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• W3000761282 abstract "To the Editor I read with great interest the review on cricoid pressure (CP) by Zdravkovic et al.1 While I wholeheartedly agree with the authors statements that it is remarkable for CP having remained the de facto standard for so many decades despite lack of evidence for its efficacy and that current data are insufficient to impose guidelines on its use, I take issue with a few of their interpretations of published data and proposed recommendations for future studies in this area. The review includes Figure 2 of the report by Sellick,2 which has often been, and still is, viewed as providing evidence for the effectiveness of CP. It depicts a lateral x-ray of the neck that shows CP-induced obliteration of the lumen of a latex tube distended with contrast medium to a pressure of 100 cm H2O at the level of fifth cervical vertebra. The use of contrast material and luminal distention of the esophagus and, considering the radiologic technique at the time of publication, the likely use of fluoroscopy optimized the production of an image demonstrating effective occlusion of the esophagus by CP. In addition, the x-ray was performed in an anesthetized and paralyzed patient during neck extension (to stretch the esophagus and prevent its lateral displacement during CP application), the applied force was not measured, and the means by which the obliteration was obtained cannot be identified on the image. In summary, the figure does not reflect clinical conditions during rapid sequence induction (RSI) of anesthesia with the head in the sniffing position and is unsuited in predicting the effects of CP application during RSI of anesthesia. The study by Noll et al3 can hardly be taken as documenting a clinically relevant benefit of CP training. Specifically, 100 clinicians applied CP on a Vernier force plate simulator. Forces were measured at 4 time points over 60 seconds (twice before and after loss of simulated consciousness). This constituted a “cycle.” Subsequently, a subset of 40 participants voluntarily underwent education and self-regulated practice, and then performed high-frequency simulation-based training consisting of thirty 1-minute cycles. “Successful” performance was defined as generation of 10 ± 5 N and 30 ± 5 N before and after loss of simulated consciousness, respectively. Although education, self-regulated practice, and high-frequency simulation-based training improved the overall success rate from 1.3% at baseline to 16%, and to 45% during the last 4 of 30 cycles, not a single participant achieved the predefined threshold for proficiency. The authors concluded, “… guideline writers should recognize that recommendations regarding levels of force during CP may not be easily taught or achievable by humans, even with technologically advanced training.” The considerable limitations of training of CP application explain its repeatedly documented marginal or absent benefit. The benefit of training is exclusively defined as improvement in producing cricoid forces in the range of recommended values (almost universally 30–40 N) under highly controlled experimental conditions using various types of laryngotracheal models and devices. Obviously, training-induced improvement under entirely nonclinical conditions cannot be equated with improved efficacy (ie, reduced incidence of gastric regurgitation and/or pulmonary aspiration), let alone the safety of CP application in the clinical setting. Even in case of some immediate benefit of a single ex-vivo training session, the skill of “correct” experimental CP application seems to be retained for only a very limited time of about 4 weeks.4 Although laryngoscopy is routinely performed during CP application, it is not part of CP training. It is implicitly assumed (without having ever been proven) that the recommended “ideal” range of cricoid forces of 30–40 N can be applied and maintained during laryngoscopy. The study by Trethewy et al5 proves this assumption to be wrong. In patients of an emergency department, a platform with a weighing scale and a mounted liquid crystal display was used to measure CP during RSI. In one of 2 groups, the CP operator monitored the digitally displayed cricoid forces and aimed at maintaining the CP between 3.060 and 4.075 kg (corresponding to a force of 30–40 N) throughout the RSI. In the control group, the CP operator was blinded to the displayed force. During the induction phase of the RSI until the start of laryngoscopy, the mean applied CP was maintained within the prescribed range of values in both groups. However, during the intubation period, the mean CPs decreased to below the prescribed low range value of 3.060 kg in both groups (2.75 and 2.86 kg in the monitored and control group, respectively). As the prescribed “ideal” range of CPs could not be maintained during laryngoscopy in either group, the trial was prematurely terminated after a total of only 54 patients instead of the priori calculated 106. The findings imply that the forces generated during laryngoscopy counter the cricoid forces to the extent that prevents the generation of “ideal” cricoid forces even with real-time force monitoring. The study results were interpreted as providing the final nail in the coffin of CP.6 It remains to be seen whether videolaryngoscopy will be associated with less counterforces. The study results of Taylor et al7 do not show that a cricoid cartilage compression device would minimize the variability in the localization and amount of applied CP force without requiring prior training. During 114 attempts, the target force of 30 N was achieved in only 15 (13%), and a range of forces of 25–35 N in only 35 attempts (31%). These less-than-optimal results occurred despite highly controlled experimental conditions (ie, application of cricoid force on a CP training simulator by practitioners familiar with both device and simulator). It is likely that the results will be even less favorable when CP is applied under less controlled conditions in humans with highly variable neck anatomy. Contrary to the authors’ statement, the study results of Birenbaum et al8 do, in fact, show a difference in Cormack-Lehane grades between groups with lower grades in the sham than in the CP group (P < .001). Furthermore, the grades improved after CP interruption in twice as many patients of the CP compared to the sham group (P < .001). This is in agreement with the experience of many practitioners that laryngoscopic view improves on reduction or release of CP. In addition, there was also evidence for more difficult intubating conditions in the CP compared to the sham group. First, more than 2 tracheal intubation attempts and use of “other” intubation techniques were more frequent in the CP than in the sham group (P < .05). Second, CP was interrupted in almost three times more patients of the CP than of the sham group (P < .001), suggesting interference of CP with laryngoscopy. Birenbaum et al8 themselves concluded that the secondary end points suggested more difficult tracheal intubation in the CP group. So did the accompanying editorial.9 With a total of 3471 participants, this is the largest controlled study on the effectiveness of CP to date. CP was applied by operators highly trained in performing this maneuver. It could be argued that the study population was at (too) low risk for pulmonary aspiration. However, RSI was performed in patients who satisfied the generally accepted criteria for such. Most patients had 2 or more criteria for high risk of aspiration. Despite several methodological limitations (underpowered to detect a difference in pulmonary aspiration between groups; debatable definition of pulmonary aspiration; no information regarding start of CP application; unmeasured cricoid forces; uncontrolled practice of removing or leaving in place a nasogastric tube before induction of anesthesia, and of administering an antacid before anesthesia), the study nevertheless documents lack of benefit of application of CP with simultaneous interference with optimal airway management. For entirely anatomic reasons, application of CP must be expected to interfere with optimal airway management.10 The authors make a plea for assuring an “optimal” CP application technique. This is a rather futile plea because the lack of proven efficacy of numerous CP application techniques precludes defining an “optimal” technique, which makes it per se doubtful that the proposed recommendations will “… obtain the optimal scientific evidence on the effectiveness and safety of CP.” The authors suggest that “appropriate” and consistent application of CP of 30 N should be taught. The continued emphasis on the cricoid force to be applied rather than on the postulated clinical benefit of CP application has always been and remains a mystery to me. The recommended “optimal” forces are mostly derived from investigations in infant and adult cadavers performed in the 1970s and 1980s and from theoretical considerations. Considering the large variations in neck anatomy and, at times, in intraluminal esophageal pressures (induced by regurgitation and vomiting), different cricoid forces will be required to effectively occlude the upper esophagus—provided an “optimal” CP will ever be established that would predictably occlude the upper esophagus in all patients. However, “for now, it seems that 30 newtons of cricoid pressure is not worth a pound of a cure.”9 The review by Zdravkovic et al1 is yet another one to acknowledge the lack of scientific evidence for the benefit of CP application. How much more (lack of) evidence needs to be provided before CP application will be abandoned altogether? CP application appears to be more of a ritual than an effective clinical measure.11 Hans-Joachim Priebe, MD, FRCA, FCAIDepartment of Anesthesiology and Critical CareMedical Center University of FreiburgFreiburg, Germany[email protected]" @default.
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• W3000761282 title "Lack of Evidence for the Benefit of Cricoid Pressure" @default.
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