The Effect of Obesity and Anesthetic Maintenance Regimen on Postoperative Pulmonary Complications

Field: Medicine (Health Sciences)

Results from the 2007 to 2008 National Health and Nutrition Examination Survey estimate that, among adults in the United States, 68% are either overweight or obese (body mass index [BMI] >25 kg m−2), with the prevalence having increased significantly over the past 20 years.a The associated health problems mean that this population is presenting for surgery in ever greater numbers. The challenges regarding the perioperative management of obese patients are well known, with the effects of obesity on the airway and the respiratory, cardiovascular, and gastrointestinal systems being of most concern to anesthetists. In 1987, Strandberg and colleagues found a correlation between obesity and the area of lung densities seen immediately after induction of anesthesia.1 A more recent study showed that during general anesthesia, morbidly obese patients suffer more atelectasis than did the nonobese, and unlike the normal population, this atelectasis can persist for 24 hours.2 Atelectasis may result in arterial hypoxemia and marked alterations in respiratory mechanics, which can be alleviated by the use of positive end-expiratory pressure during anesthesia. Applying 10 cm H2O of positive end-expiratory pressure increases oxygenation in obese patients, and this improvement is directly related to the amount of alveolar recruitment achieved.3 However, lung recruitment maneuvers do not improve postoperative oxygenation,4 and thus rapid recovery from general anesthesia is desirable to minimize postoperative respiratory complications. Agents of choice for rapid emergence and recovery in recent times have been IV propofol, owing to its large volume of distribution at steady state and high metabolic clearance, or the less-soluble volatile anesthetic, desflurane. However, confusion persists regarding drug dosing in the obese, largely because of inadequate knowledge of pharmacokinetic parameters with altered body composition. Volume of distribution, clearance, and protein binding may each be affected by obesity.5 Clinical studies have, so far, failed to define a universal size descriptor suitable for use in the obese population; therefore, titration to clinical end-points or targeted, processed, electroencephalogram (EEG) values such as bispectral index (BIS) remain advisable.6 In this edition of Anesthesia & Analgesia, Zoremba and colleagues report on the effects of propofol and desflurane on postoperative lung function in overweight patients,7 concluding that maintenance of anesthesia with propofol impairs early postoperative lung function and pulse oximetry values to a greater extent than does maintenance with desflurane. Furthermore, the reduction in pulmonary function at 2 hours appeared to increase with increasing obesity after propofol but not with desflurane anesthesia. These findings, on the face of it, seem somewhat surprising, given what is known of the pharmacokinetics of propofol. However, previous work has suggested that desflurane may have a better recovery profile than does propofol in morbidly obese patients with regard to early recovery parameters including eye opening, orientation, postextubation SpO2 values, and sedation up to 2 hours after surgery.8 In Zoremba et al.'s study, operation times were no more than 2 hours. The study group underwent procedures lasting, on average, 80 minutes. These patients had BMIs 25 to 34 kg m−2, and none was morbidly obese. It is difficult to know, under these circumstances, whether propofol and desflurane would, indeed, have similar pharmacokinetic profiles, or whether desflurane would be advantageous, as it is thought to be in the morbidly obese population. There is very little evidence available with regard to recovery profile for choice of anesthetic agent in the overweight patient group (BMI 25 to 29 kg m−2) undergoing anesthesia of short duration. Many studies involving lean patients undergoing short procedures have found little or no difference in early recovery between the 2 methods of anesthetic maintenance.9–13 A systematic review combining data from 58 studies concluded that eye opening occurred a minute earlier with desflurane than with propofol in day-case anesthesia.14 The majority of these studies used nitrous oxide as a second gas, which is likely to have affected emergence from the inhaled agent. However, the statistically significant differences in postoperative pulse oximetry and lung function values, found by Zoremba and colleagues, do suggest a difference in emergence and recovery profiles between propofol and desflurane in this population. Another interesting point that this study highlights is the possible synergistic effect between propofol and fentanyl on emergence. The patients recruited to Zoremba et al.'s study received fentanyl 2 to 3 μg kg−1 at induction, for what was (for some) a short procedure. There are many reports demonstrating that opioids can enhance the anesthetic action of propofol during induction,15–17 but there is also evidence that fentanyl can enhance the anesthetic potency of propofol during recovery from anesthesia due to a synergistic effect.18 A possible explanation for the postoperative respiratory dysfunction seen in the propofol group is that depth of anesthesia at the end of the operation, and extubation was unknown. The authors describe titration of anesthetic agents using a BIS–EEG electrode as a guide, an advantage given the controversies around drug dosing in the overweight and obese population. Indeed, there is some evidence to suggest that when continuous infusion of propofol is combined with BIS monitoring of depth of anesthesia, total propofol consumption is reduced in obese patients, leading to an even more favorable recovery profile.19,20 However, although BIS–EEG was used during the procedure, it was not used on emergence or extubation to allow comparison of conscious levels between the groups at this point. Does a rapid awakening translate to greater patient safety? It could be debated that not all recovery parameters (e.g., eye opening) are relevant when it comes to patient safety. Oxygen saturation values, lung function, and parameters such as airway reflexes may be considered clinically significant, whereas eye opening may not. Much of the literature concerning recovery from desflurane and propofol anesthesia uses varying end points, of differing clinical significance. The paper published in this edition does focus on outcome measures that are of relevance where patient safety is concerned. The final point to consider when considering not only Zoremba et al.'s paper, but patient safety-related research in general, is whether a statistically significant result translates to clinical significance. Should these findings change practice? There is no doubt that the results published by Zoremba and colleagues are statistically significant, but is a difference of oxygen saturation of 1% clinically important? Should we be considering desflurane anesthesia for the overweight population in preference to propofol on the basis of this evidence? Desflurane maintenance may offer a 1% advantage in arterial oxygen saturation over propofol, but other measures to minimize postoperative respiratory complications may satisfactorily remove any observable difference. Interestingly, published work by the same authors describes measures—such as the use of laryngeal mask airway instead of tracheal intubation, pressure support ventilation instead of pressure controlled ventilation, low inspired oxygen fractions during general anesthesia, and respiratory physiotherapy during the postanesthesia care unit stay—as ways of improving postoperative lung function in obese adults following maintenance of anesthesia with propofol.21–24 It is likely that clinically relevant differences in outcome rely on many more factors than merely the choice of anesthetic agent. DISCLOSURES Name: Kathryn R. Corrie, FRCA. Contribution: This author helped write the manuscript. Attestation: Katherine R. Corrie approved the final manuscript. Name: Shruti Chillistone, FRCA. Contribution: This author helped write the manuscript. Attestation: Shruti Chillistone approved the final manuscript. Name: Jonathan G. Hardman, FRCA. Contribution: This author helped write the manuscript. Attestation: Jonathan G. Hardman approved the final manuscript.