Department of Anaesthesiology, Kansai Medical University, 1015 Fumizono-cho, Moriguchi City, Osaka 570-8507, Japan
+LMA® is the property of Intavent Limited. This work was presented in part at the 12th World Congress of Anaesthesiologists in Montreal, Canada, June 2000.
Accepted for publication: March 14, 2000
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Abstract |
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Br J Anaesth 2000; 85: 25661
Keywords: intubation tracheal; equipment; laryngeal mask; cricoid pressure
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Introduction |
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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.
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Patients and methods |
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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 patients 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 manufacturers 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 patients 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 patients neck (i.e. bimanual cricoid pressure). When applying the test pressure, no efforts were made to extend the patients 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 patients neck and the assistants 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 patients head and neck were allowed. Only one attempt with the maximum duration of 120 s was allowed for tracheal intubation.
Statistics
Fishers 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 ShapiroFrancia 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 MannWhitney 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 9095%.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
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Results |
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Discussion |
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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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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 5090% 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.
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Footnotes |
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References |
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