Anesthesiology Clin 24 (2006) 621–636

Perioperative Considerations for Patients with Morbid Obesity Thomas J. Ebert, MD, PhD*, Hariharan Shankar, MD, Rachel M. Haake, BA The Medical College of Wisconsin, VA Medical Center, Anesthesiology/112A, 5000 W. National Avenue, Milwaukee, WI 53295, USA

Obesity is now considered a national epidemic; this fact is old news. The morbidly obese (MO) patient only has a one in seven chance of a normal life expectancy [1]. Based on the 2003 guide published by the National Institutes of Health, body mass index (BMI; kg/m2) is used to classify obesity into three classes: class I, with a BMI of 30 to 34.9 kg/m2; class II, with a BMI of 35 to 39.9 kg/m2; and extreme obesity, class 3, with a BMI of greater than 40 kg/m2 [2]. Most consider morbid obesity to refer to patients who have a BMI of 35 kg/m2 or greater. Morbid obesity is associated with an increased prevalence of numerous physical ailments, including hypertension, hypercholesterolemia, type 2 diabetes mellitus, insulin resistance, glucose intolerance, coronary heart disease, congestive heart failure, stroke, gallstones, cholecystitis, cholelithiasis, osteoarthritis, and obstructive sleep apnea. The frequency and severity of comorbid conditions are directly proportional to the weight of the patient. The cost of health care treatment for the obese population is roughly 5% to 9% of national health care expenditures and is approaching $100 billion per year [3]. As part of the preoperative evaluation of the MO patient, all ailments must be considered in terms of anesthesia (and surgical) risk and anesthetic management. Underlying conditions including cardiac disease and angina must be stable and optimally managed medically or by way of surgical or cardiology interventions. Similarly, pulmonary function, diabetes, and glucose levels should be addressed and optimized. After preoperative conditions are addressed, the immediate perioperative management of the MO patient must be carefully planned. The optimal choice of anesthetic drugs and monitors should be

* Corresponding author. E-mail address: [email protected] (T.J. Ebert). 0889-8537/06/$ - see front matter Published by Elsevier Inc. doi:10.1016/j.atc.2006.05.003

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determined. An issue that presents an early challenge is the placement of an intravenous (IV) needle. Although the anesthesia provider is highly skilled in this routine procedure, the extra adipose tissue often hides superficial veins, thereby requiring less optimal approaches in the antecubital space or in the internal or external jugular vein. Placement of monitors is constrained by the area of sterile field preparation, by the patient’s position, and by the patient’s physical size. For example, the blood pressure cuff is commonly an oversized cuff and, due to the shape of the arm, often does not fit correctly. The length of the cuff bladder should equal at least 80% of the measured arm circumference and the width should equal at least 40% of the measured arm circumference at the midpoint of the upper arm. Cuffs that are too small overestimate the true blood pressure. The anesthesia provider should have a low threshold for placing an arterial line for accurate and instantaneous blood pressure readings and for easy access to blood sampling to determine hemoglobin content, PaO2, and PaCO2. The arterial line provides an additional benefit in the postoperative period as a means to monitor these variables. In planning the perioperative care, the following common additional issues must be considered to provide optimal care:








Full stomach: need for rapid-sequence intubation Positioning Rapid desaturation if mask ventilation or intubation is unsuccessful Aids for successful intubation Rapid emergence and return of protective airway reflexes Predictable neuromuscular blockade Managing postoperative pain with short-acting opioids or nonopioid analgesics
Obstructive sleep apnea; possibility of pulmonary hypertension and right heart failure (recently reviewed in the Anesthesiology Clinics of North America [4])

Full stomach: need for rapid-sequence intubation Many consider MO patients to be at risk for acid aspiration syndrome, a serious cause of anesthesia-related morbidity and mortality. Risk for this syndrome occurs when there is a low pH of gastric contents, increased gastric volume, or gastroesophageal reflux from an inadequate esophagealto-gastric barrier pressure. Despite a high prevalence of risk factors, acid aspiration syndrome has been reported as a rare event (occurring in 1 in 3216 anesthetics [5]) and was not associated with a BMI of 35 kg/m2 or greater. Pulmonary aspiration was associated with gagging and vomiting in 67% of the cases, laryngoscopy in 33%, and extubation in 36%. Several smaller studies also failed to detect a relationship between morbid obesity and the incidence of pulmonary aspiration [6,7].

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So why are obese patients considered to be at increased risk for acid aspiration syndrome? Primarily because there are conflicting data on gastric pH, volume, and barrier pressure in MO patients. Most anesthesiologists consider a gastric pH of less than 2.5 and a residual gastric fluid volume of 25 mL or greater to be critical factors in the development of pulmonary aspiration and lung injury. It is of interest that these criteria are based on a preliminary, unpublished observation by Roberts and Shirley [8] in rhesus monkeys. In patients, Vaughan and colleagues [9] observed that more than 70% of obese individuals had a combination of gastric volumes of 25 mL or greater and a pH of 2.5 or less compared with only 5% in nonobese individuals. In another study, 16 of 30 MO patients had an esophageal pH of less than 4.0 [10]. Readdressing this issue, Harter and colleagues [11] found a substantially lower incidence of combined high gastric volume and low pH in fasted, unmedicated obese patients compared with lean patients. These investigators explained the lower incidence of ‘‘at risk’’ patients compared with earlier work as a difference in preoperative medicines. Vaughan and colleagues [9] had used Innovar, which contains fentanyl and droperidol. Gastric volumes of 50 mL or more were observed in a greater proportion of obese individuals compared with normal weight patients, although this does not necessarily translate into a greater risk of aspiration [12]. In addition, there is no correlation between gastric volume and fasting duration [13]. The rate of gastric emptying is no different in obese and nonobese patients [14]. MO patients who drank 300 mL of clear fluid before surgery had ranges of gastric volumes and pH similar to those who fasted greater than 6 hours [15]. Obese patients who received metoclopramide had lower gastric volumes than those who did not receive it, but overall, obese patients have larger gastric volumes than nonobese patients [12]. Various studies in nonobese patients have shown that intragastric pressures range from 5 to 11 mm Hg, increase during succinylcholine-induced fasciculations to 25 mm Hg, and increase during vomiting to greater than 44 mm Hg. In anesthetized cats, which have a lower esophageal sphincter similar to humans, a mean intragastric volume of 21 mL/kg was required to overcome lower esophageal sphincter barrier pressure [16]. Translating this finding to a 100-kg patient, gastric volumes would need to be 2.1 L. Barrier pressure has been shown to be the same in obese and nonobese individuals [17]. Again, there is not enough evidence to support barrier pressure as a causal link to aspiration. In addition, a recent preliminary study found gastric volume and pH to be the same in ambulatory surgery patients who did and did not have gastroesophageal reflex disease [18]. One technique to lessen the chance for pulmonary aspiration is to apply cricoid pressure. Properly applied pressure presumably compresses the esophagus against the vertebral body. In an observational study using MRI, the cricopharyngeal muscle rather than the esophagus was lying posterior to the cricoid in 18 of 19 subjects. During cricoid pressure, the esophagus was displaced laterally in 90% of the subjects [19]. These investigators

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also found that the airway was displaced with cricoid pressure in 67% of patients and the diameter of the airway was compressed at least 1 mm in 81%. Considering that the esophagus proper is usually 10 mm below the cricoid and often displaced laterally, the protection afforded by compression is questionable. Cricoid pressure has not been proven to prevent aspiration. In an elective, fasted, obese patient with no other risk factors, the need for rapid-sequence induction with cricoid pressure due to a presumed risk for regurgitation and aspiration is debatable. This information on gastric contents and risk for pulmonary aspiration has led the authors to become less anxious about the potential for ‘‘full stomach aspiration risk’’ in fasting MO patients. A preliminary study on 78 fasted obese patients of whom nearly half had gastroesophageal reflux reported gastric volumes and pH to be no different than those in lean patients [20]. The investigators argued that rapid-sequence induction is not needed. The authors concur because committing to early paralysis is outweighed by the safety of controlled induction and airway evaluation before paralysis, given the lack of effectiveness of cricoid pressure and the low incidence of ‘‘at-risk’’ gastric contents. Nonetheless, the authors pretreat the stomach simply and cheaply with IV metoclopramide (10 mg) and ranitidine (50 mg) when at least 30 minutes are available before anesthetic induction. Positioning and anesthetic induction In most nonobese patients, the anesthetic induction period focuses primarily on establishing loss of consciousness, standard laryngoscopy, and intubation; however, this period is far more complex in the MO patient. Various factors should be considered and optimized, including position of the patient, preoxygenation, induction agents, intubation device, choice and dose of relaxants, and knowledge and availability of alternate airway devices. Proper positioning of the MO patient is extremely important. A common position is to support the patient behind the upper back and head to achieve the anatomic position whereby the head is above the horizontal plane of the upper chest or whereby a horizontal plane between the sternal notch and the external auditory meatus is established (Fig. 1) [21]. This positioning improves not only pulmonary mechanics but also the alignment from mouth to glottic opening. There have been reports of various other types of patient positioning, including the lateral position, but the authors contend that maintaining the position that is routinely practiced in the nonobese patient for intubation should be preferred; that is, supine, with support under the upper back and head. Ulnar neuropathy can occur in any surgical patient; it does not seem to be related to obesity and occurs despite conventional methods of positioning and padding [22–24]. Nonetheless, careful padding of the arms and avoiding traction on the brachial plexus are important precautionary measures that should be documented on the anesthesia record.

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Fig. 1. The correct position for direct laryngoscopy in an MO patient occurs when a horizontal line exists between the sternal notch and the external auditory meatus. This position is achieved by significantly elevating the head and upper body with pillows, blankets or towels.

Preoxygenation Preoxygenation is vitally important because the MO patient has a reduced functional residual capacity, often falling below the closing capacity of the small airways, leading to atelectasis, increased intrapulmonary shunting, and impaired oxygenation. Functional residual capacity is further reduced in the supine position and after induction of anesthesia (Fig. 2) [25]. Conventional techniques to denitrogenate the lungs apply, including at least 3 minutes of breathing 100% oxygen or five vital capacity breaths of 100% oxygen. A promising new technique has been described in which patients are administered oxygen by way of 10 cm H2O of continuous positive airway pressure (CPAP) for 5 minutes before inducing anesthesia. This is followed by 10 cm H2O positive end-expiratory pressure (PEEP) by way of mask before intubation [26]. This approach, or some reasonable modification of it, adds up to 1 minute of additional time before significant desaturation occurs (Fig. 3). Ventilation by way of the facemask after anesthetic induction can be difficult in the obese patient [27]; therefore, it is possible that only preinduction CPAP might succeed, whereas mask ventilation with PEEP might fail. This outcome would lessen the benefit, but in a difficult situation, any delay in desaturation from the early CPAP would be helpful. In a separate study, the effectiveness of CPAP and PEEP was studied in a nonobese population, and atelectasis was determined with CT scanning. The nearly 4% increase in atelectasis after induction and intubation was eliminated in this CPAP/PEEP group, which resulted in over a 140-mm Hg increase in the PaO2. From these two studies, one cannot be certain that CPAP, PEEP, or the combination was critical to the documented improvement in pulmonary mechanics. One earlier study employed CPAP to MO women for 3 minutes at þ7 cm H2O and found no benefit in the mean time to desaturate to 90% [28]. Therefore, it may be the combination

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Fig. 2. Schematic representation of the effects of severe obesity on functional residual capacity. Under normal circumstances, the functional residual capacity (and therefore the tidal excursion) is clear of the closing volume of the lungs. Anesthesia and obesity are associated with a reduction in functional residual capacity, resulting in airway closure and ventilation/perfusion mismatching during normal tidal ventilation. (Adapted from Adams JP, Murphy PG. Obesity in anaesthesia and intensive care. Br J Anaesth 2000;85:92; with permission.)

of CPAP and PEEP or the higher amount of CPAP for a longer period of time (10 cm H2O for 5 minutes) that is important when considering applying this technique. Mask ventilation and intubation A BMI greater than 26 kg/m2 results in a threefold increase in difficult ventilation by way of mask [27]. The five independent risk factors for

Fig. 3. Duration of nonhypoxic apnea and PaO2 before apnea in control and in PEEP patients. *P ¼ 0.002; yP ¼ 0.038 for comparison between groups. (Adapted from Gander S, Frascarolo P, Suter M, et al. Positive end-expiratory pressure during induction of general anesthesia increases duration of nonhypoxic apnea in morbidly obese patients. Anesth Analg 2005;100:580; with permission.)

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difficult mask ventilation are age older than 55 years, BMI greater than 26 kg/m2, lack of teeth, presence of beard, and history of snoring [27]. Several investigators have suggested that the difficult intubation rate is higher in the MO patient, from 2 to 10 fold [29,30]. Not all agree. In the MO population, there were only two correlates of difficult intubation: a Mallampati score of 3 or greater and neck circumference greater than 40 cm. These investigators noted that absolute weight or increasing BMI were not associated with difficult intubation [31]. Historically, the successful intubation of the MO patient with the suspected difficult airway was approached with an awake, fiber-optic technique. Recent studies, however, have described the highly successful use of an alternate airway device called the intubating laryngeal mask airway (ILMA). In MO patients, successful tracheal intubation was achieved 96% of the time through this device on first attempt [32]. In a separate report, a comparison was made of the success rate of intubating the trachea with the ILMA in an MO group versus a lean control group [33]. There were several important findings from this new report. The first was that 100% of the patients were successfully ventilated through the laryngeal mask airway device before intubating the trachea, which means that the potential airway collapse after induction and the difficulty with mask ventilation in the MO patient can essentially be avoided with this device. Second, the study confirmed the 96% success rate of intubating the trachea through this device (Table 1). The authors contend that this success rate is at least equal to if not better than standard laryngoscopy and tracheal tube placement or successful fiber-optic intubation. Rapid emergence and return of protective airway reflexes After successful intubation, an anesthetic maintenance regimen must be chosen. When choosing a volatile anesthetic, much focus has been given to the speed of emergence, with little mention of other characteristics of the anesthetic that might be beneficial to the patient, such as its effect on the heart, blood pressure, lungs, liver, and so forth. Desflurane is one of the newer anesthetic gases (introduced in 1993) and has a low solubility in blood, which adds to its speed of emergence. It has been demonstrated in a meta-analysis of published literature that desflurane emergence is nearly 2 minutes faster than that of the next closest competitor, sevoflurane [34]. It is well known, however, that titrating down the concentration of an anesthetic near the end of the case can result in wake-up at the end of surgical dressing application with any of the inhaled (or IV) anesthetics used in clinical practice today. Desflurane has another favorable attribute: the lowest fat-to-blood solubility (approximately 50% lower than that of isoflurane or sevoflurane), which implies that it may be uniquely advantageous in the obese population. This potential advantage has never been validated.

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Table 1 Characteristics of upper airway management in obese and lean patients

Safety and efficacy of UAM Success rate (%) Ventilation through the ILMA Minimum O2 saturation (%)a Quality of UAM Number of patients requiring O1 ILMA insertion Number of patients requiring airway adjustment Number of failed, blind, tracheal tube insertion attempts Duration of airway management (sec)a

Obese group (n ¼ 50)

Lean group (n ¼ 50)

100 96 (3)

100 98 (2)

5 13 14 160 (51)

7 23* 27** 187 (114)

Abbreviations: O2, oxygen; UAM, upper airway management. a Mean  (SD). * P ! 0.025 versus obese group; ** P ! 0.010 versus obese group. Adapted from Combes X, Sauvat S, Leroux B. Intubating laryngeal mask airway in morbidly obese and lean patients. Anesthesiology 2005;102:1108; with permission.

Recently, the authors undertook research to determine whether careful titration of volatile anesthetics with different blood and fat solubilities could eliminate emergence differences between drugs [35]. The authors compared two of the newest volatile anesthetics (sevoflurane and desflurane) in MO patients who had BMIs greater than 35 kg/m2 and who were to undergo a variety of surgical procedures lasting longer than 2 hours. To manage the anesthetic drugs during the intraoperative period and during the weaning period, an additional monitor, the BIS monitor (Aspect Medical Systems, Natick, Massachusetts) was employed and a BIS number between 45 and 50 was targeted. This level of sedation/hypnosis was maintained during the surgical procedures with slightly less than 1 minimum alveolar concentration of volatile anesthetic. The average case length was 3 hours. During the last 15 minutes of the case, the authors gradually decreased the anesthetic concentration to achieve a BIS number of 60 at the end of the surgical procedure. With this technique, equivalent wake-up times between sevoflurane and desflurane were achieved, averaging from 4 to 7 minutes. Thus, the difference of several minutes in wake-up times between these two anesthetic drugs, as suggested in earlier data, was eliminated by the use of controlled titration ‘‘off’’ of the anesthetics near the end of the surgical procedure (Fig. 4). In addition, one of the more unexpected findings was the clear-headedness of the patients in the postoperative recovery room. There are several lessons to be learned from this research. One is the choice of the volatile anesthetic probably makes little difference in the management of MO patients, particularly if one attempts to maintain a moderate level of sedation and hypnosis rather than the deep level common to many practices. Furthermore, the gradual titration of the anesthetic to a lighter

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Emergence End Points 10 Desflurane

Sevoflurane

Time to : (min)

8

6

4

2

0

Response to Verbal Command

Extubation

Fig. 4. Average emergence times, defined as response to verbal command and extubation, for desflurane (dark bars) and sevoflurane (hatched bars) groups. Error bars represent SE; there were no significant differences between groups. Data are mean  SEM.

level near the end of the case resulted in excellent emergence characteristics. The authors strongly recommend the use of a processed electroencephalogram monitor when managing the MO patient to avoid anesthetic ‘‘overdoses’’ and to permit controlled titration near the end of the case.

Predictable neuromuscular blockade Full neuromuscular blockade is generally desired to get adequate relaxation and exposure for surgical interventions. Thus, the anesthesia provider uses a nondepolarizing neuromuscular blocking drug for maintenance of relaxation throughout the surgical procedure. One problem that has been identified with some of the older neuromuscular blocking drugs is the unpredictability of their duration of action. For example, pancuronium is associated with more postoperative respiratory problems than newer agents such as vecuronium and rocuronium [36]. If paralysis cannot be fully reversed at the end of the case, then sustaining intubation and mechanical or assisted ventilation is required. Recently, the authors identified a more predictable behavior of the drug cisatracurium compared with rocuronium or vecuronium [37]. This research did not focus specifically on the MO patient but focused on an older population in whom underlying organ dysfunction was not evident on laboratory testing but may have existed subclinically. Cisatracurium is a drug that is not metabolized by way of organ-dependent mechanisms. One other point with regard to neuromuscular blocking drugs

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is that they should be dosed to ideal body weight plus an additional 25%, rather than to actual body weight [38,39]. Managing postoperative pain with short-acting opioids or nonopioid analgesics The immediate postoperative care of MO patients presents several challenges, the foremost of which is treating postoperative pain without producing excessive sedation. Sedated, obese patients who have unprotected airways are more prone to upper airway obstruction from excess pharyngeal tissue. During periods of obstruction or decreased ventilation, the obese patient’s arterial oxygen saturation can drop precipitously due to a reduced functional residual capacity and elevated oxygen consumption [40–42]. Thus, in the postanesthesia care unit (PACU), it is prudent to minimize or avoid analgesic drugs associated with sedation and respiratory depression, specifically opioids. Alternative, nonsedating, nonopioid drugs have been used successfully in the postoperative period for pain management, including nonsteroidal anti-inflammatory drugs (NSAIDs), a2-adrenergic receptor agonists, N-methyl-D-aspartate (NMDA) receptor antagonists, and sodium channel blockers. Regional anesthesia also offers the benefits of improved postoperative analgesia and minimal cardiopulmonary depression. The authors offer a brief review of specific postoperative analgesic regimens that have been studied in MO patients. Perhaps the preferred method of treating postoperative pain in MO patients is with a multimodal drug regimen with minimal opioid. This approach following open gastric bypass surgery produced a lower level of sedation in the PACU and a significant decrease in the use of patientcontrolled analgesia (PCA) morphine [43]. Patients received 60 mg of methylprednisone before surgery; 30 mg of ketorolac at the beginning and end of the case; 300 to 500 mg of IV clonidine in the first hour of anesthesia; a 100-mg bolus of IV lidocaine followed by 4 mg/min for the first hour, 3 mg/min for the second hour, and 2 mg/min for the remainder of the case; 0.17 mg/kg/h of ketamine, and 80 mg/kg of magnesium sulfate. In comparison, MO patients in a control group who solely received intraoperative fentanyl were more sedated in the PACU, two patients required re-intubation, and more PCA morphine was used versus the treatment group. NSAIDs possess well-documented analgesic properties without the side effects of sedation or respiratory depression and, thus, have potential benefits for postoperative pain management in the MO population. As demonstrated by Feld and colleagues [43], NSAIDs are an efficacious adjuvant in combination with other nonopioids in MO patients. Similarly, a metaanalysis found a morphine-sparing effect of 30% to 50% and significant reductions in nausea, vomiting, and sedation when a variety of NSAIDs (aspirin, dexketoprofen, diclofenac, ibuprofen, indomethacin, ketoprofen, ketorolac, naproxen, parecoxib, piroxicam, rofecoxib, and tenoxicam)

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were used as adjuvants to PCA morphine for postoperative pain management [44]. Ketorolac is a potent NSAID (50 times more potent than naproxen) that has been promoted as a safe alternative to opioid analgesics, especially in the perioperative period [45,46]. The risk of gastrointestinal and operative site bleeding is small, although the risk is increased and clinically important at higher doses (more than 120 mg daily), in older patients, and when used for more than 5 days [47]. A study involving patients who underwent decompressive posterior lumbar laminectomy with instrumental spinal fusion also found a detrimental effect on bone union when patients were given high doses of postoperative ketorolac (120–240 mg/d for 5 days) [48]; however, patients receiving a 5-day course of low-dose ketorolac (%110 mg/d) were not at increased risk for bone nonunion. Therefore, it appears that despite its associated risks, ketorolac can be a valuable asset to an analgesic regimen when used appropriately. In addition, patients who received celecoxib (200–600 mg/d) or rofecoxib (50 mg/d) were not at increased risk of bone nonunion compared with control subjects [48]. Although the controversy over cyclooxygenase-2–specific inhibitors (coxibs) continues, there are many reports of their benefits over nonselective cyclooxygenase inhibitors [49–51]. A 2002 review of coxibs and the management of acute and perioperative pain concluded that this class of drugs improved the efficacy of pain relief and decreased the risk of side effects (mainly postoperative nausea and vomiting, sedation, and postoperative bleeding) associated with opioids and nonselective NSAIDs [49]. In one study, patients undergoing spinal fusion received celecoxib (200 mg/d) or rofecoxib (50 mg/d) before surgery without any observed increase of intraoperative bleeding for either drug [50]. A similar study of patients undergoing total joint arthroplasty showed no significant increase in perioperative bleeding or gastroduodenal perforation/bleeding when patients received rofecoxib daily for 3 days before surgery and for 2 days postoperatively [51]. Clonidine and the more selective a2-adrenergic agonist dexmedetomidine have hypnotic, sedative, sympatholytic, and analgesic properties and have been used for postoperative pain management of obese [52–54] and nonobese [35] patients. In a case report of a 433-kg man who had obstructive sleep apnea and pulmonary hypertension who underwent Roux-en-Y gastric bypass, a continuous infusion of dexmedetomidine (0.7 mg/kg/h based on estimated lean body mass) was continued through the first 24 hours after surgery in addition to PCA morphine [52]. Usage of PCA morphine increased threefold in the 24-hour period after the dexmedetomidine infusion was stopped, implying that dexmedetomidine had a significant morphinesparing effect during the first postoperative day. Clonidine is an a2-adrenergic agonist with analgesia and sedative properties, similar to dexmedetomidine. Marinangeli and colleagues [53] found that a postoperative clonidine bolus of 3 mg/kg followed by a continuous infusion of 0.3 mg/kg/h in addition to PCA morphine provided sufficient analgesia and had a dose-related reduction in morphine requirements.

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Clonidine (20 mg) has also been mixed with PCA morphine (1 mg) for postoperative pain relief [54]. Analgesia was significantly improved at 12 hours, postoperative nausea and vomiting were decreased, and morphine consumption was slightly reduced. Ketamine is a noncompetitive NMDA receptor antagonist with analgesic and psychomimetic properties [55]. A recent systematic review of ketamine and the treatment of postoperative pain found that pain intensity at rest was significantly decreased at 6 hours postoperatively but that the dose of ketamine was not related to the analgesic efficacy [55]. Patients receiving ketamine used, on average, 16 mg less of morphine and had a 16-minute delay to the first request for additional analgesia. The use of ketamine, however, was not associated with a decrease in opioid-related side effects. Although ketamine is often avoided because of its association with hallucinations, patients receiving a general anesthetic in the studies reviewed in this article had only a minimal risk for ketamine-induced hallucinations. Although this review found a significant morphine-sparing effect, ketamine did not demonstrate the benefit of reduced sedation and respiratory depression when combined with opioids, but this benefit might be achieved if ketamine is combined with nonopioid analgesics. Ketamine enhances the analgesic effects of morphine by inhibiting the opioid activation of NMDA receptors [56]. Giving small doses of postoperative ketamine and morphine (250 mg/kg and 15 mg/kg, respectively) has been shown to significantly decrease pain intensity and total opioid usage by 35%, increase wakefulness, increase oxygen saturation as measured by pulse oximetry, and decrease the incidence of postoperative nausea and vomiting compared with morphine alone, with minimal occurrence of ketamine-related adverse effects [56]. Systemically administered sodium channel blockers in the perioperative period play a role in decreasing postoperative pain by preventing central and peripheral hyperalgesia and by inhibiting inflammation [57–60]. To be effective in inhibiting hyperalgesia and inflammation, lidocaine must be given preoperatively. A morphine-sparing effect and improved postoperative pain scores in the first two days after surgery have been demonstrated when a continuous low-dose infusion of IV lidocaine (2 mg/min) was started 30 minutes before surgery and continued for 24 hours after surgery [57]. Koppert and colleagues [59] produced similar results, with significantly improved analgesia lasting through the third postoperative day, when patients undergoing major abdominal surgery received a preoperative bolus of IV lidocaine (1.5 mg/kg in 10 minutes) followed by a continuous infusion of 1.5 mg/kg/h through 60 minutes after skin closure. Some studies report that IV lidocaine decreases the incidence of postoperative ileus and hastens the return of bowel function in surgical patients, thus shortening the hospital stay [60,61], which is clearly beneficial to MO patients. Not all studies concur on the bowel benefits of lidocaine [59]. The final topic to cover on the subject of treating postoperative pain in MO patients is that of regional anesthesia. Currently, many obese patients

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are not deemed eligible for regional techniques because these patients are difficult to position, the bony and muscular landmarks are hard to identify, and the right equipment might not be available. In one study, 6920 overweight or obese patients received an upper- or lower-extremity peripheral nerve block, paravertebral block, central neuraxial block, or combined block (ie, lumbar plexus and sciatic nerve blocks) [62]. Although the overall success rate was high (89.1%), patients who had a BMI greater than 30 kg/mg2 were 1.62 times more likely to have a failed block and a higher unadjusted rate of acute complications (0.3%). Lumbar plexus blocks and continuous interscalene blocks were associated with the highest failure rates (9.5% and 15.2%, respectively). Obese patients who had successful blocks, however, had improved pain with movement in the PACU. Postoperative pain at rest, total opioid use, postoperative nausea and vomiting, unanticipated admissions, and overall satisfaction were similar to nonobese patients. The authors have discussed in some detail the nonopioid alternatives for postoperative pain management in MO patients. A preferred approach is the multimodal nonopioid analgesics similar to the description by Feld and colleagues [43], including methylprednisone, ketorolac, clonidine, lidocaine, ketamine, and magnesium. In most of the reviewed studies, some patients still required opioids, but overall, the amounts used were significantly reduced and, thus, patients were less sedated and the risk for airway obstruction was presumably reduced. Summary The anesthetic management of the MO patient requires an important focus on a number of issues beginning with a careful preoperative evaluation and synthesizing pre-existing disease processes with the anesthetic management plan. The common misperception that all MO patients are ‘‘full stomach’’ has been challenged and may be a nonissue. New approaches to pre-oxygenation to lessen the likelihood of desaturation during apnea may be a valuable tool if difficulty is encountered in tracheal intubation. In addition, promising results have been demonstrated with the use of the ILMA for ventilation and for blindly establishing tracheal tube placement. Proper patient positioning is essential to aid in successful intubation when a laryngoscope is employed. Intraoperative anesthetic management can be guided with a processed electroencephalogram monitor to help improve emergence and to enhance wakefulness in the PACU. Careful consideration must be given to postoperative analgesic needs by minimizing the use of opioids and employing nonopioid analgesics including NSAIDs, a2-adrenergic agonists, and low doses of ketamine. References [1] Baxter J. Obesity surgerydanother unmet need: it is effective but prejudice is preventing its use. BMJ 2000;321:523–4.

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