Cricoid pressure applied after placement of laryngeal mask impedes subsequent fibreoptic tracheal intubation through mask{dagger}

T. Asai*, K. Murao and K. Shingu

Department of Anaesthesiology, Kansai Medical University, 10–15 Fumizono-cho, Moriguchi City, Osaka 570-8507, Japan

+LMA® is the property of Intavent Limited. {dagger}This work was presented in part at the 12th World Congress of Anaesthesiologists in Montreal, Canada, June 2000.

Accepted for publication: March 14, 2000


    Abstract
 Top
 Abstract
 Introduction
 Patients and methods
 Results
 Discussion
 References
 
We studied 70 patients to see if cricoid pressure applied after insertion of the laryngeal mask altered the success rate of tracheal intubation through the mask. After induction of anaesthesia and neuromuscular blockade, patients were randomly allocated to have either cricoid pressure (Group C) or sham pressure (Group S). The view of the glottis through the laryngeal mask was assessed before and after the test pressure, and tracheal intubation through the mask was attempted using a fibreoptic bronchoscope. The test pressure did not alter the view of the glottis in any patient in group S, whereas it narrowed the glottic aperture in 16 out of 35 patients in group C. The fibrescope was inserted into the trachea in all patients in group S and in 25 patients in group C. The success rate of tracheal intubation in group S (31 patients) was significantly higher than in group C (21 patients, P<<0.001; 95% CI for difference: 9–48%). The time for insertion of the fibrescope in group S (median (95% CI): 12 (11–12) s) was significantly faster than in group C (16 (14–17) s, P<<0.001; 95% CI for difference: 3–6 s), and the time for tracheal intubation in group S (16 (15–18) s) was significantly faster than in group C (22 (19–24) s, P<0.0005; 95% CI for difference: 3–7 s). Cricoid pressure after insertion of the laryngeal mask makes tracheal intubation through the mask significantly more difficult.

Br J Anaesth 2000; 85: 256–61

Keywords: intubation tracheal; equipment; laryngeal mask; cricoid pressure


    Introduction
 Top
 Abstract
 Introduction
 Patients and methods
 Results
 Discussion
 References
 
The laryngeal mask+ has a potential role in patients with difficult airways.1 2 Cricoid pressure is necessary if such a patient is at an increased risk of pulmonary aspiration, as the laryngeal mask cannot reliably prevent gastric insufflation, regurgitation or pulmonary aspiration.3–5 However, cricoid pressure, when it is applied before insertion, impedes the correct positioning of the laryngeal mask6–8 and may prevent adequate ventilation.6 7 9 It has been suggested that cricoid pressure should be released temporarily during insertion of the laryngeal mask and, once the mask has been inserted, cricoid pressure should be reapplied.7 9

Cricoid pressure applied after insertion of the laryngeal mask effectively prevents regurgitation10 and gastric insufflation.5 The patient is woken up or the trachea intubated through the laryngeal mask, because it is difficult to maintain sufficient force of cricoid pressure for a prolonged period of time,11 and because ventilation via the laryngeal mask may be insufficient because of the presence of cricoid pressure.5 Although one of the authors claimed12 that cricoid pressure applied after insertion of the laryngeal mask did not usually hamper tracheal intubation through the mask, there has been no formal study to confirm this. We studied if this claim was supported.


    Patients and methods
 Top
 Abstract
 Introduction
 Patients and methods
 Results
 Discussion
 References
 
We studied 70 patients (ASA physical status class I or II, aged 18–80 yr) undergoing elective surgery, in whom tracheal intubation was indicated. Patients with any pathology of the neck, upper respiratory or upper alimentary tracts, or at risk of pulmonary aspiration of gastric contents were excluded. Patients with Mallampati class 313 or 414 were also excluded. The institutional research ethics committee approved the study and written informed consent was obtained from all patients.

In the operating theatre, an electrocardiograph, a pulse oximeter and an arterial pressure cuff were attached. A firm pad (7 cm in height) was placed under the patient’s occiput, but not under the neck. After pre-oxygenation, anaesthesia was induced with intravenous thiopental or propofol, and neuromuscular block was produced with vecuronium. Neuromuscular block was monitored with a peripheral nerve stimulator. Anaesthesia was maintained with sevoflurane in oxygen during the study period.

A laryngeal mask was inserted using the method described in the manufacturer’s instruction manual.15 The size 4 was used in all patients for the following reasons. First, several studies1619 have shown that size selection based on sex (size 5 in males and size 4 in females) is more appropriate than the weight-based selection described in the instruction manual.15 Second, the size 4, rather than size 5, was used in males, as the internal diameter of the size 5 is larger than that of size 4,15 and this difference might alter the success rate of passage of a 6.0-mm-ID tracheal tube through the laryngeal mask into the trachea. The cuff of the mask was inflated with 25 ml of air. This volume was selected because it was the approximate mean minimum volume of air which just prevented gas leak around the mask,16 and because further inflation of the cuff by the maximum volume (30 ml) provides less effective sealing.17

Adequacy of ventilation was assessed by manual ventilation. Ventilation was judged as adequate when chest inflation suggested a satisfactory compliance. If it was not possible to ventilate the lungs adequately, one more attempt at placement of the laryngeal mask was allowed. If unsuccessful, the patient was excluded from the study. After successful insertion, a bite-block (a wad of gauze) was inserted, and both the laryngeal mask and the bite-block were fixed to the patient’s face with tape.

A 6.0-mm reinforced tracheal tube (Mallinckrodt, Athlone, Ireland) was inserted into the laryngeal mask, and a fibreoptic bronchoscope (Olympus, outer diameter: 3.5 mm) was passed through them. The view of the larynx was assessed by looking through the bronchoscope with its tip positioned at the level of the grille of the mask. The patient was included if only the glottis or the glottis and tip of the epiglottis were seen; the patient was excluded if the epiglottis was pressed downward or if the glottis was not seen.

Patients were randomly allocated to one of two groups by block randomization (in blocks of 10). In one group (Group C), cricoid pressure was applied by an assistant, whereas in the other group (Group S), the assistant placed fingers on the cricoid cartilage, but applied no pressure (sham pressure). In both groups, the assistant placed a free hand under the patient’s neck (i.e. bimanual cricoid pressure). When applying the test pressure, no efforts were made to extend the patient’s head on the neck. The assistant who applied cricoid pressure had been trained to generate ~30 N by practising on a weighing scale,20 and the scale was used before each case to standardize the pressure as much as possible. The investigator who attempted to intubate through the laryngeal mask was blind as to whether or not cricoid pressure was being applied, by covering the patient’s neck and the assistant’s hands with a drape. The change in the patency of the glottis was assessed after application of the test pressure (1 = no or mild change; 2 = the anterior part of the glottis was narrowed; 3 = the entire part of the glottis was narrowed; 4 = glottis not seen).

The fibreoptic bronchoscope was then inserted into the trachea and the tracheal tube was passed over the fibrescope into the trachea. If it was difficult to insert the tracheal tube into the trachea, rotation of the tube (clockwise, possibly followed by anticlockwise rotation) and alteration of the position of the patient’s head and neck were allowed. Only one attempt with the maximum duration of 120 s was allowed for tracheal intubation.

Statistics
Fisher’s exact test was used to compare the success rate of tracheal intubation between groups. A value of P<0.05 was considered significant. The 95% confidence intervals (CI) for difference in the success rate of tracheal intubation between groups were also calculated.

Time for insertion of the fibrescope into the trachea and time for tracheal intubation over the fibrescope were measured in patients in whom tracheal insertion of the fibrescope or tracheal intubation succeeded. The normal plots (plots of normal scores) and Shapiro–Francia W' test (which analyses if the data are normally distributed)21 showed that the time for insertion of the fibrescope or tracheal intubation was generally not normally distributed. The Mann–Whitney U-test was therefore used to compare the time for insertion of the fibrescope and time for tracheal intubation between groups. The 95% CI for each time and CI for time difference between groups were also calculated.

In our previous studies, when cricoid pressure was applied, the success rate of tracheal intubation through the laryngeal mask was 90–95%.6 22 Our hypothesis was that cricoid pressure applied after insertion of the mask would not alter the success rate of tracheal intubation through the mask, with a difference in the success rate of up to 20%. Sixty to seventy patients would be required to assess this hypothesis, with a power of 90% and a one-sided 95% confidence interval, using an equivalence test.23


    Results
 Top
 Abstract
 Introduction
 Patients and methods
 Results
 Discussion
 References
 
In 72 of the patients recruited, adequate ventilation through the laryngeal mask was obtained at the first attempt; however, in two patients the epiglottis was pressed downward by the mask and these patients were excluded from the study. The remaining 70 patients were studied. The height, weight, sex ratio and age of patients were similar in the two groups (Table 1).


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Table 1 Patients’ characteristics (mean (SD); age (range)) and the proportion of the view of the larynx through the laryngeal mask before application of the test pressure. A = the glottis, but not the tip of the epiglottis, seen; Ba = the glottis and tip of the epiglottis seen7
 
After application of the test pressure, the view of the glottis was not altered in any patient in group S, whereas it deteriorated in 16 out of 35 patients (46%) in group C (score 2: five patients; score 3: three patients; and score 4: eight patients). The fibrescope was inserted into the trachea in all patients in group S, whereas it succeeded in 25 patients in group C (Fig. 1). Tracheal intubation succeeded in 31 patients (89%) in group S and in 21 patients (60%) in group C; there was a significant difference in its success rate between groups (P<<0.001; 95% CI for difference: 9–48%, Fig. 1).



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Fig 1 Success rate of insertion of tracheal intubation through the laryngeal mask using a fibre optic bronchoscope with and without cricoid pressure applied after insertion of the mask. Tracheal intubation through the laryngeal mask succeeded (filled box); tracheal insertion of a fibrescope succeeded, but tracheal intubation failed (hatched box) and tracheal insertion of a fibrescope failed (clear box).

 
In patients in whom insertion of the fibrescope into the trachea succeeded, the time for insertion of the fibrescope in group S (median (95% CI): 12 (11–12) s) was significantly faster than in group C (16 (14–17) s, P<<0.001; 95% CI for difference: 3–6 s). Similarly, in patients in whom tracheal intubation succeeded, the time for tracheal intubation in group S (16 (15–18) s) was significantly faster than in group C (22 (19–24) s, P<0.0005; 95% CI for difference: 3–7 s).


    Discussion
 Top
 Abstract
 Introduction
 Patients and methods
 Results
 Discussion
 References
 
Contrary to our hypothesis, cricoid pressure applied after insertion of the laryngeal mask significantly decreased the success rate and the ease of tracheal intubation through the laryngeal mask.

When cricoid pressure is applied without an undue force (~30 N) in the absence of the laryngeal mask, the pressure usually does not worsen and may improve the view of the glottis at laryngoscopy.20 In contrast, in our study in the presence of the laryngeal mask, cricoid pressure applied at 30 N frequently narrowed the glottis and made tracheal intubation through the mask more difficult (Fig. 2). This difference is likely to be produced by the laryngeal mask.




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Fig 2 A typical view of the glottis through the laryngeal mask, before (upper figure) and after (lower figure) application of cricoid pressure. The presence of the laryngeal mask shifts the entire part of the larynx anteriorly. Cricoid pressure shifts the lower part of the larynx back towards the posterior pharyngeal wall, while the upper part remained shifted anteriorly, causing the anterior tilting of the larynx. A = tissue covering the arytenoid cartilages.

 
The distal part of the laryngeal mask occupies the hypopharynx (laryngeal part of the pharynx) and the tip of the mask reaches the caudal border of the cricoid cartilage. Inflation of the cuff shifts the larynx anteriorly.2 When cricoid pressure is applied in the presence of the laryngeal mask, the caudal part of the larynx (or the cricoid cartilage) is shifted back against the posterior pharyngeal wall while theoretically the cranial part of the larynx (or more specifically, the arytenoid cartilage) is being shifted anteriorly by the mask, tilting the larynx anteriorly and closing the glottis (Fig. 2). In contrast, when the laryngeal mask is not inserted, because there is little space between the larynx and pharyngeal wall, there would only be a slight displacement of the larynx with cricoid pressure (Fig. 3).




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Fig 3 A typical view of the glottis, before (upper figure) and after (lower figure) application of cricoid pressure (without the presence of the laryngeal mask). There is little difference in the patency of the glottis.

 
In one report, a tilted larynx with a closed glottis was observed in one out of 85 patients in whom cricoid pressure was applied before insertion of the laryngeal mask.24 It seems likely that the mask wedged behind the cricoid cartilage despite application of cricoid pressure and the mask caused the obstruction. In our study, where cricoid pressure was applied after insertion of the laryngeal mask, the glottis was narrowed more frequently, supporting this theory. This narrowing of the glottis is also consistent with a previous report that cricoid pressure applied after insertion of the laryngeal mask inhibited ventilation via the mask.5

Fibrescope-aided tracheal intubation through the laryngeal mask is significantly easier and faster than conventional fibreoptic intubation (without the aid of the laryngeal mask). With conventional fibreoptic tracheal intubation, advance of the tracheal tube over the fibrescope into the trachea is difficult in 50–90% of patients even if the patients do not have difficult airways.22 25 26 In contrast, insertion of the fibrescope through the laryngeal mask into the trachea and advance of the tracheal tube over the fibrescope are usually easy.6 20 In our current study, the success rate of fibreoptic intubation through the laryngeal mask (without application of cricoid pressure) was 89%, consistent with previous reports (success rates 90 or 95%).6 22 There have also been case reports of successful and smooth fibreoptic intubation through the laryngeal mask in patients in whom conventional fibreoptic intubation had failed or was extremely difficult.27 28 Therefore, the laryngeal mask is a useful aid to fibrescope-aided tracheal intubation.

In our previous report in which cricoid pressure was applied before insertion of the laryngeal mask, the success rate of fibreoptic tracheal intubation through the laryngeal mask was 15% (three out of 20 patients) and release of the pressure (after insertion of the mask) only allowed tracheal intubation in another four patients (20%).6 In our current study, in which cricoid pressure was applied after insertion of the laryngeal mask, the success rate of tracheal intubation was 60% (21 out of 35 patients). Therefore, cricoid pressure, regardless of the timing of application, impedes tracheal intubation through the laryngeal mask, and it can be concluded that the usefulness of the laryngeal mask as the aid to tracheal intubation is markedly reduced in patients at increased risk of pulmonary aspiration.

The role of the laryngeal mask has been established in patients with difficult airways, but there has been uncertainty about its role when patients are also at increased risk of pulmonary aspiration. We propose its use in such patients in the following three situations. The first situation is when tracheal intubation has failed but ventilation via a facemask is possible after induction of anaesthesia. Although there is generally no need to insert the laryngeal mask in this situation, the laryngeal mask might be usefully inserted prior to intubation through it.1 2 However, in patients at increased risk of pulmonary aspiration, attempt should not be made to insert the laryngeal mask, because cricoid pressure often impedes its correct insertion, and because even if the laryngeal mask has been successfully inserted, cricoid pressure often prevents tracheal intubation through the mask.

The second situation is when tracheal intubation using a laryngoscope has failed and adequate ventilation through a facemask is impossible after induction of anaesthesia. In this situation, insertion of the laryngeal mask may be attempted while equipment for transtracheal ventilation is being prepared. Cricoid pressure should be temporarily loosened during insertion of the laryngeal mask to increase the success rate of insertion, although this temporary release may allow regurgitation and pulmonary aspiration.6 7 9 Cricoid pressure should be reapplied after insertion of the mask, as it effectively prevents regurgitation of gastric contents.10 If tracheal intubation is required, it may be attempted through the laryngeal mask while cricoid pressure is kept applied, and if there is difficulty in advancing the tracheal tube over the fibrescope, cricoid pressure might be temporarily loosened and tracheal intubation over the fibrescope reattempted.

The third situation is when a difficult tracheal intubation is predicted. One of the authors reported previously the method of awake insertion of the laryngeal mask, and subsequent tracheal intubation through the mask before or after induction of anaesthesia, in patients with full stomachs.29 However, we no longer recommend the latter method, as our current study has shown that tracheal intubation through the laryngeal mask becomes more difficult when cricoid pressure is applied. Therefore, it is safer to intubate the trachea before induction of anaesthesia.


    Footnotes
 
* Corersponding author Back


    References
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 Abstract
 Introduction
 Patients and methods
 Results
 Discussion
 References
 
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