VOLUME 31, ISSUE 2

Kristina L. Goff, MD

Assistant Professor of Anesthesiology
University of Texas Southwestern Medical Center
Dallas, TX

Changing Perspectives in Modern Anesthesia Practice

As our field evolves, incorporating new techniques and medications, many of the age-old assumptions of anesthesia practice are being overturned in favor of more updated methods. Here we will discuss several widely held beliefs worth rethinking, review the latest evidence and suggest more modern approaches to anesthetic management.

Patient Selection in the Ambulatory Setting

As ambulatory surgical centers have become more prevalent and the American people have grown sicker, the discussion surrounding appropriate selection criteria has become quite complex. Morbid obesity and obstructive sleep apnea have both been considered contraindications to surgery in an ambulatory center, due to the associated risk of postoperative complications. However, given the increasing incidence of both of these conditions, many anesthesiologists have questioned the practicality of this and after careful research, are finding caveats to this approach.

The Society for Ambulatory Anesthesia (SAMBA) developed a consensus statement in 2012, looking specifically at the management of patients with Obstructive Sleep Apnea (OSA).1 Rather than excluding all patients with OSA from having surgery at ambulatory centers, they found that well-optimized patients who are able to use a continuous positive airway pressure machine and patients who can be managed with an opioid-sparing technique in the operative period, may both be suitable for surgery in the ambulatory setting. The SAMBA statement also encourages the use of the STOP-BANG questionnaire to assess the presence of undiagnosed sleep apnea. They also recommend taking other comorbid conditions into consideration when making decisions about the appropriate surgical setting for this patient population.

Morbid obesity is another condition that has been the focus of a great deal of attention in recent years. Based on the current evidence, it seems clear that patients with a BMI less than 40kg/m can safely have surgery in the ambulatory setting, assuming other conditions are optimized. Patient who are super obese (BMI>50kg/m), on the other hand, do have a significantly increased risk of postoperative complications and should only be cared for in an ambulatory surgery environment, under very specific conditions where extra caution can be exercised. Between these two extremes exists a gray zone, where the data suggests careful consideration of comorbid conditions, surgical technique, and anticipated postoperative management when determining the appropriateness of patients for ambulatory surgical centers.

Resurgence of Nitrous Oxide?

Many anesthesiologists avoid the routine use of nitrous oxide because of a long-held belief that it causes postoperative nausea and vomiting. The interaction between nitrous oxide and vitamin B12, leading to elevated plasma homocysteine levels, has also been a source of concern, as this is felt to be a potential contributor to increased cardiovascular complications in the perioperative setting. The data on this subject has been confusing for many, but the ENIGMA II Trial, published in 2015, laid much of the debate to rest, failing to show any association between the use of nitrous oxide and the incidence of myocardial infarction.3 Additional studies have looked at the incidence of nausea in relation to nitrous oxide use. These studies find that the risk of postoperative nausea is likely dose dependent,4 is dependent on duration of exposure (no clinically significant risk with duration < 1 hour),5 and is essentially nullified by the administration of antiemetic prophylaxis.6

Because nitrous oxide inhibits the N-Methyl-D-aspartate receptor, it may be a beneficial adjunct when employing a multimodal, opioid-sparing approach to anesthesia. Given the evidence, it seems we can reasonably use nitrous oxide without significant fear of causing patient harm.

Lung Protective Ventilation in the Operating Room

The ventilators anesthesiologists use in the operating room(OR) are significantly different from those used in most intensive care units. It turns out that the settings we have traditionally used are too. Although intensivists have been using a lung protective strategy of smaller, ideal-body-weight-based tidal volumes and higher levels of positive end-expiratory pressure (PEEP) to prevent lung trauma for nearly two decades, only recently have anesthesiologists in the OR become interested in the potential benefits of this technique. Several studies have recently looked at lower tidal volumes during abdominal surgery, finding improved clinical outcomes with tidal volumes of 6-8mL/kg ideal body weight as compared to larger tidal volumes.7-8 Given that there is little potential downside to this approach, it seems appropriate that we begin tailoring our ventilation strategy to consider lung trauma, particularly in patients at risk for postoperative lung injury. Data regarding optimal levels of PEEP is less clear. Appropriate PEEP settings are dependent on the type of surgery, the patient’s hemodynamics, and risk for atelectasis and lung injury. Considering a PEEP setting of 6-8cmH20 in patients with existing (or at high risk for developing) lung injury seems to be a wise approach.9

The Appropriate Reversal of Neuromuscular Blocking Agents

When it comes to reversal of neuromuscular blockade, an all-or-none strategy has often prevailed. Neuromuscular blockade, until recently, required reversal with neostigmine, an anticholinesterase with many unfortunate physiologic effects. For this reason, many anesthesiologists have avoided using reversal agents when possible. In deciding whether or not to give neostigmine, anesthesiologists may consider how long it has been since the last dose was given, subjective evaluation of return of twitches using a train of four (TOF) peripheral nerve stimulator, or observed strength using a set of basic criteria such as the ability to sustain a head lift for >5 seconds. In patients who are felt to have residual paralysis, neostigmine has often been administered in a standard dose of 4 or 5mg.

Unfortunately, we are learning that our ability to determine residual paralysis using traditional techniques is likely inadequate10 and indiscriminate dosing of neostigmine carries potential harm. In fact, giving anticholinesterase reversal when it isn’t indicated, or giving too much even when it is, can paradoxically cause weakness.

Ideally, subjective neuromuscular blockade monitoring using TOF assessment would be replaced by newer quantitative monitors based on electro- and acceleromyography, which are much more accurately able to determine the degree of residual paralysis. In any case, dosing of neostigmine should be adjusted to the extent of residual paralysis, with doses ranging from 0.02mg/kg to 0.06mg/kg.11

Appropriate Fasting before Anesthesia

The days of “nothing to eat or drink after midnight” are quickly fading. Although a fasting period of at least 8 hours after a heavy meal is still recommended prior to anesthesia, NPO (from the latin nil per os) guidelines have become much more nuanced in recent years. In 2017, the American Society of Anesthesiologists (ASA) published a revised set of fasting guidelines, providing new recommendations on a variety of subtleties.12 For example, clear liquids may safely be consumed up to 2 hours prior to anesthesia. In fact, many enhanced recovery protocols encourage patients to consume an oral carbohydrate drink 2 hours before surgery in hopes of decreasing perioperative insulin resistance and improving patient comfort.13 Although the evidence in support of this practice is somewhat mixed, the data is clear that gastric volume and pH are not adversely affected by this decrease in fasting time and the risk of clinically significant aspiration is not higher.

The ASA recommendations also allow for a decreased fasting time of 6 hours after a “light meal,” suggesting anesthesiologists give careful thought to both the amount and type of food a patient consumes when making decisions about appropriate NPO status.

Not uncommonly, our longest-held beliefs prove to be the most resistant to necessary change. Even when faced with solid supportive data, it may feel more comfortable to stick with an old habit than to try a new approach. Nevertheless, as new evidence emerges, it is imperative that we adjust our anesthetic techniques in order to continue to provide our patients with the very best possible care. The topics in this article represent only a small subset of the practice changes we need to consider. Imagine the evolution we might see if each of us chose just one assumption of our daily practice, and sought to question it more closely.

References

  1. Joshi, GP, et al. Society for Ambulatory Anesthesia consensus statement on preoperative selection of adult patients with obstructive sleep apnea scheduled for ambulatory surgery. Anesth Analg 2012 Nov;115(5):1060-8.
  2. Moon, TS, Joshi, GP. Are morbidly obese patients suitable for ambulatory surgery? Curr Opin Anaesthesiol 2016 Feb;29(1):141-5.
  3. Leslie, Kate, et al. Nitrous Oxide and Serious Long Term Morbidity and Mortality in the Evaluation of Nitrous Oxide in the Gas Mixture for Anaesthesia (ENIGMA)-II Trial. Anesthesiology 2015 Dec;123(12):1267-1280.
  4. Mraovic, B, et al. The dose-response of nitrous oxide in postoperative nausea in patients undergoing gynecologic laparoscopic surgery: a preliminary study. Anesth Analg 2008 Sep;107(3):818-23.
  5. Peyton, PJ, Wu, CY. Nitrous oxide-related postoperative nausea and vomiting depends on the duration of exposure. Anesthesiology 2014 May;120(5):1137-45.
  6. Myles, Paul, et al. Severe Nausea and Vomiting in the Evaluation of Nitrous Oxide in the Gas Mixture for Anaesthesia II Trial. Anesthesiology 2016;124(5):1032-1040.
  7. Severgnini, P, et al. Protective mechanical ventilation during general anesthesia for open abdominal surgery improves postoperative pulmonary function. Anesthesiology 2013;118:1307-21.
  8. Futier, E, et al. IMPROVE Study Group: A trial of intraoperative low-tidal-volume ventilation in abdominal surgery. N Engl J Med 2013;369:428-37.
  9. Guldner, A, et al. Intraoperative protective mechanical ventilation prevention of postoperative pulmonary complications: A comprehensive review of the role of tidal volume, positive end-expiratory pressure, and lung recruitment maneuvers. Anesthesiology 2015;123:696-713.
  10. Thilen, SR, et al. Intraoperative neuromuscular monitoring site and residual paralysis. Anesthesiology 2012;117:964-72.
  11. Brull, SJ, Kopman, AF. Current Status of Neuromuscular Reversal and Monitoring: Challenges and Opportunities. Anesthesiology 2017; 126(1):173-190.
  12. American Society of Anesthesiologists. Practice Guidelines for Preoperative Fasting and the use of Pharmacologic Agents to Reduce the Risk of Pulmonary Aspiration: Application to Healthy Patients Undergoing Elective Procedures: An updated report. Anesthesiology 2017;126:376-93.
  13. Bilku, DK, et al. Role of preoperative carbohydrate loading: a systematic review. Ann R Coll Surg Engl 2014 Jan;96(1):15-22.

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