The use of cricoid pressure is a polarizing issue. It is not uncommon for experts in emergency airway management to come close to blows when discussing the efficacy of Sellick's famous maneuver. Until recently, there has been a paucity of data to support or discredit its use, but the publication of the IRIS trial represents the first large, high-quality empiric examination of this historic piece of airway dogma.
The IRIS (Sellick Interest in Rapid Sequence Induction) trial examined using cricoid pressure in surgical patients undergoing rapid sequence induction (RSI) at 10 centers across France. (JAMA Surg 2018 Oct 17. doi: 10.1001/jamasurg.2018.3577; http://bit.ly/2zuJKid.) The authors enrolled 3,472 adult patients requiring RSI for any type of surgical procedure under general anesthesia who were considered to have a full stomach (less than six hours of fasting) or at least one risk factor for pulmonary aspiration. Patients were randomized to receive a sham procedure or true cricoid pressure, defined as an expected pressure equivalent to 30 newtons applied using the first three fingers on the cricoid cartilage. Operators were trained to apply the procedure correctly before being permitted to participate in the study.
This study was technically negative, but the authors reported the sham cricoid procedure failed to demonstrate noninferiority when compared with the true Sellick technique. This is most likely an error in the authors' statistical analysis rather than any clinical benefit of cricoid pressure. The rate of primary endpoint, pulmonary aspiration (detected at the glottis level during laryngoscopy or by tracheal aspiration just after tracheal intubation), was essentially identical in both groups. It occurred in 10 patients (0.6%) in the Sellick group and in nine (0.5%) in the sham group. The rates were similar for suspected pneumonia within 24 hours of intubation (0.9% vs 0.6%), aspiration pneumonia (0.2% vs 0.2%), and severe pneumonia (0.1% vs 0.1%).
The only element that was noticeably different between the groups was the difficulty of intubation. Patients randomized to the cricoid pressure group had a higher incidence of grade 3 and 4 Cormack and Lehane views. Interruption of the maneuver also occurred more frequently in the cricoid pressure group. Abandoning these attempts more often improved the view after its release. The cricoid pressure group required longer times to intubation and more frequently experienced intubations exceeding 30 seconds. The incidence of difficult tracheal intubation did not reach statistical significance, but it was numerically higher in the cricoid pressure group (72 v. 51).
This was a negative trial from a frequentist perspective because of the form of hypothesis testing utilized in the primary analysis. The authors used a noninferiority trial design, which asked a different question from the traditional superiority trials to which we are accustomed. Rather than presenting a null hypothesis that states no difference between the groups, the noninferiority trial design operates under the assumption that the novel intervention is inferior to the standard treatment. The alternative hypothesis states that the treatment options are equivalent. The novel treatment must demonstrate a nearly equal efficacy within a degree of certainty to reject the null hypothesis. This means the point estimate and surrounding 95% confidence interval must fall above an a priori selected noninferiority margin. (JAMA 2015;313:2371; http://bit.ly/2F2lN8c; Ann Intern Med 2006;145:62.)
The authors in this case designated the true cricoid pressure group as the established approach and the sham maneuver as the novel comparator, and in doing so, designed a trial in which cricoid pressure could not fail. The sham group at worst would be found to be noninferior to the traditional approach. The authors chose an inferiority margin of 50 percent worse than the cricoid group, or a relative risk of 1.5. They predicted that the rate of their primary endpoint would occur in 2.8 percent of the patients in the cricoid pressure arm based on previous literature, meaning the sham control group could have a rate of aspiration no greater than 4.2 percent to be considered noninferior.
The actual rate of aspiration events in the cricoid pressure group was far lower than the authors anticipated (0.6%). The rate of aspiration in the sham control group was numerically lower at 0.5 percent and clinically equivalent due to the paucity of aspiration events, but the confidence interval surrounding this outcome was larger than anticipated. Despite the relative risk of aspiration falling in favor of the sham group at 0.9, the confidence interval surrounding this point estimate crossed the noninferiority margin (95% CI, 0.33-2.38).
We utilize frequentist statistics as a tool to estimate the risk of sampling error in any given cohort. But its single-minded dichotomous temperament at times limits our ability to interpret what is otherwise in plain sight. From a frequentist perspective in the IRIS trial, we are unable to demonstrate the noninferiority of sham cricoid pressure. A simple inspection of the results, however, demonstrated that true aspiration events are uncommon and cricoid pressure fails to prevent them.
Our futile attempts to prevent this rarity also actively thwart our own efforts to secure an airway. The applicability of these results to the ED or ICU patient population where the risk of aspiration is much higher is unclear, but given the obvious harm demonstrated in this study, the onus should now fall on us as clinicians to demonstrate its utility. Until then, it should be considered nothing more than a melancholy specimen of medical nostalgia.