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Patient Safety: Original Clinical Research Report

The Cricoid Force Necessary to Occlude the Esophageal Entrance: Is There a Gender Difference?

Zeidan, Ahed M. MD*†; Salem, M. Ramez MD‡§; Bamadhaj, Munir MD; Mazoit, Jean-Xavier MD, PhD; Sadek, Hussein MD*#; Houjairy, Hassan MD*; Abdulkhaleq, Kamal MD**; Bamadhaj, Nabil IT Specialist††

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doi: 10.1213/ANE.0000000000001631
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Since the introduction of cricoid pressure (CP) into clinical practice,1 investigators have attempted to quantify the applied cricoid force (CF) necessary to prevent regurgitation. Although Wraight et al2 suggested the use of at least 44 N, follow-up studies revealed that far less CF was effective in occluding the esophageal lumen.3,4 Excessive force can produce deformation of the cricoid cartilage, apposition of the vocal cords, interfere with the passage of a tracheal tube, and may result in ineffective gas exchange.2,5–11 Thus, the appropriate use of CP demands the application of an adequate force to prevent regurgitation without causing airway-related complications.

Because of differences in the size of the cricoid plates12 and the reported increase in the incidence of airway obstruction with the use of 30 N in women,6,13 it is conceivable that women actually may require less CF to occlude the esophageal entrance compared with men. We have observed that occlusion of the esophageal entrance tends to occur with less CF in women. The primary aim of this investigation is to test the hypothesis that gender differences exist in the applied CF necessary to prevent regurgitation. Real-time visual and dynamic means were used to assess the effectiveness of different CFs in occluding the esophageal entrance in men and women. The patency of the esophageal entrance was directly visualized with Glidescope video laryngoscope (GVL; Glidescope, Verathon Co, Canada) in anesthetized and paralyzed patients. Evidence of closure of the esophageal entrance was confirmed by the inability to introduce a gastric tube (GT) into the esophagus under direct vision.14 In this study, we used the Dixon and Mood up-and-down sequential allocation technique to calculate the median cricoid force (CF 50, the force that occludes the esophageal entrance in 50% of patients) in men and women. A new accurate method for measuring the applied CF was used in the current study.


This protocol was approved by the Research and Ethical Committee of Procare Riaya Hospital, Al-Khobar, Saudi Arabia ( identifier: NCT02618863). Three anesthesia operators participated in the study: the first operator, who was in charge of the overall anesthetic management, directed the study; the second operator performed CP; and the third operator performed both laryngoscopy using the GVL and the GT insertion trials. Training of the operators performing CP and GT insertions commenced 3 months before the study began. Competence in use of CP was assured by 10 consecutive applications of 20 N and 10 consecutive applications of 30 N force (±2N) on a weighing scale. Training of the GT insertions was completed after demonstration of successful insertion of GT (20 F) with the aid of the GVL in 20 patients. The operator also was trained to use the same force in introducing the GT in all the insertion trials.

Informed consent was obtained from 73 patients. Subjects were divided into 2 groups: group 1, 39 men and group 2, 34 women. Inclusion criteria were American Society of Anesthesiologists physical status I and II patients, ages 19 to 50 years, body mass index < 28 kg/m2, scheduled to undergo surgical operations requiring general anesthesia and endotracheal intubation. Exclusion criteria were patients who had symptoms suggestive of gastroesophageal reflux; respiratory or cardiac diseases; anticipated airway difficulties; inability to visualize the esophageal entrance; changes in mean arterial blood pressure or heart rate of >30%; arrhythmias requiring treatment; oxyhemoglobin saturation decreasing to <95% measured by pulse oximetry; and when it became apparent that the study period would last longer than 2 minutes.

After the administration of midazolam (2–3 mg), preoxygenation was commenced by the use of 10 L/min of oxygen, in a semiclosed circle absorber system. When maximal preoxygenation was achieved (end-tidal oxygen >90%), anesthesia was induced with propofol 2 mg/kg, fentanyl 0.002 mg/kg, and sevoflurane in oxygen. Cisatracurium 0.2 mg/kg was given to provide muscle relaxation. Gentle mask manual ventilation (peak airway pressure < 20 cm H2O) was then initiated after the placement of an oropharyngeal airway. Neuromuscular blockade was monitored by eliciting mechanically evoked responses (thumb adduction) with ulnar nerve stimulation by use of Neuromuscular Transmission Module E-NMT (Datex-Ohmeda-GE Healthcare Monitor). A decrease in twitch height by 90% or disappearance of 3 twitches using the train-of-four mode was indicative of adequate muscle relaxation. Direct laryngoscopy using the GVL (blade size 4) was performed, and the view was video recorded. The first operator, who stood behind the patient, signaled to the second operator to apply or release CP.

In this investigation, a novel instrument was used to measure the applied CF (cricometer, applied patent number 114360056, [email protected], KSA; Figure 1). It consists of a portable, small electronic device, which includes a thin flexible force sensor (Interlink Electronics Inc, FSRTM 406, Force Sensing Resistor TM USA. Size 43.69 ×43.69 mm; thickness 0.4 mm; force measurement range 0.1–102 N; force sensitivity calibration and accuracy range ±1–2 N). The sensor is connected to a central electronic unit, which contains a microprocessor, small liquid-crystal display screen, and a power supply source.

Figure 1.
Figure 1.:
The used instrument in the study: cricometer.

Before anesthetic induction, the operator rolled his fingers from the thyroid cartilage in a caudad direction until the cricoid cartilage was found. The sensor was fixed to the skin over the cricoid cartilage. The selected force was applied on the sensor via the 3-finger maneuver as described by Sellick.1 The microprocessor analyzes the change in the signal instantly, converts it to Newtons, and the force is displayed on the liquid-crystal display screen. The desired CF was reached by the operator adjusting the force of his fingers on the sensor. Before use, the accuracy of the device was determined as follows: the sensor was fixed on an anatomical model placed on a weighing scale and a predetermined force was applied on the sensor. When the device and the weighing scale showed similar readings, the accuracy of the device was verified.

CP was performed with the operator using his dominant (right) hand while standing to the right side of the patient with his back toward the video monitor. The hand was maintained in the same position whether or not CP was applied. The third operator, who attempted the GT insertions, was unaware of the applied CF used. To blind the operator, a screen separated him from the operator performing the CP maneuver. Visualization of the esophageal entrance was a prerequisite for attempting cannulation. In each patient, a trial was made to introduce a lubricated GT (20 F, SMMP Co, Riyadh, Kingdom of Saudi Arabia), into the esophagus during CP. Up to a maximum of 3 attempts (5 s/attempt) were conducted before cannulation was considered a failure. Successful insertion of the GT, indicative of a patent esophagus, was defined as the insertion of the GT 15 cm beyond the esophageal inlet.

The first patient in each group received a 20-N force. The applied CF in successive patients was determined by the response of the previous patient within the same group, using an up-and-down sequential allocation technique.15–17 The adjustment force interval was 2 N. Successful insertion of the GT with 20-N force was considered ineffective CP, and the next patient received 22 N force. When the effective applied CF was reached, the force was reduced by 2 N in the patient that followed. Conversely, an unsuccessful GT insertion with 20 N cricoid force was considered effective CP, and the next patient received 18 N. In patients in whom cannulation of the esophagus was possible, no more attempts of cannulations were made. In all patients in whom cannulation could not be done (effective CP), cannulation was attempted again following the release of CP, and the result was noted. Tracheal intubation was performed with 7.0 or 7.5 mm (ID) endotracheal tube while the same CF was used, initially tested, and the surgical procedure was begun. Relevant observations and complications were noted.


We calculated the CF50 (the median force in 50% of patients) and the 95% confidence interval (95% CI) in both men and women.15 The predetermined CF was applied in the first patient in each group. If the patient responded positively (successful CP), the force was decreased by 2 N for the next patient, and conversely, if the patient responded negatively (failed CP), the force was increased by 2 N for the next patient.

The number of subjects needed and the testing interval were calculated considering a 24% applied CF standard deviation similar for males and females,2 a clinically relevant effect size (difference in median value between male and female patients) of 10 N, a power of 80%, and the fact that the CI of the median would never be greater than twice the testing interval when the up-and-down technique was used.15–17 This calculated number of patients in each group (20) was increased to 30 because the starting point might have been far from the median cricoid force (CF 50).17 The 2 groups (male versus female patients) were compared with the Student t test. Demographic data are reported as mean ± SD.


From the 73 patients initially recruited for the study, 13 (9 men and 4 women) were excluded because of difficulty in visualizing the esophageal entrance before the study began. The demographic data of the patients that completed the study (30 men and 30 women) are shown in Table. Despite the differences in height and weight, the body mass index was similar in both groups. Manual ventilation was accomplished easily after placement of an oropharyngeal airway. There were no instances of unexpected difficult intubation in the 60 patients who qualified for the study.

Demographic Data and Results

Figure 2 shows the effectiveness of the applied CF in occluding the esophageal entrance. The CF 50 necessary to occlude the esophageal entrance in men was 30.8 N (95% CI = 28.15–33.5), whereas it was 18.7 N in women (95% CI = 17.1–20.3; P < .0001, Student t test; Figure 3). Patency of the esophageal entrance was observed when CP was not applied and also was observed when an inadequate applied CF was used, a force that permitted esophageal introduction of a GT. The use of an effective CF that prevented esophageal cannulation was always associated with inability to visualize the esophageal entrance because of its closure. In all 28 patients (12 men and 16 women), where CP effectively occluded the esophageal entrance, a GT was easily inserted into the esophagus after the release of CP.

Figure 2.
Figure 2.:
Images demonstrating the effect of CP when the esophageal entrance is in a middle position without CP (A) and with CP (B), and inability to introduce GT (20 F) into the esophagus with CP (C) and the easy insertion of the GT without CP (D). CP indicates cricoid pressure; GT, gastric tube.
Figure 3.
Figure 3.:
CF necessary to occlude the esophageal entrance in males (top) and females (bottom; median and 95% confidence interval). Filled circles, Effective CF as evidence by inability to introduce a GT; open squares, failed CF. CF indicates cricoid force; CP, cricoid pressure; GT, gastric tube.

There were no complications related to the anesthetic management, use of the GVL, CP, esophageal placement of the GTs, or tracheal intubation. The time required to obtain the data was <2 minutes in all the patients studied.


This investigation provides evidence that the median CF necessary to occlude the esophageal entrance in anesthetized and paralyzed patients is less in women than in men. The median CF that occluded the esophageal entrance was 18.7 N in women and 30.8 in men. Previous investigations did not adequately address gender differences and the applied CF. In an early study of 12 men and 12 women, Wraight et al2 reported that the CF required to occlude the esophagus is independent of sex; however, in their study, the operators performing the maneuver remarked that they would not use as much force in a female as in a male patient.2

The use of excessive force, or the routine use of 30 N in women, may be accompanied by various degrees of airway obstruction.6 This can make ventilation difficult, cause difficulties in placing an endotracheal tube or a laryngeal mask airway, and alter visualization by a fiberoptic scope.5–10,18–21 Airway obstruction induced by CP can occur at the level of the glottis, cricoid cartilage, or both.6 Vocal cord tightening (approximation of the cords) occurs because of posterior displacement of the arytenoids.6,9 Deformation of the cricoid cartilage also may occur, reducing its anteroposterior diameter.6 Because the anteroposterior diameter of the cricoid cartilage is smaller in women (13.9 ± 1.8 mm) compared with men (17.6 ± 1.9 mm), deformation of the cricoid cartilage is more likely to occur in women than in men.6,22 It is conceivable that during CP, especially in women, an endotracheal tube may readily pass through the glottis but cannot be advanced into the trachea.6

The finding that less applied CF occludes the esophageal entrance in women compared with men may be explained by the differences in the size of the cricoid plate. Although cricoid configuration varies substantially from patient to patient and from population to population, significant differences also exist between the sexes: the cricoid cartilage is larger in men than in women.12,23–25 Assuming that the compression area is 3 times the surface area of the cricoid cartilage,26 we calculated the CF necessary to prevent regurgitation at an intraesophageal pressure of 40 mm Hg (3 N/cm2) and 25 mm Hg (2 N/cm2) in both men (cricoid surface area = 4.1 cm2) and women (cricoid surface area = 2.87 cm2). In men, the theoretically calculated forces were 36 and 24 N to prevent regurgitation at an intraesophageal pressure of 40 and 25 mm Hg, respectively, whereas in women, the calculated forces were 25 and 17 N. Because the intragastric pressure rarely exceeds 25 mm Hg,26–29 17 N applied CF in women and 24 N applied CF in men should theoretically be adequate to prevent regurgitation. These calculated values are in the range of the median forces measured in our study, with the limitation that 32 N was ineffective in 1 male and 20 N in 1 female patient (Figure 3).

The use of the GVL yielded panoramic view of the glottis and esophageal entrance and allowed real-time assessment of the functional status of the esophageal lumen with and without CP. Patency of the esophageal entrance was observed when CP was not applied, but occlusion was observed when the appropriate applied CF was reached. Occlusion of the esophageal entrance was confirmed by the inability to cannulate the esophagus, whereas without CP and with the use of inadequate CF, cannulation was possible. In the current study, the use of the cricometer yielded accurate measurements of the applied CF. In magnetic resonance imaging study, Rice et al30 clearly demonstrated (in awake volunteers) anatomic occlusion of the postcricoid hypopharynx during CP. Our studies in anesthetized and paralyzed patients provided assessment of the dynamic status of the esophageal inlet during CP.14 We believe that both magnetic resonance imaging and GVL studies complement each other and provide evidence of the effectiveness of CP.

The current investigation has some limitations that should be recognized. Because we used the up-and-down sequential allocation methodology, the study was not randomized. Traditionally, the up-and-down methodology has been used in research related to general and local anesthetics. Advantages of this method include simplicity and smaller sample size. Sequential designs outperform nonsequential designs by having a smaller mean square error for the same sample size. In addition, it has been demonstrated that this method is more sensitive for efficacy comparison and requires less attempts compared with other traditional statistical method.31 Because of these advantages, the use of the up-and-down methodology has been extended to other research areas including efficacy of the defibrillation waveforms.31 It may be argued that measuring the CF95 would be more appropriate than measuring the CF50. It is possible that the CF95 could be higher (up to 25%) than the CF50. It is also possible that both forces could be equal; however, this study focused primarily on differences in the CF necessary to occlude the esophageal entrance in men and woman.

The study was conducted in normal adult men and women; our findings should not be extrapolated to morbidly obese patients, children, or patients with esophageal pathology, such as achalasia or esophageal pouch.28 The result should only apply to patients with normal body habitus. It may be argued that it would be preferable to have more than 1 operator performing CP and the GT insertion trials; however, to minimize the variability between operators, 1 single trained operator applied CP in all patients and another trained operator performed the GT insertions.

In conclusion, the current study provides evidence that the median force necessary to occlude the esophageal entrance to prevent regurgitation is less in women compared with men. Applying the appropriate CF in women should also decrease airway-related problems that tend to occur with the use of excessive forces.


Name: Ahed M. Zeidan, MD.

Contribution: This author helped to design and conduct the study, analyze the data, and write the manuscript.

Name: M. Ramez Salem, MD.

Contribution: This author helped to design the study, analyze the data, and write the manuscript.

Name: Munir Bamadhaj, MD.

Contribution: This author helped to conduct the study and write the manuscript.

Name: Jean-Xavier Mazoit, MD, PhD.

Contribution: This author helped to design the study, analyze the data, and was responsible for the statistic section and results.

Name: Hussein Sadek, MD.

Contribution: This author helped to conduct the study and write the manuscript.

Name: Hassan Houjairy, MD.

Contribution: This author helped to conduct the study and write the manuscript.

Name: Kamal Abdulkhaleq, MD.

Contribution: This author helped to conduct the study and write the manuscript.

Name: Nabil Bamadhaj, IT Specialist.

Contribution: This author helped to manufacture the instrument (cricometer) and participated in analyzing the data.

This manuscript was handled by: Richard C. Prielipp, MD.


The authors thank Ms. Manal Ghandour for her skilled technical assistance.


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