Cricoid pressure is typically applied during the induction of anaesthesia in patients at high risk of aspirating their stomach contents.1 Previous studies have evaluated the cricoid pressure effect on tracheal intubation using direct laryngoscopy or videolaryngoscopy.2,3 Few studies to date have evaluated the effects of cricoid pressure on mask ventilation, the primary form of airway management.4 As quantitative evaluation of manual mask ventilation is difficult, we measured the effects of cricoid pressure during pressure-controlled ventilation (PCV), setting a fixed ventilatory rate and a standardised amount of pressure, and hypothesised that cricoid pressure would impede mask ventilation. This hypothesis was tested by assessing ventilation volume using mechanical ventilation in a randomised crossover trial.
The research ethics committee of Osaka Medical College approved this study (2 May 2016, Head: Masashi Neo). We registered the study in the UMIN Clinical Trials Registry on 6 July 2015 (Registration number: UMIN000022183). In June and July 2016, 22 female patients were assessed for eligibility; none refused to participate and two were excluded because of morbid obesity (BMI > 35) in whom difficult mask ventilation was anticipated. Male patients were excluded, as it is known that performing mask ventilation can be more difficult.
After written informed consent was obtained, 20 female patients aged 20 to 75 years who were to undergo general anaesthesia in the supine position were recruited. Without any premedication, anaesthesia was induced with propofol 1.5 to 2 mg kg−−1 and remifentanil 0.3 to 0.5 μg kg−1 min−1, and neuromuscular blockade was achieved with rocuronium 0.9 mg kg−1. Doses of propofol and remifentanil were determined by anaesthetists on the basis of the patient's age or condition. After loss of consciousness, mask ventilation was performed using 3 to 5% sevoflurane in oxygen with the double hand technique which performs the E-C clamp technique with both hands for yielding sealing pressure. The E-C clamp technique is a technique for holding a mask on the face of a manually ventilated patient, in which the thumb and index finger hold the mask down over the nose and mouth of the patient, forming a C while the other three fingers of the hand grasp the patient's mandible, forming an E. Cases with less than 20 cmH2O pressure or unstable ventilation were excluded. Cricoid pressure was applied at a standardised force of 30 N, which sufficiently prevents regurgitation into the pharynx, even in patients with a full stomach undergoing caesarean section.5 An independent anaesthetist was trained to apply the force (within an error of 5%) by practising on a weighing scale with a top board.2 Sham pressure was applied by placing a finger on the cricoid cartilage without exerting force. Both the patient's neck and the assistant's hands were covered by a nontransparent sheet to blind the investigator. The intervention started after confirming full muscle relaxation as the zero count of train-of-four with a TOF (train-of-four) watch (NIHON KOHDEN, Tokyo, Japan).
Ventilation volume was measured with or without cricoid pressure using the ventilator attached to the anaesthesia machine (Fabius GS, Dräger, Germany) in PCV mode with peak inspratory pressures (PIP) of 10, 15 and 20 cmH2O, and at a ventilatory rate of eight breaths per minute and a 1 : 2 inspiratory to expiratory ratio. Ventilation volume was the average volume measured during 1 min. Measurement was performed in ascending order of PIP (10, 15, 20 cmH2O), and the order of cricoid or sham pressure was randomly assigned using the envelope method at each pressure.
Statistical analysis was performed using JMP 11 (SAS Institute Inc., Cary, North Carolina, USA). Ventilation volume, the primary outcome, was assessed using the Wilcoxon matched-pairs signed-rank test for comparisons between groups. Data are expressed as mean (SD), or median [interquartile range]. A P value less than 0.05 was considered statistically significant.
We performed a preliminary study in which five patients underwent mask ventilation during PCV with or without cricoid pressure. Ventilation volume at PIP 15 cmH2O was about 500 ± 200 ml without cricoid pressure and 300 ± 100 ml with cricoid pressure. We considered this 200 ml difference to be clinically significant, as it was approximately 40% of the ventilation volume without cricoid pressure. To detect this difference with 80% power at a 5% significance level, 16 patients were required. To adjust for potential missing data, we planned to recruit 20 patients for each group.
None of the participants dropped out or were lost to follow-up. Mean weight, height and age of the 20 female patients were 56.1 (10.3) kg, 155.9 (4.7) cm and 59.5(11.1) years, respectively. Ventilation volume was significantly smaller with cricoid pressure than without at PIP of 10, 15 and 20 cmH2O during PCV (Table 1).
Cricoid pressure is a technique used to occlude the oesophagus by compressing it between the cricoid cartilage and cervical vertebrae. As cricoid pressure physically compresses the pharyngeal structures, the technique inevitably affects airway device insertion. In the present study, cricoid pressure significantly impeded PCV at various pressures. One possible reason for this is airway deviation or oral space reduction resulting from cricoid pressure, which increases ventilation resistance. Given our findings that cricoid pressure impedes mask ventilation, it may be effective to increase the ventilation rate to prevent hypoxia or accumulation of carbon dioxide when applying cricoid pressure.
One limitation of this study is that we excluded male patients. Evaluation in male patients is warranted in the future. The measurement of airway resistance change by cricoid pressure is also needed in the future study.
In conclusion, our findings suggest that cricoid pressure reduces PCV volume, thereby impeding mask ventilation.
Acknowledgements relating to this article
Assistance with the study: none.
Financial support and sponsorship: funding was provided by our institution and department.
Conflicts of interest: none.
Trial Registration: UMIN Clinical Trials Registry Identifier: UMIN000022183.
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