Cricoid pressure was first described in the modern medical literature in 1961  and the maneuver was named after its inventor (Sellick's maneuver). However, this classic report actually represented a rediscovery of a technique first used and described in Britain in 1774 as a means of preventing gastric distention and regurgitation during resuscitation of drowning victims . Sellick's maneuver is an effective method of preventing the aspiration of passively regurgitated fluids from the stomach in both adult and pediatric patients by occluding the esophagus with posterior displacement of the cartilaginous cricothyroid ring [1,3,4]. Sellick suggested that firm pressure would successfully accomplish the desired effect but made no effort to quantify the force . Others have suggested pressure "sufficient to cause pain if applied to the bridge of your nose ." Studies using a cricoid yoke in anesthetized tracheally intubated patients or cadavers have determined that the optimal amount of force necessary to achieve the occlusion of the esophagus without obstruction of the trachea is between 30 and 44 newtons (N) [6-8]. In addition, a recent study by Vanner et al.  has demonstrated a rapid loss of upper esophageal barrier pressure with intravenous induction of anesthesia, before loss of consciousness. This led them to suggest early definitive cricoid pressure. However, 40 N of force is uncomfortable to the awake patient and may induce vomiting. They recommended placement of a moderate amount of cricoid force, i.e., 20 N, prior to loss of consciousness and increasing to 40 N after loss of consciousness .
It is not in the routine experience of the average anesthesiologist to measure force in newtons. Most anesthesiologists surveyed for this study did not know the definition of a newton. This makes training of the cricoid pressure maneuver to a recommended force difficult and haphazard at best. No commercial system is presently available to teach the recommended level of force for cricoid pressure. Studies with a test rig consisting of a 60-mL syringe connected to a pressure manometer have been performed that reported 47% of anesthesiologists and assistants do not attain the recommended level of force considered to be safe in at risk patients . This apparatus, however, does not adequately simulate the patient anatomy and correct hand position. Use of a weighing scale to practice quantifying applied force has been previously suggested . No study has been presented demonstrating its efficacy in teaching the proper amount of cricoid pressure.
The goals of this present study were to survey the adequacy of cricoid pressure in the anesthetists and assistants in a major university anesthesiology department using a laryngotracheal model and a tared infant scale and, in addition, to determine whether this system could be used to teach the recommended force and whether this learned skill could be retained over time.
Fifty-three anesthesia providers, residents rotating from other medical or surgical services (MD), medical students, obstetric (OB) nurses, and operating room (OR) technicians were recruited to participate in this four-phased teaching/testing protocol. The participants were separated into groups dependent upon their level of training in an attempt to assess whether educational training bias affected the ability to learn the recommended intensity of cricoid pressure: MD anesthesiology faculty, anesthesiology residents (CA-1, CA-2, and CA-3), certified registered nurse anesthetists (CRNAs), and others (OB nurses, offservice MD rotators, medical students, and OR technicians).
A teaching model for assessing the force of cricoid pressure consisted of a life-size laryngotracheal model and a tared infant scale (Model N-10; Air-Shields Vickers, Hatboro, PA). The scale readout was programmed so that pressure applied to the cricoid ring of the model was registered in kilograms. The kilogram value for force obtained for each participant test were converted to newtons using the conversion factor 1 kg = 9.8 N.
Each participant was first briefed on the purpose of the study and asked to commit to the full extent of the study (to the best of their ability) which included a 3-mo follow-up period to assess technique retention. After consent, each was asked to apply cricoid pressure to the model, first to simulate the level of force applied to an "awake" patient and then to the "unconscious" patient. The participants were blinded to the results of these Preinstructional values. Each individual was then informed of the recommended level of force to be given to a patient both before and after unconsciousness (20 and 40 N, respectively). Each was again asked to apply cricoid pressure to the blinded model in light of these recommendations (Postinstructional test). The scale was then unblinded so that each study participant could view the force applied. Each was instructed that 2 and 4.1 kg of measured force on the scale represented 20 and 40 N, respectively, and then allowed to practice as long as desired until each felt confident of the ability to replicate these recommended pressures. After completion of the practice phase, the participants were again blinded to their results and asked to apply "awake" and "unconscious" cricoid pressure to the model to obtain Post-practice values. Three months after the initial phase of this protocol, each participants was asked to return to obtain Follow-up values. Each was again blinded to the scale, reminded of the recommended levels of cricoid pressure, and asked to apply these levels (Follow-up values).
Data obtained for this study were height of each participant and newton values of force "awake" and "anesthetized" for the four different stages of the protocol (Preinstructional, Postinstructional, Postpractice, and Follow-up). Mean values (+/- SE) were calculated for each of the six participant groups (MD Faculty, CA-1 Residents, CA-2 Residents, CA-3 Residents, CRNA, and Others) for each of the four testing phases. Two-way repeated measures analysis of variance (ANOVA) was performed to assess intergroup differences at each testing stage and intragroup differences throughout the course of the protocol. The a priori level of significance was established at P <or=to 0.05. Significant ANOVA differences were further evaluated by multiple comparison analysis using Bonferroni's method to detect the specific differences between or within groups .
The 53 participants in the study were unevenly dispersed into the six groups preestablished prior to the start of the protocol. There were 9 MD faculty, 20 anesthesiology residents (9 CA-1 residents, 5 CA-2 residents, and 6 CA-3 residents), 9 CRNAs, and 15 Other0s (including 1 off-service MD rotators, 3 medical students, 9 OB nurses, and 2 OR technicians). Forty-three of the study participants (81%) were retested at 3 mo after the initial session. The five missing enrollees in the resident groups (2 CA-1, 2 CA-2, and 1 CA-3) and five in the Other group were lost to Follow-up because of away rotations, graduation, termination, or were off-service.
The height of each study member was measured to assess whether stature was a factor in the placement of cricoid pressure because model height was fixed. The heights, in inches, were: MD faculty, 66.9 +/- 1.3; CA-1 residents, 67.0 +/- 1.3; CA-2 residents, 68.2 +/- 2.6; CA-3 residents, 71.2 +/- 1.5; CRNA, 65.8 +/- 1.2; and Others, 66.1 +/- 0.5. No significant differences were detected for the heights of the participants between any of the groups using single-factor ANOVA. Therefore participant height was not considered a factor influencing the subsequent outcome of this study.
The data for the cricoid force applied in the "awake" scenario for each group and for the cumulative values of all the groups are displayed in Figure 1. The cumulative Preinstruction (baseline) force (16.2 +/- 1.7 N) was less than the 20 N pressure recommended by Vanner et al.  in the awake patient. The MD anesthesiology faculty recorded the lowest values for force in the initial survey phase at 6.6 +/- 2.0 N. Several of these attending faculty members exerted little to no force when asked to give cricoid pressure to a patient who was awake just prior to induction and confirmed that they did not under actual clinical conditions.
The measured force for all participants after being told that the recommended cricoid force for the awake patient was 20 N was then obtained (Postinstruction). The mean force of 20.3 +/- 1.7 N was on the mark; however, this belies the findings seen with the MD faculty. The force sustained in this group of experienced anesthesiologists was well below the pressure they were asked to achieve.
After a period of practice with the model and scale, the Postpractice cricoid pressure was measured. The mean pressure 22.2 +/- 0.1 N was again on target with the recommended force and had a smaller SE because the MD faculty successfully learned to recreate this amount of pressure on the model. Follow-up testing yielded a cumulative mean of 26.7 +/- 1.7 N, with all groups surpassing the 20 N mark. There was a greater SE in all groups when compared with the Postpractice values.
There was a statistical difference (P <or=to 0.05) between the Preinstructional and the Postpractice and Follow-up values for cumulative data for the "awake" scenario (see Figure 1). Otherwise there were no other statistical intragroup or intergroup differences noted for either the cumulative data or the individual groups.
The applied cricoid force data obtained for the "anesthetized" scenario, i.e., after intravenous induction and loss of consciousness, are presented in Figure 2. The cumulative mean for all groups in the Preinstruction phase of the study was almost 20 N less than the recommended force for esophageal occlusion and 10 N less than the lower limit of an accepted range of 30-44 N. Again the attending faculty value was very low at 12.5 +/- 1.8 N. No single group reached into the lower accepted limit of 30 N.
After the verbal instruction of the recommended value of 40 N in the anesthetized patient, the overall mean increased to 30.8 +/- 2.1 N (Postinstruction). This was within the lower accepted range. The CA-1, CA-2, CRNA, and Other groups all had mean values within the range; however, no group reached the recommended value of 40 N.
After a period of practice, the pooled participant mean value was at 39.0 +/- 1.4 N, statistically indistinguishable from the recommended value. The knowledge and technique gained by this teaching session was maintained at the 3-mo Follow-up, with a mean cumulative pressure of 38.6 +/- 2.9 N.
As in the "awake" scenario, there were no significant intergroup differences for any of the phases of the "anesthetized" arm of this study. There were intragroup differences detected by the Bonferroni multiple comparison test (see Figure 2) between the Preinstruction phase and the Postpractice and Follow-up phases for both the MD faculty and Other groups (P <or=to 0.05). There were also significant differences detected between the following: The Preinstruction phase and the Postinstruction, Postpractice and Follow-up phases; and the Postinstruction phase and the Postpractice and Follow-up phases for the cumulative means of all the participants (P <or=to 0.05). No differences were found between the Postpractice and Follow-up portions of the study for any of the individual groups or the total cumulative means.
The results of this study suggest that the application of cricoid pressure can be learned over a short period. In addition, after a teaching session, the knowledge is fairly well retained after three months.
Most of the study members had no knowledge of the recommended force of 20 N and 40 N, before and after induction, respectively. Most were taught the Sellick maneuver using the relative comparison method, i.e., "sufficient to cause pain if applied to the bridge of your nose" . No group individually nor all participants in total applied adequate pressure in the "anesthetized" scenario (Figure 2, Preinstruction). These results suggest that this former method of teaching cricoid force is inadequate.
The "anesthetized" cricoid pressure data suggest another important finding: Few participants knew that a newton was a unit of force and no one knew how much force it represented. When given the appropriate level prior to the Postinstruction attempt, the participants significantly increased their applied force (P <or=to 0.05 in the "anesthetized" scenario), some even into an acceptable range. However, none reached the recommended force of 40 N necessary to assure esophageal obstruction  until after they were allowed to practice applying the necessary amount of force. It probably is less important for anesthesia practitioners and assistants to know that a newton is a unit of force required to accelerate a 1-kg mass by 1 m centered dot s-1 centered dot s-1 than it is to know practically how much force it takes to achieve the proper esophageal occlusion. Therefore, future researchers in this area should not assume universal understanding of the newton and instead attempt to teach their colleagues an important and practical lesson.
There were no differences between the three training levels of anesthesiology residents for any of the test periods of this study. All residents, junior and senior, were receptive and easily trained using this instruction technique. The same was found for all the varying participants in the Other group.
The suboptimal early results in the attending faculty group for both the "awake" and "anesthetized" scenarios were illustrative of the pervasive need for an easy and reliable method of teaching the recommended amount of cricoid pressure to all anesthesia practitioners, not simply residents and assistants. Since the attending anesthesiologist is most likely to be the one holding cricoid pressure while young, eager (and less experienced) residents and medical students learn to intubate patients, the results here suggest that they are an important target group. This is particularly important in light of the newer evidence of the loss of upper esophageal barrier pressure before loss of consciousness after intravenous induction of anesthesia .
The results after three months show a trend toward an increase in the amount of applied force, especially in the "anesthetized" scenario. Too much cricoid pressure in an anesthetized patient can distort and/or occlude the trachea. It would appear advisable for practitioners and assistants to use the model every three to six months to refresh their technique.
In summary, an effective and easy model for instructing, practicing, and refreshing the recommended force to apply and achieve esophageal occlusion using the Sellick maneuver was presented. The use of an infant scale and laryngotracheal model, items generally present at most hospitals, makes this an easily assembled teaching tool. Even though the plastic laryngotracheal model may not have the compressibility and elasticity of the actual human laryngeal cartilage and neck, it allows one to better obtain a more quantifiable and reproducible means of assessing effort. The laryngotracheal model is practical for teaching the suggested amount and position of cricoid force, as well as for examining airway anatomy.