Cricoid Pressure Does Not Increase the Rate of Failed Intubation by Direct Laryngoscopy in Adults

This double-blind randomized controlled study was conducted at the Centre Hospitalier Affilié  Universitaire de Québec (Hôpital de l’Enfant-Jésus) and was approved by the hospital ethics committee. Patients were evaluated for eligibility the day before or on the morning of surgery, and written informed consent was obtained. Patients older than 18 yr undergoing elective surgery under general anesthesia with orotracheal intubation were eligible. Exclusion criteria were contraindication to the medication used for induction, contraindication to a CP,14upper respiratory tract abnormalities, patients known to be impossible to ventilate by mask, history of a difficult intubation requiring an alternative to direct laryngoscopy, pregnancy, surgery requiring a double-lumen endotracheal tube, symptomatic gastroesophageal reflux, morbid obesity (body mass index >35 kg/m²), and definite indications for a CP (e.g. , a full stomach). The following risk factors for difficult intubation were recorded: modified Mallampati class, dental status, presence of retrognathism, ability to prognate, interincisal distance (or intergingival in toothless patients), and thyromental and sternomental distances.15–19 

Immediately before the induction of anesthesia, patients were randomly assigned to receive either CP or a sham cricoid pressure (SCP). The randomization sequence was prepared with the Maple software (version 6.0; Maplesoft, Waterloo, Ontario, Canada) and sealed in prenumbered opaque envelopes. Standard monitoring was used for all patients. Neuromuscular blockade was monitored at the adductor pollicis by stimulating the ulnar nerve at the wrist. End-tidal carbon dioxide was sampled at the Y-piece of the breathing circuit. The patient’s head was positioned in the sniffing position using a 7-cm thick uncompressible pillow.14,20After preoxygenation with 100% oxygen for 3 min by face mask, anesthesia was induced with propofol 1.0–3.0 mg/kg and sufentanil 0.2–1.0 μg/kg. Neuromuscular blockade was obtained either with rocuronium 0.6–1.2 mg/kg or with rocuronium 0.03 mg/kg followed by succinylcholine 1.5 mg/kg. Patient lungs were manually ventilated with 100% oxygen by mask until complete paralysis was achieved.

In the experimental group, a standardized CP was applied using the single hand technique as originally described by Sellick.1Seven anesthesia assistants took part in the study and were trained by one of the investigators (AFT) to correctly identify the cricoid cartilage and apply a pressure of 30 newtons (∼3 kg) with the first three fingers of their dominant hand.1,21The pressure was applied with the thumb and the middle finger at 10 o’clock and 2 o’clock, respectively. The index finger was located above the cricoid cartilage to control the direction of the force. To optimize the learning of the correct pressure, a simulator was devised with a 20-ml syringe mounted on an electronic scale. Anesthesia assistants trained daily on this simulator. In every patient, correct identification of the cricoid cartilage and appropriate positioning of the fingers were confirmed by one of the investigators. In the SCP group, the cricoid cartilage was identified and the fingers were positioned as in the CP group but no pressure was applied. A screen was hung over the upper part of the patient’s neck to keep both the anesthesiologist and the data collector unaware of the patient’s CP or SCP status. In both groups, one of eight certified anesthesiologists who participated in the study intubated the trachea by direct laryngoscopy with a Macintosh No. 3 laryngoscope blade.

The intubation time was defined as the interval between the insertion of the laryngoscope blade into the mouth up to the inflation of the endotracheal tube cuff and was measured with a chronometer. Correct positioning of the tube was confirmed by capnography. A 30-s period was allowed to complete tracheal intubation. If capnography did not confirm tracheal intubation, the attempt could be resumed only if there was time remaining in this 30-s period. The anesthesiologist rated the grade of laryngoscopic view on the Cormack and Lehane scale22and the complexity of intubation on the Intubation Difficulty Scale23and was asked whether the larynx was in midline position or shifted laterally. If the intubation could not be completed within 30 s, the intubation attempt was aborted and recorded as a failure and the patient was entered in the crossover phase of the study. These patients were then ventilated for 30 s with 100% oxygen by mask. In the second intubation attempt, patients of the CP group had a SCP applied and patients of the SCP group had a CP applied. This was done following the same algorithm as the original attempt. The grade of laryngoscopy was again rated by the anesthesiologist. If the trachea could not be intubated within 30 s of that second attempt, the protocol was discontinued and the airway was managed following the difficult airway algorithm of the American Society of Anesthesiologists.24If arterial oxygen saturation decreased to less than 90% at any time during the study, CP or SCP was released and the protocol was terminated. This attempt was recorded as a failure, blinding was maintained at all times, and data were analyzed according to the patient’s group allocation.

The primary endpoint was the failure to intubate the trachea within the first 30-s attempt. Sample size was determined to identify an increase in the rate of failed intubation in the CP group compared with the SCP group. The incidence of failed orotracheal intubation by direct laryngoscopy within 30 s is unknown. However, Cormack grades III and IV are good predictors of failed intubation,22and it was decided to use their incidence as a surrogate in sample size determination. Wide variations of this incidence are reported in the literature (1.1–11.3%).16,25,26We considered that a reasonable estimate would be between 4% and 5%. Therefore, it was determined that a sample size of 350 patients per group would be required to identify an increase in the incidence of failed intubation to 10% with Type I and Type II errors of 5% and 20%, respectively. Continuous variables are reported as mean ± SD. Statistical analysis was performed with the Student t  test or the Wilcoxon test for continuous data and with the chi-square test or the Fisher exact test for proportions. All tests were two-sided and P < 0.05 was considered significant.

Over a 7-month period, 830 patients were evaluated for inclusion in the study. Of those, 700 were enrolled and randomly assigned to the CP or the SCP group (fig. 1). Groups did not differ for gender, age, ASA physical status, anthropometric characteristics, or risk factors for difficult intubation (table 1). The distribution of the anesthesiologists (n = 8) and the anesthesia assistants (n = 7) involved in the study was comparable between the two groups. All patients received the CP or SCP as determined by randomization.

The proportion of patients who could not be intubated within the first 30-s attempt was comparable between the CP and the SCP groups (4.4% and 3.7%, respectively; P = 0.70) (table 2). No difference was observed between groups for grade of laryngoscopic view or for Intubation Difficulty Scale score (table 2and fig. 2). The intubation time for the successful intubations at the first attempt was slightly longer in the CP group compared with the SCP group (table 2). In patients who could not be intubated during the first intubation attempt and entered the crossover phase of the study (n = 28), the rate of failed intubation was again comparable for the two groups (table 2). Among these patients, no change was observed for grade of laryngoscopic view in 20 patients, whereas it was improved in four and three patients from the CP and the SCP groups, respectively (P = not significant). In one patient of the SCP group, glottic exposure worsened when CP was applied.

In one patient of the CP group, oxygen saturation dropped to less than 90% during the second intubation attempt. The patient was ventilated by mask and his trachea was intubated by direct laryngoscopy with a rigid hockey-shaped stylet. Intubation was recorded as unsuccessful for both attempts.

In this study, CP had no influence on the rate of failed orotracheal intubation with a Macintosh No. 3 laryngoscope blade in adult patients. Furthermore, CP had no effect on glottic exposure during laryngoscopy or on the complexity of intubation as assessed by the Intubation Difficulty Scale score.

The clinical effectiveness of CP should be determined by weighing its efficacy in preventing pulmonary aspiration of gastric content against the risk of impeding tracheal intubation. The protection against pulmonary aspiration of gastric content provided by CP is very difficult to assess. Considering the low incidence of pulmonary aspiration, a huge number of patients would be required. Obvious ethical considerations would also be raised. On the other hand, this study shows that concerns about impeding intubation are not justified because tracheal intubation can be achieved as successfully with CP.

Two previous randomized studies evaluating the effect of CP on laryngoscopic view with a standard laryngoscope blade have yielded conflicting results. In a group of 100 patients, Brimacombe et al.  reported no effect of CP on laryngoscopy grade.12In another study, 50 patients had a standard CP, an upward and backward CP, or no CP applied.13These authors concluded that laryngoscopic view, assessed in millimeters of visible vocal cords, was worse with standard CP than without CP. Other studies have evaluated the success of intubation with CP using devices other than a common laryngoscope blade. With a lightwand27and the Wu-Scope System (Pentax Precision Instruments, Orangeburg, NY),28a lower success of tracheal intubation was reported with the application of CP. Using the fiberoptic bronchoscope, the rate of successful intubation was increased in one study and decreased in another.29,30Finally, one study has reported that the success of intubation with the Bullard laryngoscope is not altered by CP.29 

Although most previous studies used laryngoscopic view as their primary endpoint, we elected to use the rate of failed intubation within a fixed time period. This was preferred because orotracheal intubation is the final objective of direct laryngoscopy. Moreover, it is an objective binary variable, as opposed to more subjective variables such as the grade of laryngoscopic view. However, the duration of a reasonable intubation attempt is not well defined. The choice of a 30-s duration was made a priori  because it represents a reasonable duration for an intubation attempt based on experts’ opinion.31,32Allowing a longer period for intubation would probably have resulted in a few more successful intubations, but it is doubtful that it would have favored one group over the other. The application of a cricoid pressure had two minor adverse effects on laryngoscopy. First, median intubation time was slightly increased by 0.9 s in the CP group. Second, lateral shift of the larynx was more frequent in the CP group (table 2). However, these effects had no influence on the failure rate of intubation or on the Intubation Difficulty Scale score and are probably not clinically significant.

The results of this study are in contradiction with the common clinical impression that CP impedes visualization of the larynx and with case reports of difficult intubation.5–8This discrepancy might be explained by the frequent use of a less than optimal technique for the application of CP in the usual clinical setting. Indeed it has been reported that anesthesia personnel have a limited knowledge of CP and that most of them are not aware of any recommendation on the force to be applied on the cricoid cartilage.33,34Herman et al.  reported that a wide variation in the actual force was applied when the personnel were not previously trained for this task.35However, they have also shown that, with training on a simulator, performance is reproducible within a range of 2 newtons with a good retention time. It can be presumed that excessive force, wrong (lateral) direction of the force or, more importantly, application of the pressure on the larynx rather than on the cricoid ring would make visualization of the larynx and intubation difficult or impossible. In our study, the anesthesia assistants were taught the correct CP technique and they trained daily on a simulator to apply the recommended pressure of 30 newtons. Thus it is possible that different results might be obtained in a clinical setting where the application of CP is not correctly done. It must also be mentioned that a Macintosh laryngoscope blade was used in our study and that the use of a straight laryngoscope blade might yield different results.

In conclusion, our results indicate that CP, when applied by trained personnel, does not increase the rate of failed orotracheal intubation and has no impact on the difficulty of laryngoscopy and intubation in an adult surgical population. Therefore the application of CP should not be avoided for fear of increasing the difficulty of intubation by direct laryngoscopy when its use is indicated.

The authors thank the clinical anesthesiologists François Caron, M.D., F.R.C.P.C., Pierre Côté, M.D., F.R.C.P.C., Dary Croft, M.D., F.R.C.P.C., Jacques-Jules Côté, M.D., F.R.C.P.C., and Gilles Lacroix, M.D., F.R.C.P.C., the research assistant Ms. Diane Martineau, R.N., B.Sc., and the anesthesia assistants who participated in this study, Centre Hospitalier affilié  Universitaire de Québec (Hôpital Enfant-Jésus), Quebec City, Quebec, Canada.