Cricoid pressure applied after placement of laryngeal mask impedes subsequent fibreoptic tracheal intubation through mask†
2000; Elsevier BV; Volume: 85; Issue: 2 Linguagem: Inglês
10.1093/bja/85.2.256
ISSN1471-6771
AutoresTakashi Asai, Kohei Murao, Koh Shingu,
Tópico(s)Tracheal and airway disorders
ResumoWe 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. 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. The laryngeal mask +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. +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. has a potential role in patients with difficult airways.1Benumof JL Laryngeal mask airway and the ASA difficult airway algorithm.Anesthesiology. 1996; 84: 686-699Crossref PubMed Scopus (424) Google Scholar 2Asai T Morris S The laryngeal mask airway: its features, effects and role.Can J Anaesth. 1994; 41: 930-960Crossref PubMed Scopus (173) Google Scholar 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.3Devitt JH Wenstone R Noel AG O'Donnell MP The laryngeal mask airway and positive-pressure ventilation.Anesthesiology. 1994; 80: 550-555Crossref PubMed Scopus (201) Google Scholar, 4Nandi M Maltby JR Vomiting and aspiration pneumonitis with the laryngeal mask airway.Can J Anaesth. 1992; 39: 69-70Crossref PubMed Scopus (88) Google Scholar, 5Asai T Barclay K McBeth C Vaughan RS Cricoid pressure applied after placement of the laryngeal mask prevents gastric insufflation but inhibits ventilation.Br J Anaesth. 1996; 76: 772-776Crossref PubMed Scopus (50) Google Scholar However, cricoid pressure, when it is applied before insertion, impedes the correct positioning of the laryngeal mask6Asai T Barclay K Power I Vaughan RS Cricoid pressure impedes placement of the laryngeal mask airway and subsequent tracheal intubation through the mask.Br J Anaesth. 1994; 72: 47-51Crossref PubMed Scopus (70) Google Scholar, 7Asai T Barclay K Power I Vaughan RS Cricoid pressure impedes placement of the laryngeal mask airway.Br J Anaesth. 1995; 74: 521-525Crossref PubMed Scopus (89) Google Scholar, 8Aoyama K Takenaka I Sata T Shigematsu A Cricoid pressure impedes positioning and ventilation through the laryngeal mask airway.Can J Anaesth. 1996; 43: 1035-1040Crossref PubMed Scopus (70) Google Scholar and may prevent adequate ventilation.6Asai T Barclay K Power I Vaughan RS Cricoid pressure impedes placement of the laryngeal mask airway and subsequent tracheal intubation through the mask.Br J Anaesth. 1994; 72: 47-51Crossref PubMed Scopus (70) Google Scholar 7Asai T Barclay K Power I Vaughan RS Cricoid pressure impedes placement of the laryngeal mask airway.Br J Anaesth. 1995; 74: 521-525Crossref PubMed Scopus (89) Google Scholar 9Ansermino JM Blogg CE Cricoid pressure may prevent insertion of the laryngeal mask airway.Br J Anaesth. 1992; 69: 465-467Crossref PubMed Scopus (80) Google Scholar 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.7Asai T Barclay K Power I Vaughan RS Cricoid pressure impedes placement of the laryngeal mask airway.Br J Anaesth. 1995; 74: 521-525Crossref PubMed Scopus (89) Google Scholar 9Ansermino JM Blogg CE Cricoid pressure may prevent insertion of the laryngeal mask airway.Br J Anaesth. 1992; 69: 465-467Crossref PubMed Scopus (80) Google Scholar Cricoid pressure applied after insertion of the laryngeal mask effectively prevents regurgitation10Strang TI Does the laryngeal mask airway compromise cricoid pressure?.Anaesthesia. 1992; 47: 829-831Crossref PubMed Scopus (41) Google Scholar and gastric insufflation.5Asai T Barclay K McBeth C Vaughan RS Cricoid pressure applied after placement of the laryngeal mask prevents gastric insufflation but inhibits ventilation.Br J Anaesth. 1996; 76: 772-776Crossref PubMed Scopus (50) Google Scholar 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,11Meek T Vincent A Duggan JE Cricoid pressure: can protective force be sustained?.Anaesthesia. 1998; 80: 672-674Crossref Scopus (42) Google Scholar and because ventilation via the laryngeal mask may be insufficient because of the presence of cricoid pressure.5Asai T Barclay K McBeth C Vaughan RS Cricoid pressure applied after placement of the laryngeal mask prevents gastric insufflation but inhibits ventilation.Br J Anaesth. 1996; 76: 772-776Crossref PubMed Scopus (50) Google Scholar Although one of the authors claimed12Asai T The laryngeal mask airway as an aid to intubate in patients at risk of aspiration?.Anesthesiology. 1993; 78: 1198Crossref Google Scholar 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. 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 313Mallampati SR Gatt SP Gugino LD et al.A clinical sign to predict difficult tracheal intubation: a prospective study.Can Anaesth Soc J. 1985; 32: 429-434Crossref PubMed Scopus (1689) Google Scholar or 414Samsoon GLT Young JRB Difficult tracheal intubation: a retrospective study.Anaesthesia. 1987; 42: 487-490Crossref PubMed Scopus (1118) Google Scholar 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.15Brimacombe JR Brain AIJ Berry AM The Laryngeal Mask Airway Instruction Manual. Intavent Research Limited, Berkshire, UK1997Google Scholar The size 4 was used in all patients for the following reasons. First, several studies16Asai T Howell TK Koga K Morris S Appropriate size and inflation of the laryngeal mask airway.Br J Anaesth. 1998; 80: 470-474Crossref PubMed Scopus (65) Google Scholar, 17Brimacombe J Keller C Laryngeal mask airway size selection in males and females: ease of insertion, oropharyngeal leak pressure, pharyngeal mucosal pressures and anatomical position.Br J Anaesth. 1999; 82: 703-707Crossref PubMed Scopus (59) Google Scholar, 18Berry AM Brimacombe JR McManus KF Goldblatt M An evaluation of the factors influencing selection of the optimal size of the laryngeal mask airway in normal adults.Anaesthesia. 1998; 53: 565-570Crossref PubMed Scopus (89) Google Scholar, 19Voyagis GS Batzioulis PG Secha-Doussaitou PN Selection of the proper size of laryngeal mask airway in adults.Anesth Analg. 1996; 83: 663-664Crossref PubMed Google Scholar 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.15Brimacombe JR Brain AIJ Berry AM The Laryngeal Mask Airway Instruction Manual. Intavent Research Limited, Berkshire, UK1997Google Scholar 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 4,15Brimacombe JR Brain AIJ Berry AM The Laryngeal Mask Airway Instruction Manual. Intavent Research Limited, Berkshire, UK1997Google Scholar 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,16Asai T Howell TK Koga K Morris S Appropriate size and inflation of the laryngeal mask airway.Br J Anaesth. 1998; 80: 470-474Crossref PubMed Scopus (65) Google Scholar and because further inflation of the cuff by the maximum volume (30 ml) provides less effective sealing.17Brimacombe J Keller C Laryngeal mask airway size selection in males and females: ease of insertion, oropharyngeal leak pressure, pharyngeal mucosal pressures and anatomical position.Br J Anaesth. 1999; 82: 703-707Crossref PubMed Scopus (59) Google Scholar 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,20Vanner RG Asai T Safe use of cricoid pressure.Anaesthesia. 1999; 54: 1-3Crossref PubMed Scopus (130) Google Scholar 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. 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)21Altman DG Practical Statistics for Medical Research. Chapman and Hall, London1991Google Scholar 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%.6Asai T Barclay K Power I Vaughan RS Cricoid pressure impedes placement of the laryngeal mask airway and subsequent tracheal intubation through the mask.Br J Anaesth. 1994; 72: 47-51Crossref PubMed Scopus (70) Google Scholar 22Koga K Asai T Latto IP Vaughan RS Effect of the size of a tracheal tube and the efficacy of the use of the laryngeal mask for fibrescope-aided tracheal intubation.Anaesthesia. 1997; 52: 131-135Crossref PubMed Scopus (56) Google Scholar 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.23Armitage P Berry G Statistical Methods in Medical Research. 3rd edn. Blackwell Scientific, Oxford1994Google Scholar 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).Table 1Patients' 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 seen7Sham pressureCricoid pressuren3535Age (yr)50 (20–76)54 (23–80)Sex (M:F)21:1423:12Weight (kg)61 (11)61 (11)Height (cm)162 (7)162 (9)View of the glottis (A:Ba)33:232:3 Open table in a new tab 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). 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). 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.20Vanner RG Asai T Safe use of cricoid pressure.Anaesthesia. 1999; 54: 1-3Crossref PubMed Scopus (130) Google Scholar 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.Fig 2A 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.View Large Image Figure ViewerDownload (PPT) 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.2Asai T Morris S The laryngeal mask airway: its features, effects and role.Can J Anaesth. 1994; 41: 930-960Crossref PubMed Scopus (173) Google Scholar 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).Fig 3A 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.View Large Image Figure ViewerDownload (PPT) 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.24Brimacombe J Berry A Mechanical airway obstruction after cricoid pressure with the laryngeal mask airway.Anesth Analg. 1994; 78: 604-605Crossref PubMed Scopus (1) Google Scholar 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.5Asai T Barclay K McBeth C Vaughan RS Cricoid pressure applied after placement of the laryngeal mask prevents gastric insufflation but inhibits ventilation.Br J Anaesth. 1996; 76: 772-776Crossref PubMed Scopus (50) Google Scholar 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.22Koga K Asai T Latto IP Vaughan RS Effect of the size of a tracheal tube and the efficacy of the use of the laryngeal mask for fibrescope-aided tracheal intubation.Anaesthesia. 1997; 52: 131-135Crossref PubMed Scopus (56) Google Scholar 25Schwartz D Johnson C Roberts J A maneuver to facilitate flexible fiberoptic intubation.Anesthesiology. 1989; 71: 470-471Crossref PubMed Scopus (69) Google Scholar 26Marsh NJ Easier fiberoptic intubations.Anesthesiology. 1992; 76: 860-861Crossref PubMed Scopus (36) Google Scholar 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.6Asai T Barclay K Power I Vaughan RS Cricoid pressure impedes placement of the laryngeal mask airway and subsequent tracheal intubation through the mask.Br J Anaesth. 1994; 72: 47-51Crossref PubMed Scopus (70) Google Scholar 20Vanner RG Asai T Safe use of cricoid pressure.Anaesthesia. 1999; 54: 1-3Crossref PubMed Scopus (130) Google Scholar 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%).6Asai T Barclay K Power I Vaughan RS Cricoid pressure impedes placement of the laryngeal mask airway and subsequent tracheal intubation through the mask.Br J Anaesth. 1994; 72: 47-51Crossref PubMed Scopus (70) Google Scholar 22Koga K Asai T Latto IP Vaughan RS Effect of the size of a tracheal tube and the efficacy of the use of the laryngeal mask for fibrescope-aided tracheal intubation.Anaesthesia. 1997; 52: 131-135Crossref PubMed Scopus (56) Google Scholar 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.27Asai T Fiberoptic tracheal intubation through the laryngeal mask in an awake patient with cervical spine injury.Anesth Analg. 1993; 77: 404Crossref PubMed Scopus (4) Google Scholar 28Asai T Use of the laryngeal mask for fibrescope-aided tracheal intubation in an awake patient with a deviated larynx.Acta Anaesthesiol Scand. 1994; 38: 615-616Crossref PubMed Scopus (9) Google Scholar 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%).6Asai T Barclay K Power I Vaughan RS Cricoid pressure impedes placement of the laryngeal mask airway and subsequent tracheal intubation through the mask.Br J Anaesth. 1994; 72: 47-51Crossref PubMed Scopus (70) Google Scholar 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.1Benumof JL Laryngeal mask airway and the ASA difficult airway algorithm.Anesthesiology. 1996; 84: 686-699Crossref PubMed Scopus (424) Google Scholar 2Asai T Morris S The laryngeal mask airway: its features, effects and role.Can J Anaesth. 1994; 41: 930-960Crossref PubMed Scopus (173) Google Scholar 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.6Asai T Barclay K Power I Vaughan RS Cricoid pressure impedes placement of the laryngeal mask airway and subsequent tracheal intubation through the mask.Br J Anaesth. 1994; 72: 47-51Crossref PubMed Scopus (70) Google Scholar 7Asai T Barclay K Power I Vaughan RS Cricoid pressure impedes placement of the laryngeal mask airway.Br J Anaesth. 1995; 74: 521-525Crossref PubMed Scopus (89) Google Scholar 9Ansermino JM Blogg CE Cricoid pressure may prevent insertion of the laryngeal mask airway.Br J Anaesth. 1992; 69: 465-467Crossref PubMed Scopus (80) Google Scholar Cricoid pressure should be reapplied after insertion of the mask, as it effectively prevents regurgitation of gastric contents.10Strang TI Does the laryngeal mask airway compromise cricoid pressure?.Anaesthesia. 1992; 47: 829-831Crossref PubMed Scopus (41) Google Scholar 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.29Asai T Use of the laryngeal mask for tracheal intubation in patients at increased risk of aspiration of gastric contents.Anesthesiology. 1991; 77: 1029-1030Crossref Scopus (29) Google Scholar 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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