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Better understanding of the effectiveness of cricoid pressure and the rapid sequence induction

Dear Editor, As clinicians with vested interest in cricoid pressure (CP), we read with keen interest the publication entitled “Pressure measurement in the upper esophagus during cricoid pressure: A high‐resolution solid‐state manometry study”.1 The authors justified their report, which is an “overshoot” of a previously published 2012 study, on the basis that “it is difficult to determine whether CP can occlude the esophageal lumen”.2 They added, “the variability in the methods and approaches in previous studies, make comparisons difficult”. The authors asserted that high‐resolution solid‐state manometry (HRSM) has several advantages including enabling exact measure‐ ments of the intended areas and eliminating errors that could result from catheter sliding. Modern modalities and instrumentation have provided undisputable evidence of the effectiveness of CP in oc‐ cluding the esophageal entrance (post‐cricoid hypopharynx).3 This can be easily visualized by performing CP prior to tracheal intuba‐ tion while using the Glidescope® Video Laryngoscope, or similar devices. The panoramic view will allow easy viewing of closure of the esophageal entrance during CP.4 The insinuation that HRSM is far superior to other manometric methods is a misleading exag‐ geration. Distinguished features between the two methods include catheter design and data display. In HRSH, the catheter has multi‐ ple (up to 30) pressure sensors, whereas conventional manometry catheters typically have 5 sensors (which are adequate for studying the influence of CP on the esophageal sphincters).5 Although some studies demonstrated a quick learning curve with both manometric measurements, other studies documented various learning curves with HRSM and a majority of trainees did not achieve competency in interpreting the results, even after 50 cases. In their study, the authors reported that they used a modified rapid sequence induction. However, from their description, it is ap‐ parent that no muscle relaxants were given. It is common knowledge that in both the original and modified rapid sequence induction, mus‐ cle relaxation is an important component of the technique.6 In fact, the principle of use of CP is to substitute for the loss of the upper esophageal sphincter tone associated with muscular relaxation. The upper esophageal tone should be close to 0 mm Hg, with complete muscular relaxation. With intact upper esophageal tone in the cur‐ rent study, interpretation of the effect of CP becomes doubtful. Many factors can influence the effectiveness of CP, including the location of the esophageal entrance relative to the cricoid cartilage and vertebral body. It has been hypothesized that a 30 Newton force should provide a pressure around 200 mm Hg under the 10 cm2 area of the lamina of the cricoid cartilage. When the esophageal entrance is located in a midline position (approximately one third of subjects), it becomes squeezed between the cricoid cartilage and vertebral body and a cricoid force as low as 10 N can result in total occlusion. In contrast, if the esophageal entrance is in a lateral position, it will be pressed against the longus coli muscle during CP (rather than the vertebral body) and a greater force is required to achieve occlusion.7 Thus, the use of 30 N in men (and possibly 25 N in women), is recom‐ mended to ensure occlusion of the esophageal entrance regardless of its location.