Anesthesia & Analgesia:
The Search for an Evidence-Based Method of Reducing Aspiration
Maile, Michael D. MD; Blum, James M. MD
From the Department of Anesthesiology, Division of Critical Care, University of Michigan Cardiovascular Center, Ann Arbor, Michigan.
Supported by departmental funds.
The authors declare no conflicts of interest.
Reprints will not be available from the authors.
Address correspondence to James M. Blum, MD, Department of Anesthesiology, University of Michigan Health System, 4172 Cardiovascular Center, 1500 E. Medical Center Dr., Ann Arbor, MI 48109-5861. Address e-mail to firstname.lastname@example.org.
Accepted January 17, 2012
Dr. Curtis L. Mendelson is the eponym for Mendelson syndrome, which is better known today as “pulmonary aspiration syndrome.” He originally wrote that, “The clue to etiology of the liquid aspiration syndrome was apparent when I inadvertently inhaled gastric fluid following an evening of relative intemperance.”1,2 More than 60 years later, the optimal anesthetic technique to avoid this complication during the induction of general anesthesia is still debated. Virtually all aspects of anesthetic induction have been investigated, such as whether to induce anesthesia with patients in the lateral, head-up, or head-down position, or whether to use volatile or IV medications during the induction. Based primarily on clinical experience, some techniques have become so widespread that they are now considered “standard of care.”
During a standard anesthetic induction, anesthesiologists commonly avoid cricoid pressure, administer preinduction anxiolytics, and perform mask ventilation. The concept of using a special anesthetic induction procedure for patients at risk for aspiration began to take shape in 1951 after the introduction of succinylcholine. Currently, most clinicians would agree that a neuromuscular blocking drug with a fast onset time and short duration of action is preferred during induction of anesthesia in patients at risk for aspiration, because this would eliminate the need for mask ventilation and potential stomach insufflation. A decade after the first use of succinylcholine, in 1961, Sellick3 described cricoid pressure as a maneuver to decrease aspiration during the induction of anesthesia. This has also gained widespread acceptance as a valuable technique to reduce stomach insufflation and regurgitation of gastric contents. These and other concepts were integrated by Stept and Safar4 into the first written description of a rapid sequence induction and intubation (RSII). Their protocol included preoxygenation, a semisitting position, precurarization, a fixed dose of thiopental, cricoid pressure, succinylcholine, and the avoidance of mask ventilation. Although newer medications may now be used, this sequence has been relatively unchanged over the past 40 years.
Despite the widespread acceptance of Stept and Safar's concept, there remains little clinical evidence supporting the various components of the RSII.5 Even cricoid pressure, which has become a cornerstone of RSII, lacks clinical evidence supporting its use. For example, several recent studies have shown that cricoid pressure may decrease the lower esophageal sphincter tone6,7 and may frequently fail to compress the esophagus.8 There is also evidence that the use of cricoid pressure may increase the difficulty of mask ventilation9 or placement of a laryngeal mask airway.10 Because there is no definitive evidence supporting the components of an RSII, many clinicians feel comfortable altering the classic RSII. This has given rise to the term “modified RSII.” For example, many clinicians will use sedatives before bringing the patient into the operating room or will “test” their ability to oxygenate the patient's lungs via bag mask ventilation before administering a neuromuscular blocking agent despite the perceived increased risk of aspiration. Unfortunately, neither a standard definition of a “classic” RSII nor a classification of the different types of “modified” RSII exists. This makes it difficult to communicate and conduct clinical research in this area.
In this issue of Anesthesia & Analgesia, Ehrenfeld et al.11 present the results of a national cross-sectional survey conducted to study the current techniques used to induce anesthesia in adult patients at risk for aspiration. The goal was to better define what anesthesiologists consider a “modified” RSII and explore the reasons a modified technique is used. The study population consisted of attending and resident anesthesiologists at 131 hospitals in the United States that contain an anesthesiology residency program accredited by the Accreditation Council for Graduate Medical Education. Each institution received the same number of questionnaires irrespective of the size of their residency programs, and no incentives were used to enhance survey completion rates. No data from private practice anesthesiologists, certified registered nurse anesthetists, or anesthesia assistants were collected. They received responses from 44% of institutions, and responding institutions returned an average of 84% of surveys.
Their definition of a “classic” RSII was derived from previous surveys of anesthesia practice and consisted of preoxygenation, cricoid pressure, IV induction, and avoidance of mask ventilation. Other items, such as patient positioning and the use of premedication were not included; participants had to choose from a fixed list of modifications. Participants were asked how they “most often” or “usually” modified an RSII: choice of muscle relaxant, timing of muscle relaxant, preoxygenation, use of cricoid pressure, ventilating with a bag-mask apparatus, and administering pharmacologic aspiration prophylaxis. No information regarding the use of sedatives before induction of anesthesia or the use of other modifications was obtained.
It appears that in certain situations, the presumed increased risk of aspiration with mask ventilation is outweighed by risks of hypoxemia. The authors propose a new definition of a modified RSII, which consists of preoxygenation, cricoid pressure, and mask ventilation. These data show that many anesthesiologists believe that cricoid pressure is indispensable, whereas avoiding mask ventilation does not have as much benefit for patients perceived to have an increased risk of aspiration. This provides guidance for future research of anesthesia induction practices.
The challenge of this future research will be to examine relevant outcomes when studying RSII. As mentioned previously, even cricoid pressure may not be a benign maneuver. Likewise, mask ventilation, use of succinylcholine, using fixed doses of medications, etc., may cause more problems than they solve. If future studies use pulmonary aspiration of gastric contents as an end point, it is possible that iatrogenic complications caused by attempts to reduce aspiration will be missed. If outcomes such as mortality or length of hospital stay are used as outcome end points, it will be very difficult to collect enough patients to find a significant difference between techniques, given the low incidence of poor outcomes in patients receiving general anesthesia.
The field of anesthesiology is full of examples in which it is unclear whether our current practice improves or worsens patient outcome. The various techniques used to decrease pulmonary aspiration all have logical explanations, but medicine is full of examples in which a logical treatment was found to worsen patient outcome. Use of aprotinin during cardiac surgery,12 striving to achieve tight glycemic control,13 and the liberal use of perioperative β-blockade14 are just a few examples of practices that now have evidence suggesting that we may have been causing more harm than benefit. Therefore, we must continuously examine the quality of evidence that either supports or refutes our practices and continue to look for evidence to support or dispute our logical assumptions. Ehrenfeld et al. have provided new information that can help to guide future research in assessing the existing practice of modified RSII. Going forward, it will be important to focus on relevant outcomes so that we are not led astray by surrogate end points.
Name: Michael D. Maile, MD.
Contribution: This author helped prepare the manuscript.
Name: James M. Blum, MD.
Contribution: This author helped prepare the manuscript.
This manuscript was handled by: Sorin J. Brull, MD, FCARCSI (Hon).
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