiazepine, and oversedation for premedication purposes [46]. Sedation must be appropriate to the patient, the surgery, and the individual clinical protocol. The intramuscular route of administration must be avoided because of unpredictable absorption and effect. Gastric aspiration prevention is an important issue. Histamine (H2) receptor blockers, metoclopramide, and sodium citrate are appropriate to reduce the risk of aspiration during anesthesia induction and endotracheal intubation.
Strategies of airway management
Obese patients must be examined for predictive signs of potential difficult intubation [47, 48], which may include Mallampati classification, thyromental and sternomental distances, small mouth opening, large protuberant teeth, limited neck mobility, retrognathia, and Wilson sum score. Based on several studies [49] on airway management of obese patients, the ramped position can guarantee easy intubation using conventional direct laryngoscopy. The equipment for emergency airway management, including laryngeal masks and a fiberoptic bronchoscope, should be immediately available. An awaked intubation sequence can be performed when a difficult intubation is predicted; the fiberoptic bronchoscope is a suitable choice for this purpose.
Effects of patient positioning
The supine position makes respiration more difficult by compressing the diaphragm, so the patient may need to have his/her back elevated. Abdominal pressure and weight can compress the inferior vena cava, which reduces venous return. The aorta increases the afterload, increasing blood pressure and decreasing stroke volume (when exaggerated). These changes are more significant when the Trendelenburg position is applied [50, 51]. The prone position is also not well tolerated and may have potential complications (skin breakdown, reduced respiration and circulation, nerve damage, eye or ear damage, damage to the breasts in women or genitals in men) [50, 51]. The potential hazards in the lithotomy position include skin breakdown, peroneal nerve damage, musculoskeletal injury (improper raising and lowering of the legs), and circulatory compromise (hypotension by lowering the legs). The lateral position is usually well tolerated by obese patients.
Anesthetic pharmacokinetics and dosage in obese patients
Several factors can affect the pharmacokinetic in obese patients. These include changes in the volume of distribution (decreased body water; increased fat, lean body mass, cardiac output, and total blood volume), increased free-drug availability, and changes in clearance (increased renal blood flow and glomerular filtration rate; decreased hepatic blood flow). Drug dosage can be calculated on total body weight, BMI, lean body mass (ideal body mass +20 %), or ideal body weight. It is well known that pharmacokinetics of most anesthetics are usually determined by adipose tissue, producing a prolonged effect. The dosage of a lipophilic drug is connected to volume of distribution. The increased volume of distribution may prolong elimination—benzodiazepines and barbiturates—and dosages of these drugs must be calculated on ideal body weight or lean body mass if an infusion is to be used and according to total body mass when a single endovenous dose is needed [52]. For some muscle relaxants (vecuronium, rocuronium, cis-atracurium), ideal body mass should be used. Atracurium and succinylcholine are both calculated according to total body weight. Propofol has fast onset and short duration, making it the induction anesthetic agent of choice. Propofol, as a highly lipophilic agent, seems to have the volume of distribution and clearance proportional to total weight, so its dosage for continuous infusion should be calculated based on total body weight. Other studies [53, 54] conclude that for anesthesia induction, lean body mass is a more appropriate weight-based scalar for propofol. Fentanyl and sufentanil should be calculated on total body weight, whereas remifentanil [55, 56] should be based on ideal body weight. Dosages of almost all anesthetic drugs must be reduced. This is particularly important for obese patients with sleep apnea, who may be very sensitive to general anesthetics [57]. Table 4 summarizes the recommended weight-based dosing scalar for commonly used anesthetics.
Respiratory support during intubation, anesthesia, and postoperative period
Respiratory changes in obese patients discussed in the previous sections suggest that obese patients are prone to faster desaturation, increased rate of atelectasis, and need for ventilator support. There are many options to reduce respiratory complications. These include preoxygenation, continuous positive airway pressure (CPAP) application, and positive end-expiratory pressure (PEEP) during the intraoperative period. Preoxygenation prevents desaturation, which is much faster in obese than in normal-weight patients. The preoxygenation effect is augmented by 25° head-up position or by applying CPAP [58–60].
Tidal volumes based on actual body weight often cause alveolar overdistention and volutrauma. Using low tidal volumes (6 ml/kg) based on ideal body weight can decrease mortality risk in patients with acute respiratory distress syndrome, but several studies [61, 62] concluded that even the highest BMI group can benefit from low tidal volumes. The use of PEEP is also beneficial while ventilating morbidly obese patients, resulting in significant improvement in lung volumes, compliance, oxygenation, ventilation, and lower intra-abdominal pressure compared with no obese controls [63]. The addition of PEEP [64] to the head-up body position in 20 morbidly obese patients undergoing bariatric surgery decreased the alveolar–arterial oxygenation difference and increased respiratory system compliance. The obese patient needs careful ventilation management because of decreased thoracic compliance and secondary, possibly restrictive, respiratory dysfunction. These pathophysiological changes can contribute to increased airway pressure during intraoperative ventilation. The increased airway pressure may make the patient prone to ventilator-induced injury. The obese patient may be complicated by hypoventilation and atelectasis if not adequately ventilated but also by ventilator-induced injury if ventilator parameters are not properly chosen. The anesthesiologist must fix the ventilator parameters in order to adequately ventilate the patient and prevent injury, such as volutrauma, barotrauma. Adequate ventilation can prevent postoperative respiratory complications and is attained by a logical combination of tidal volume, respiratory rate, and PEEP. It is well known that airway pressure can be increased using an increased tidal volume, increased respiratory rate, and PEEP. In our institution, two rational ventilation options may be available, both guided by end-tidal carbon dioxide concentration in expired air (ETCO2) and blood gases and by maintaining airway pressure at <30–35 cmH2O. The first option includes standard tidal volume (8–10 ml/kg, ideal body weight), respiratory rate 10–12 min, and moderate PEEP (5–8 cmH2O). The second option is based on low tidal volume (6–8 ml/kg), respiratory rate 12–18 min, and PEEP (8–10 cmH2O). Pelosi et al. [65] recommended tidal volume of 6–10 ml/kg based on ideal body weight, increased respiratory rate to maintain physiological partial pressure of carbon dioxide in arterial blood (PaCO2), and a recruitment maneuver (35–55 cmH2O for 6 s), followed by PEEP application of 10 cmH2O. The anesthesiologist must choose case by case how to ventilate the patient, taking care to minimize intraoperative (ventilator-induced injury by “aggressive ventilation”) and postoperative (atelectasis by hypoventilation) respiratory complications. Neuromuscular blockade must be fully reversed before extubation. Muscle relaxant monitoring by nerve stimulation or using clinically aspects (muscular strength, following commands, lifting the head for 5 s) can give reliable information about safety during extubation. After extubation, CPAP or a pressure support mask should be available and, if necessary, delivered to the patient. These maneuvers may ameliorate oxygenation, increase functional residual capacity, and improve respiratory performance.
Fluid therapy
During surgery, the opening of the cavities makes evaporation possible. This explains temperature and fluid loss. Major surgery is also associated with extravasations and third-space loss. Obese patients undergoing bariatric or nonbariatric surgery may have protracted volume in the preoperative period as a result of preoperative fasting, forced urine output induced by diabetes, and antihypertensive drugs (e.g., diuretics). These problems are the reasons for postoperative renal failure. The predisposing factors are weight loss, BMI >50 kg/m2, prolonged surgery, pre-existent renal disease, and intraoperative hypotension [66]. Fluid requirements may be greater than predicted, and in even a relatively short (2- to 3-h case), 4–5 l of crystalloid fluid may be needed to prevent acute tubular necrosis in the kidneys.
Role of regional anesthesia in obese individuals
Regional anesthesia offers many advantages to obese individuals undergoing to surgery. These include no airway manipulation, no general anesthetic drugs, no cardiorespiratory depression, and effective control of postoperative pain. Several studies [66–68], however, found that obesity makes regional anesthesia difficult. Hidden anatomic landmarks, difficulty in palpating bony landmarks or indentifying the midline, and the relatively short needle are possible causes [69]. Success rate can be improved by using ultrasound [70, 71]. Another feature of spinal anesthesia in obese patients is a tendency for cephalic spread of local anesthetic, more so than in a nonobese patient [68]. This phenomenon can be explained by smaller cerebrospinal fluid volume [69], allowing the anesthesiologist to reduce the local anesthetic dose. The majority of studies are performed in pregnant women, and data are controversial in the nonpregnant obese patient. So the recommendation to use a low dose remains controversial [72], carrying the risk of block failure and the need for general anesthesia during the surgical procedure [73].
Postsurgery complications
Obese patients are prone to respiratory complications after extubation. These complications vary from reintubation and MV to hypoxemia and atelectasis. Therefore, the patient’s trachea must be extubated when fully recovery of neuromuscular activity is present. Respiratory physiotherapy is an important step in preventing hypoxemia, atelectasis, and pneumonia. Respiratory physiotherapy is attained by encouraging the patient to take deep breaths and to cough. These exercises cannot be achieved without aggressive pain therapy. Adequate pain therapy controls pain, prevents respiratory complication, and reduces thromboembolic events. The main goal is to adequately control pain and avoid oversedation. Several pain therapy regimes are recommended, but a multimodal analgesic strategy using the combination of opiate-sparing patient-controlled analgesia with epidural analgesia seems to be the most preferred technique [74].
Thromboembolic events can be prevented using anticoagulants in the preoperative period. The selected anticoagulant agent is a protocol matter. Stress ulcer prophylaxis seems to be reasonable and effective, especially if a gastric bypass is to be performed. If the patient rests immobilized for a long period, pressure ulcers may occur in several parts of the body. Activating and periodically rotating the patient can be effective measures to prevent this complication. Surgery-related complications are bleeding, infection, perforation, anastomotic leaks, dehydration, peritonitis, pneumonia, DVT, malnutrition, vitamin K and B12 deficiency, and polyneuropathies.
Conclusions
The obese patient presents a great challenge to the anesthesiologist, who must deal with obesity-induced comorbidities and unique anesthetic considerations such as airway management, different drug dosage regimens, difficulties in monitoring and vascular access, respiratory and cardiac complications, and patient positioning.
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Domi, R., Laho, H. Anesthetic challenges in the obese patient. J Anesth 26, 758–765 (2012). https://doi.org/10.1007/s00540-012-1408-4
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DOI: https://doi.org/10.1007/s00540-012-1408-4