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Cricoid pressure applied after placement of laryngeal mask impedes subsequent fibreoptic tracheal intubation through mask†
British Journal of Anaesthesia 85 (2): 256±61 (2000)
Cricoid pressure applied after placement of laryngeal mask impedes subsequent ®breoptic tracheal intubation through mask² T. Asai*, K. Murao and K. Shingu Department of Anaesthesiology, Kansai Medical University, 10±15 Fumizono-cho, Moriguchi City, Osaka 570-8507, Japan *Corresponding author 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 ®breoptic 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 ®brescope 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 signi®cantly higher than in group C (21 patients, P<<0.001; 95% CI for difference: 9±48%). The time for insertion of the ®brescope in group S (median (95% CI): 12 (11±12) s) was signi®cantly 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 signi®cantly 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 signi®cantly more dif®cult. Br J Anaesth 2000; 85: 256±61 Keywords: intubation tracheal; equipment; laryngeal mask; cricoid pressure Accepted for publication: March 14, 2000
The laryngeal mask+ has a potential role in patients with dif®cult 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 insuf¯ation, 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 insuf¯ation.5 The patient is woken up or the trachea intubated through the laryngeal mask, because it is dif®cult to maintain suf®cient force of cricoid pressure for a prolonged period of time,11 and because ventilation via the laryngeal mask may be insuf®cient because of the +
LMAâ is the property of Intavent Limited.
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 con®rm this. We studied if this claim was supported.
Patients and methods 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. ²This work was presented in part at the 12th World Congress of Anaesthesiologists in Montreal, Canada, June 2000.
Ó The Board of Management and Trustees of the British Journal of Anaesthesia 2000
Cricoid pressure and laryngeal mask
In the operating theatre, an electrocardiograph, a pulse oximeter and an arterial pressure cuff were attached. A ®rm 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 sevo¯urane 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 studies16±19 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 in¯ated 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 in¯ation 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 in¯ation 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 ®xed 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 ®breoptic 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 ®ngers 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 ®breoptic bronchoscope was then inserted into the trachea and the tracheal tube was passed over the ®brescope into the trachea. If it was dif®cult 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 signi®cant. The 95% con®dence intervals (CI) for difference in the success rate of tracheal intubation between groups were also calculated. Time for insertion of the ®brescope into the trachea and time for tracheal intubation over the ®brescope were measured in patients in whom tracheal insertion of the ®brescope 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 ®brescope or tracheal intubation was generally not normally distributed. The Mann±Whitney U-test was therefore used to compare the time for insertion of the ®brescope 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% con®dence interval, using an equivalence test.23
Results In 72 of the patients recruited, adequate ventilation through the laryngeal mask was obtained at the ®rst 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).
257
Asai et al. 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
n Age (yr) Sex (M:F) Weight (kg) Height (cm) View of the glottis (A:Ba)
Sham pressure
Cricoid pressure
35 50 (20±76) 21:14 61 (11) 162 (7) 33:2
35 54 (23±80) 23:12 61 (11) 162 (9) 32:3
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: ®ve patients; score 3: three patients; and score 4: eight patients). The ®brescope 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 signi®cant 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 ®brescope into the trachea succeeded, the time for insertion of the ®brescope in group S (median (95% CI): 12 (11±12) s) was signi®cantly 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 signi®cantly faster than in group C (22 (19±24) s, P<0.0005; 95% CI for difference: 3±7 s).
Discussion Contrary to our hypothesis, cricoid pressure applied after insertion of the laryngeal mask signi®cantly 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 dif®cult (Fig. 2). This difference is likely to be produced by the laryngeal mask. 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. In¯ation 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 speci®cally, the arytenoid cartilage) is being shifted
Fig 1 Success rate of insertion of tracheal intubation through the laryngeal mask using a ®bre optic bronchoscope with and without cricoid pressure applied after insertion of the mask. Tracheal intubation through the laryngeal mask succeeded (®lled box); tracheal insertion of a ®brescope succeeded, but tracheal intubation failed (hatched box) and tracheal insertion of a ®brescope failed (clear box).
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). 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 signi®cantly easier and faster than conventional ®breoptic intubation (without the aid of the laryngeal mask). With conventional ®breoptic tracheal intubation, advance of the tracheal tube over the ®brescope into the trachea is dif®cult in 50±90% of patients even if the patients do not have dif®cult airways.22 25 26 In contrast, insertion of the ®brescope through the laryngeal mask into the trachea and advance of the tracheal tube over the ®brescope are usually easy.6 20 In our current study, the success rate of ®breoptic 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 ®breoptic intubation through the laryngeal mask in patients in whom conventional ®breoptic intubation had failed or was extremely
258
Cricoid pressure and laryngeal mask
Fig 2 A typical view of the glottis through the laryngeal mask, before (upper ®gure) and after (lower ®gure) 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.
Fig 3 A typical view of the glottis, before (upper ®gure) and after (lower ®gure) application of cricoid pressure (without the presence of the laryngeal mask). There is little difference in the patency of the glottis.
259
Asai et al.
dif®cult.27 28 Therefore, the laryngeal mask is a useful aid to ®brescope-aided tracheal intubation. In our previous report in which cricoid pressure was applied before insertion of the laryngeal mask, the success rate of ®breoptic 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 dif®cult 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 ®rst 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 dif®culty in advancing the tracheal tube over the ®brescope, cricoid pressure might be temporarily loosened and tracheal intubation over the ®brescope reattempted. The third situation is when a dif®cult 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 dif®cult when cricoid pressure is applied. Therefore, it is safer to intubate the trachea before induction of anaesthesia.
References
260
1 Benumof JL. Laryngeal mask airway and the ASA dif®cult airway algorithm. Anesthesiology 1996; 84: 686±99 2 Asai T, Morris S. The laryngeal mask airway: its features, effects and role. Can J Anaesth 1994; 41: 930±60 3 Devitt JH, Wenstone R, Noel AG, O'Donnell MP. The laryngeal mask airway and positive-pressure ventilation. Anesthesiology 1994; 80: 550±5 4 Nandi M, Maltby JR. Vomiting and aspiration pneumonitis with the laryngeal mask airway. Can J Anaesth 1992; 39: 69±70 5 Asai T, Barclay K, McBeth C, Vaughan RS. Cricoid pressure applied after placement of the laryngeal mask prevents gastric insuf¯ation but inhibits ventilation. Br J Anaesth 1996; 76: 772±6 6 Asai 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±51 7 Asai T, Barclay K, Power I, Vaughan RS. Cricoid pressure impedes placement of the laryngeal mask airway. Br J Anaesth 1995; 74: 521±5 8 Aoyama K, Takenaka I, Sata T, Shigematsu A. Cricoid pressure impedes positioning and ventilation through the laryngeal mask airway. Can J Anaesth 1996; 43: 1035±40 9 Ansermino JM, Blogg CE. Cricoid pressure may prevent insertion of the laryngeal mask airway. Br J Anaesth 1992; 69: 465±7 10 Strang TI. Does the laryngeal mask airway compromise cricoid pressure? Anaesthesia 1992; 47: 829±31 11 Meek T, Vincent A, Duggan JE. Cricoid pressure: can protective force be sustained? Anaesthesia 1998; 80: 672±4 12 Asai T. The laryngeal mask airway as an aid to intubate in patients at risk of aspiration? Anesthesiology 1993; 78: 1198 13 Mallampati SR, Gatt SP, Gugino LD, et al. A clinical sign to predict dif®cult tracheal intubation: a prospective study. Can Anaesth Soc J 1985; 32: 429±34 14 Samsoon GLT, Young JRB. Dif®cult tracheal intubation: a retrospective study. Anaesthesia 1987; 42: 487±90 15 Brimacombe JR, Brain AIJ, Berry AM. The Laryngeal Mask Airway Instruction Manual. Berkshire, UK: Intavent Research Limited, 1997 16 Asai T, Howell TK, Koga K, Morris S. Appropriate size and in¯ation of the laryngeal mask airway. Br J Anaesth 1998; 80: 470±4 17 Brimacombe 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±7 18 Berry AM, Brimacombe JR, McManus KF, Goldblatt M. An evaluation of the factors in¯uencing selection of the optimal size of the laryngeal mask airway in normal adults. Anaesthesia 1998; 53: 565±70 19 Voyagis GS, Batzioulis PG, Secha-Doussaitou PN. Selection of the proper size of laryngeal mask airway in adults. Anesth Analg 1996; 83: 663±4 20 Vanner RG, Asai T. Safe use of cricoid pressure. Anaesthesia 1999; 54: 1±3 21 Altman DG. Practical Statistics for Medical Research. London: Chapman and Hall, 1991 22 Koga K, Asai T, Latto IP, Vaughan RS. Effect of the size of a tracheal tube and the ef®cacy of the use of the laryngeal mask for
Cricoid pressure and laryngeal mask
23 24 25 26
®brescope-aided tracheal intubation. Anaesthesia 1997; 52: 131±5 Armitage P, Berry G. Statistical Methods in Medical Research, 3rd edn. Oxford: Blackwell Scienti®c, 1994 Brimacombe J, Berry A. Mechanical airway obstruction after cricoid pressure with the laryngeal mask airway. Anesth Analg 1994; 78: 604±5 Schwartz D, Johnson C, Roberts J. A maneuver to facilitate ¯exible ®beroptic intubation. Anesthesiology 1989; 71: 470±1 Marsh NJ. Easier ®beroptic intubations. Anesthesiology 1992; 76: 860±1
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27 Asai T. Fiberoptic tracheal intubation through the laryngeal mask in an awake patient with cervical spine injury. Anesth Analg 1993; 77: 404 28 Asai T. Use of the laryngeal mask for ®brescope-aided tracheal intubation in an awake patient with a deviated larynx. Acta Anaesthesiol Scand 1994; 38: 615±6 29 Asai T. Use of the laryngeal mask for tracheal intubation in patients at increased risk of aspiration of gastric contents. Anesthesiology 1991; 77: 1029±30
Cricoid pressure applied after placement of laryngeal mask impedes subsequent ®breoptic tracheal intubation through mask² T. Asai*, K. Murao and K. Shingu Department of Anaesthesiology, Kansai Medical University, 10±15 Fumizono-cho, Moriguchi City, Osaka 570-8507, Japan *Corresponding author 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 ®breoptic 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 ®brescope 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 signi®cantly higher than in group C (21 patients, P<<0.001; 95% CI for difference: 9±48%). The time for insertion of the ®brescope in group S (median (95% CI): 12 (11±12) s) was signi®cantly 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 signi®cantly 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 signi®cantly more dif®cult. Br J Anaesth 2000; 85: 256±61 Keywords: intubation tracheal; equipment; laryngeal mask; cricoid pressure Accepted for publication: March 14, 2000
The laryngeal mask+ has a potential role in patients with dif®cult 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 insuf¯ation, 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 insuf¯ation.5 The patient is woken up or the trachea intubated through the laryngeal mask, because it is dif®cult to maintain suf®cient force of cricoid pressure for a prolonged period of time,11 and because ventilation via the laryngeal mask may be insuf®cient because of the +
LMAâ is the property of Intavent Limited.
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 con®rm this. We studied if this claim was supported.
Patients and methods 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. ²This work was presented in part at the 12th World Congress of Anaesthesiologists in Montreal, Canada, June 2000.
Ó The Board of Management and Trustees of the British Journal of Anaesthesia 2000
Cricoid pressure and laryngeal mask
In the operating theatre, an electrocardiograph, a pulse oximeter and an arterial pressure cuff were attached. A ®rm 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 sevo¯urane 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 studies16±19 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 in¯ated 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 in¯ation 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 in¯ation 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 ®xed 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 ®breoptic 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 ®ngers 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 ®breoptic bronchoscope was then inserted into the trachea and the tracheal tube was passed over the ®brescope into the trachea. If it was dif®cult 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 signi®cant. The 95% con®dence intervals (CI) for difference in the success rate of tracheal intubation between groups were also calculated. Time for insertion of the ®brescope into the trachea and time for tracheal intubation over the ®brescope were measured in patients in whom tracheal insertion of the ®brescope 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 ®brescope or tracheal intubation was generally not normally distributed. The Mann±Whitney U-test was therefore used to compare the time for insertion of the ®brescope 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% con®dence interval, using an equivalence test.23
Results In 72 of the patients recruited, adequate ventilation through the laryngeal mask was obtained at the ®rst 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).
257
Asai et al. 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
n Age (yr) Sex (M:F) Weight (kg) Height (cm) View of the glottis (A:Ba)
Sham pressure
Cricoid pressure
35 50 (20±76) 21:14 61 (11) 162 (7) 33:2
35 54 (23±80) 23:12 61 (11) 162 (9) 32:3
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: ®ve patients; score 3: three patients; and score 4: eight patients). The ®brescope 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 signi®cant 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 ®brescope into the trachea succeeded, the time for insertion of the ®brescope in group S (median (95% CI): 12 (11±12) s) was signi®cantly 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 signi®cantly faster than in group C (22 (19±24) s, P<0.0005; 95% CI for difference: 3±7 s).
Discussion Contrary to our hypothesis, cricoid pressure applied after insertion of the laryngeal mask signi®cantly 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 dif®cult (Fig. 2). This difference is likely to be produced by the laryngeal mask. 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. In¯ation 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 speci®cally, the arytenoid cartilage) is being shifted
Fig 1 Success rate of insertion of tracheal intubation through the laryngeal mask using a ®bre optic bronchoscope with and without cricoid pressure applied after insertion of the mask. Tracheal intubation through the laryngeal mask succeeded (®lled box); tracheal insertion of a ®brescope succeeded, but tracheal intubation failed (hatched box) and tracheal insertion of a ®brescope failed (clear box).
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). 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 signi®cantly easier and faster than conventional ®breoptic intubation (without the aid of the laryngeal mask). With conventional ®breoptic tracheal intubation, advance of the tracheal tube over the ®brescope into the trachea is dif®cult in 50±90% of patients even if the patients do not have dif®cult airways.22 25 26 In contrast, insertion of the ®brescope through the laryngeal mask into the trachea and advance of the tracheal tube over the ®brescope are usually easy.6 20 In our current study, the success rate of ®breoptic 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 ®breoptic intubation through the laryngeal mask in patients in whom conventional ®breoptic intubation had failed or was extremely
258
Cricoid pressure and laryngeal mask
Fig 2 A typical view of the glottis through the laryngeal mask, before (upper ®gure) and after (lower ®gure) 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.
Fig 3 A typical view of the glottis, before (upper ®gure) and after (lower ®gure) application of cricoid pressure (without the presence of the laryngeal mask). There is little difference in the patency of the glottis.
259
Asai et al.
dif®cult.27 28 Therefore, the laryngeal mask is a useful aid to ®brescope-aided tracheal intubation. In our previous report in which cricoid pressure was applied before insertion of the laryngeal mask, the success rate of ®breoptic 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 dif®cult 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 ®rst 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 dif®culty in advancing the tracheal tube over the ®brescope, cricoid pressure might be temporarily loosened and tracheal intubation over the ®brescope reattempted. The third situation is when a dif®cult 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 dif®cult when cricoid pressure is applied. Therefore, it is safer to intubate the trachea before induction of anaesthesia.
References
260
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