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The Oesophageal Vent-Laryngeal Mask
British Journal of Anaesthesia 1994; 72: 726-735
CORRESPONDENCE The Oesophageal Vent-Laryngeal Mask
T. ASAI
Kansai Medical University Osaka, Japan
1. Akhtar TM. Oesophageal Vent-Laryngeal Mask to prevent aspiration of gastric contents. British Journal of Anaesthesia 1994; 72: 52-54. 2. Brain AIJ. The development of the laryngeal mask—a brief history of the invention, early clinical studies and experimental work from which the laryngeal mask evolved. European Journal of Anaesthesiology 1991; (Suppl. 4): 5-17. 3. Barker P, Murphy P, Langton JA, Rowbotham DJ. Regurgitation of gastric contents during general anaesthesia using the laryngeal mask airway. British Journal of Anaesthesia 1992; 69: 314-315. 4. El Mikatti N, Luthra AD, Healy TEJ, Mortimer AJ. Gastric regurgitation during general anaesthesia in the supine position with the laryngeal and face mask airways. British Journal of Anaesthesia 1992: 69: 529-530P. 5. Vanner RG. Gastro-esophageal reflux and regurgitation during general anesthesia for termination of pregnancy. International Journal of Obstetric Anesthesia 1992; 1: 123-128.
Sir,—I was interested to read the article by Akhtar [1] describing a prototype for a combined laryngeal mask-oesophageal obturator. I note the reported success in preventing oesophageal reflux in patients not at risk, but at what cost? Previous studies [2, 3] have shown that partial airway obstruction after insertion of the laryngeal mask airway (LMA) may occur, which is difficult to detect by clinical observation alone. Fibreoptic laryngoscopy has been shown to be a valuable technique in assessing such cases, resulting in a 10% detection rate in adults. In one study [3], 60 % of these partial obstructions were caused by back-folding of the epiglottis and 40 % by forward displacement of the LMA tip into the posterior portion of the laryngeal inlet. One could argue that an LMA attached to an oesophageal tube will not follow its intended route through the pharynx to the laryngeal inlet, resulting in partial airway obstruction which may or may not be evident on clinical grounds alone. Perhaps significantly, of 17 patients in this study, one complete obstruction and two cases each of laryngeal spasm and hiccup were reported. S. S. SOROOSHIAN
University Department of Anaesthetics Newcastle upon Tyne 1. Akhtar TM. Oesophageal Vent-Laryngeal Mask to prevent aspiration of gastric contents. British Journal of Anaesthesia 1994; 72: 52-54. 2. Brodrick PM, Webster NR, Nunn JF. The laryngeal mask airway. Anaesthesia 1989; 44: 238-241. 3. Payne J. The use of fibreoptic laryngoscopy to confirm the position of the laryngeal mask. Anaesthesia 1989; 44: 865.
Sir,—We read with interest the paper by Dr Akhtar [1] regarding the Oesophageal Vent-Laryngeal Mask and suggesting its possible use for patients at risk of pulmonary aspiration. However, use of the laryngeal mask airway (LMA) for such patients is unsuitable unless cricoid pressure is applied and this may impede successful placement [2, 3]. Placement of the Oesophageal Vent-Laryngeal Mask would certainly be difficult while effective cricoid pressure is used. If cricoid pressure is released, then pulmonary aspiration could occur. We have conducted a study in paralysed patients in whom an LMA and oesophageal tube were placed separately [4]. With a tracheal tube in the oesophagus, it was possible to insert an LMA successfully. Therefore, if tracheal intubation fails because of oesophageal intubation, the tube could be left in the oesophagus with its cuff inflated to prevent aspiration, while further attempts are made to intubate the trachea. If a tracheal tube cannot be subsequently inserted successfully, an LMA can be used to provide effective oxygenation of the patient while the tracheal tube remains inflated in the oesophagus and protects against aspiration. Furthermore, it is possible to use a face mask to ventilate
Downloaded from http://bja.oxfordjournals.org/ at New York University on May 30, 2015
Sir,—Dr Akhtar proposed a modified laryngeal mask, the "Oesophageal Vent-LaryngeaJ Mask", for prevention of aspiration [1]. The modified device described by him, which had been considered and abandoned previously by Dr Brain [2], has several theoretical problems. One of the crucial points in the safe use of the laryngeal mask airway (LMA) is that the tip of the mask is inserted correctly into the hypopharynx. Because the modified device consists of a trachea] tube (obturator) which is fixed to the dorsum of the LMA, the mask is likely to be driven in an anterior direction during placement. In addition, because the large-bore obturator is inserted through the hypopharynx into the oesophagus before placement of the mask, the obturator may prevent the tip of the mask from occupying its correct position in the hypopharynx. In both situations, the tip of the mask is likely to press down the epiglottis, obstruct the glottis or impact upon the arytenoid cartilages, leading to failed ventilation or laryngospasm. In fact, laryngospasm occurred in two of 16 patients in his study. Furthermore, placement of the modified LMA would be more difficult than the standard one in patients in whom movements of the head and neck are restricted because of the greater length of the two combined tubes and reduced flexibility. Dr Ahktar suggests using the modified LMA after failed tracheal intubation in patients at increased risk of aspiration. Cricoid pressure is necessary when anaesthesia is induced in this group of patients, because, as he reports, regurgitarion can occur immediately after induction of anaesthesia. However, temporary release of cricoid pressure is necessary for placement of the modified LMA, because the obturator cannot be inserted when the oesophagus is compressed by cricoid pressure. In addition, although the number of patients studied is too small to make comparisons between the success rate of placement of the standard LMA and the modified masks, it seems that the success rate is lower and the incidence of complications higher with the modified advice. This may be true because of the above-mentioned theoretical problems. Therefore, placement of the modified device may require release of cricoid pressure and cessation of ventilation for a longer period than the standard one. This implies that placement of the modified device may be associated with a higher incidence of both aspiration and hypoxia. The incidence of regurgitation while the LMA is in place in patients at low risk of regurgitation is not clear [3, 4]. The incidence in his study cannot be attributed entirely to the presence of the LMA, as the presence of the obturator in the oesophagus also may induce regurgitation. Two patients hiccuped in the study. Although he did not state if the patients who hiccuped also regurgitated, hiccups are frequently associated with regurgitation [5]. When the LMA is placed correctly, that is when the tip of the mask occupies the hypopharyx, the oesophagus should not be seen in the aperture of the mask. It is doubtful that preventing regurgitation by placing a tube into the oesophagus can be justified in this group of patients. Another theoretical problem is that the obturator may be inserted into the trachea. This might not be a problem as the lungs can be ventilated through the obturator, but the length of the protruding part of the obturator is too short so that the cuff of the obturator may be positioned between the vocal cords. The incorrectly positioned cuff cannot always prevent aspiration and may damage the vocal cords, particularly when the cuff is inflated with 20 ml of air. Unless the above-mentioned problems are overcome, attempts at placement of any modified device in patients at increasedriskof aspiration is not advisable. Dr Brain has considered another modified LMA in which a tube is passed through the LMA with the orifice of the tube in the truncated tip of the mask [2]. Regurgitated material can be drained through the tube and, if required, a soft cannula with a balloon can be passed through the drainage tube into the oesophagus. This modified device may be more useful.
CORRESPONDENCE
727
the lungs of a patient who has a tube in the oesophagus if the proximal end of the tube is brought under the rim of the face masks. In patients with limited mouth opening, our technique may be easier than insertion of the Oesophageal Vent-Laryngeal Mask, as the oesophageal rube can be moved over to the left side of the mouth. Clearly, further assessment and comparison of these techniques in cases of difficult and failed tracheal intubation would be interesting. N. M. GAJRAJ N. A. PACE J. H. PENNANT
University of Texas Southwestern Medical Center Dallas, Texas
Sir,—The modified laryngeal mask described by Akhtar [1] reflects a line of enquiry pursued by the inventor of the laryngeal mask airway (LMA) between 1983 and 1987. Figure 1 shows one of the patented designs, which closely resembles Akhtar's modification, but has a communication between the two cuffs, permitting simultaneous inflation from a single inflation port. The device, which was made entirely from silicone, can be seen in the LMA Museum at the Royal Berkshire Hospital, Reading. The reasons for not continuing with this development may be of interest. The addition of an integral oesophageal tube increases the difficulty of insertion, the invasiveness of the procedure and the complexity (and hence the likely price) of the device. Such a development therefore is not likely to be attractive as a generalpurpose airway. If it is desired to pass a rube into the oesophagus, this can be done easily after placing a standard LMA using a welllubricated tracheal rube. The LMA is only moderately inflated (for example 25 ml in a size 4) and the trachcal tube is passed blindly and gently with the head extended. Not only is this easier,
A. I. J. BRAIN Northvrick Park Hospital Middlesex
1. Akhtar TM. Oesophageal Vent-Laryngeal Mask to prevent aspiration of gastric contents. British Journal of Anaesthesia 1994; 72: 52-54. Sir,—Thank you for the opportunity to reply to the comments made on the Oesophageal Vent-Laryngeal Mask. The device described is only a prototype that requires refinement to allow better conformation to the oropharyngeal anatomy which could make placement no more difficult than the standard laryngeal mask airway (LMA). I note Dr Brain's comments but remember him describing a new modification of the LMA that has a tube attached to the dorsum of the LMA (not penetrating the oesophagus) for the drainage of regurgitated contents, at a conference on " The Use of Laryngcal Mask for Resuscitation" in London, January 13, 1994. Hiccups under general anaesthesia may increase the incidence of regurgitation but two patients that hiccuped in my study did not regurgitate. The presence of the LMA anterior to the oesophageal tube encourages it to slide along the posterior wall of the pharynx and the oesophagus, making trachcal intubation most unlikely. Search for a better design of the LMA (and various other techniques) to prevent regurgitation however, continues. T. AKHTAR King's College Hospital London
Ferrous distortion during MRI Sir,—The presence of ferromagnetic material in the proximity of magnets during magnetic resonance imaging (MRI) may produce unwanted interference and degradation of image quality [1]. Standard tracheal tubes may become kinked in patients undergoing head and neck procedures or in association with changes in patient position. Reinforced tracheal tubes are used in these situations to prevent intraoperative hypoxaemia and increases in airway pressure. We report a case of image distortion caused by a stainless steel reinforced tracheal rube. A 49-yr-old female presented for stereotactic biopsy of a right temporal space-occupying lesion. Anaesthesia was induced with fentanyl 2 |ig kg"1 and thiopentone 4 mg kg"1, and vecuronium 0.1 mg kg"1 was administered to facilitate tracheal intubation with a 7.5-mm cuffed reinforced tracheal tube. Anaesthesia was maintained with nitrous oxide and isoflurane in oxygen and normocapnic controlled ventilation. A stereotactic frame was applied and imaging commenced. However, marked image distortion was observed. A biopsy specimen of the targeted lesion was reported as normal brain tissue. An open brain biopsy was required to obtain histological specimens which revealed glioblastoma multiforma. This is the first reported case of a reinforced tracheal tube producing image distortion during an MRI procedure. Image distortion associated with armoured tracheal tubes has major clinical implications. We suggest the use of a nylon reinforced tube when a non-kinkable tube is indicated during MRI procedures. Extra anaesthetic vigilance is warranted because such tubes may be more prone to kinking. This case report confirms previous experience that any metal object, if sufficiently close to the region to be scanned, may cause image distortion [2]. M. CARROLL M. ELJAMEL A. J. CUNNINGHAM Departments of Anaesthesia and Neurosurgery Beaumont Hospital, Dublin
FIG. 1. One of the patented designs of the laryngeal mask airway.
1. Menon DK, Peden CJ, Hall AS, Sargentoni J, Whirwam JG. Magnetic resonance for the anaesthetist. Part I: physical
Downloaded from http://bja.oxfordjournals.org/ at New York University on May 30, 2015
1. Akhtar TM. Oesophageal Vent-Laryngeal Mask to prevent aspiration of gastric contents. British Journal of Anaesthesia 1994; 72: 52-54. 2. Asai T, Barclay K, Power I, Vaughan RS. Cricoid pressure impedes placement of the laryngeal mask airway and subsequent txachcal intubation through the mask. British Journal of Anaesthesia 1994; 72: 47-51. 3. Ansermino JM, Blogg CE. Cricoid pressure may prevent insertion of the laryngeal mask airway. British Journal of Anaesthesia 1992; 69: 465-^67. 4. Gajraj NM, Pace NA, Pennant JH, Victory RA, Johnson ER, White PF. Use of the laryngeal mask airway following unintentional oesophageal intubation. Anesthesia and Analgesia 1993; 76: S112.
but there is a further advantage in that the mask serves to shield the glottis from the tube as the latter is passed downwards, guiding it into the upper oesophageal sphincter. A characteristic pressure is felt as the tube encounters and passes through the sphincter, so that it is not difficult to judge the level of tube placement.
CORRESPONDENCE The Oesophageal Vent-Laryngeal Mask
T. ASAI
Kansai Medical University Osaka, Japan
1. Akhtar TM. Oesophageal Vent-Laryngeal Mask to prevent aspiration of gastric contents. British Journal of Anaesthesia 1994; 72: 52-54. 2. Brain AIJ. The development of the laryngeal mask—a brief history of the invention, early clinical studies and experimental work from which the laryngeal mask evolved. European Journal of Anaesthesiology 1991; (Suppl. 4): 5-17. 3. Barker P, Murphy P, Langton JA, Rowbotham DJ. Regurgitation of gastric contents during general anaesthesia using the laryngeal mask airway. British Journal of Anaesthesia 1992; 69: 314-315. 4. El Mikatti N, Luthra AD, Healy TEJ, Mortimer AJ. Gastric regurgitation during general anaesthesia in the supine position with the laryngeal and face mask airways. British Journal of Anaesthesia 1992: 69: 529-530P. 5. Vanner RG. Gastro-esophageal reflux and regurgitation during general anesthesia for termination of pregnancy. International Journal of Obstetric Anesthesia 1992; 1: 123-128.
Sir,—I was interested to read the article by Akhtar [1] describing a prototype for a combined laryngeal mask-oesophageal obturator. I note the reported success in preventing oesophageal reflux in patients not at risk, but at what cost? Previous studies [2, 3] have shown that partial airway obstruction after insertion of the laryngeal mask airway (LMA) may occur, which is difficult to detect by clinical observation alone. Fibreoptic laryngoscopy has been shown to be a valuable technique in assessing such cases, resulting in a 10% detection rate in adults. In one study [3], 60 % of these partial obstructions were caused by back-folding of the epiglottis and 40 % by forward displacement of the LMA tip into the posterior portion of the laryngeal inlet. One could argue that an LMA attached to an oesophageal tube will not follow its intended route through the pharynx to the laryngeal inlet, resulting in partial airway obstruction which may or may not be evident on clinical grounds alone. Perhaps significantly, of 17 patients in this study, one complete obstruction and two cases each of laryngeal spasm and hiccup were reported. S. S. SOROOSHIAN
University Department of Anaesthetics Newcastle upon Tyne 1. Akhtar TM. Oesophageal Vent-Laryngeal Mask to prevent aspiration of gastric contents. British Journal of Anaesthesia 1994; 72: 52-54. 2. Brodrick PM, Webster NR, Nunn JF. The laryngeal mask airway. Anaesthesia 1989; 44: 238-241. 3. Payne J. The use of fibreoptic laryngoscopy to confirm the position of the laryngeal mask. Anaesthesia 1989; 44: 865.
Sir,—We read with interest the paper by Dr Akhtar [1] regarding the Oesophageal Vent-Laryngeal Mask and suggesting its possible use for patients at risk of pulmonary aspiration. However, use of the laryngeal mask airway (LMA) for such patients is unsuitable unless cricoid pressure is applied and this may impede successful placement [2, 3]. Placement of the Oesophageal Vent-Laryngeal Mask would certainly be difficult while effective cricoid pressure is used. If cricoid pressure is released, then pulmonary aspiration could occur. We have conducted a study in paralysed patients in whom an LMA and oesophageal tube were placed separately [4]. With a tracheal tube in the oesophagus, it was possible to insert an LMA successfully. Therefore, if tracheal intubation fails because of oesophageal intubation, the tube could be left in the oesophagus with its cuff inflated to prevent aspiration, while further attempts are made to intubate the trachea. If a tracheal tube cannot be subsequently inserted successfully, an LMA can be used to provide effective oxygenation of the patient while the tracheal tube remains inflated in the oesophagus and protects against aspiration. Furthermore, it is possible to use a face mask to ventilate
Downloaded from http://bja.oxfordjournals.org/ at New York University on May 30, 2015
Sir,—Dr Akhtar proposed a modified laryngeal mask, the "Oesophageal Vent-LaryngeaJ Mask", for prevention of aspiration [1]. The modified device described by him, which had been considered and abandoned previously by Dr Brain [2], has several theoretical problems. One of the crucial points in the safe use of the laryngeal mask airway (LMA) is that the tip of the mask is inserted correctly into the hypopharynx. Because the modified device consists of a trachea] tube (obturator) which is fixed to the dorsum of the LMA, the mask is likely to be driven in an anterior direction during placement. In addition, because the large-bore obturator is inserted through the hypopharynx into the oesophagus before placement of the mask, the obturator may prevent the tip of the mask from occupying its correct position in the hypopharynx. In both situations, the tip of the mask is likely to press down the epiglottis, obstruct the glottis or impact upon the arytenoid cartilages, leading to failed ventilation or laryngospasm. In fact, laryngospasm occurred in two of 16 patients in his study. Furthermore, placement of the modified LMA would be more difficult than the standard one in patients in whom movements of the head and neck are restricted because of the greater length of the two combined tubes and reduced flexibility. Dr Ahktar suggests using the modified LMA after failed tracheal intubation in patients at increased risk of aspiration. Cricoid pressure is necessary when anaesthesia is induced in this group of patients, because, as he reports, regurgitarion can occur immediately after induction of anaesthesia. However, temporary release of cricoid pressure is necessary for placement of the modified LMA, because the obturator cannot be inserted when the oesophagus is compressed by cricoid pressure. In addition, although the number of patients studied is too small to make comparisons between the success rate of placement of the standard LMA and the modified masks, it seems that the success rate is lower and the incidence of complications higher with the modified advice. This may be true because of the above-mentioned theoretical problems. Therefore, placement of the modified device may require release of cricoid pressure and cessation of ventilation for a longer period than the standard one. This implies that placement of the modified device may be associated with a higher incidence of both aspiration and hypoxia. The incidence of regurgitation while the LMA is in place in patients at low risk of regurgitation is not clear [3, 4]. The incidence in his study cannot be attributed entirely to the presence of the LMA, as the presence of the obturator in the oesophagus also may induce regurgitation. Two patients hiccuped in the study. Although he did not state if the patients who hiccuped also regurgitated, hiccups are frequently associated with regurgitation [5]. When the LMA is placed correctly, that is when the tip of the mask occupies the hypopharyx, the oesophagus should not be seen in the aperture of the mask. It is doubtful that preventing regurgitation by placing a tube into the oesophagus can be justified in this group of patients. Another theoretical problem is that the obturator may be inserted into the trachea. This might not be a problem as the lungs can be ventilated through the obturator, but the length of the protruding part of the obturator is too short so that the cuff of the obturator may be positioned between the vocal cords. The incorrectly positioned cuff cannot always prevent aspiration and may damage the vocal cords, particularly when the cuff is inflated with 20 ml of air. Unless the above-mentioned problems are overcome, attempts at placement of any modified device in patients at increasedriskof aspiration is not advisable. Dr Brain has considered another modified LMA in which a tube is passed through the LMA with the orifice of the tube in the truncated tip of the mask [2]. Regurgitated material can be drained through the tube and, if required, a soft cannula with a balloon can be passed through the drainage tube into the oesophagus. This modified device may be more useful.
CORRESPONDENCE
727
the lungs of a patient who has a tube in the oesophagus if the proximal end of the tube is brought under the rim of the face masks. In patients with limited mouth opening, our technique may be easier than insertion of the Oesophageal Vent-Laryngeal Mask, as the oesophageal rube can be moved over to the left side of the mouth. Clearly, further assessment and comparison of these techniques in cases of difficult and failed tracheal intubation would be interesting. N. M. GAJRAJ N. A. PACE J. H. PENNANT
University of Texas Southwestern Medical Center Dallas, Texas
Sir,—The modified laryngeal mask described by Akhtar [1] reflects a line of enquiry pursued by the inventor of the laryngeal mask airway (LMA) between 1983 and 1987. Figure 1 shows one of the patented designs, which closely resembles Akhtar's modification, but has a communication between the two cuffs, permitting simultaneous inflation from a single inflation port. The device, which was made entirely from silicone, can be seen in the LMA Museum at the Royal Berkshire Hospital, Reading. The reasons for not continuing with this development may be of interest. The addition of an integral oesophageal tube increases the difficulty of insertion, the invasiveness of the procedure and the complexity (and hence the likely price) of the device. Such a development therefore is not likely to be attractive as a generalpurpose airway. If it is desired to pass a rube into the oesophagus, this can be done easily after placing a standard LMA using a welllubricated tracheal rube. The LMA is only moderately inflated (for example 25 ml in a size 4) and the trachcal tube is passed blindly and gently with the head extended. Not only is this easier,
A. I. J. BRAIN Northvrick Park Hospital Middlesex
1. Akhtar TM. Oesophageal Vent-Laryngeal Mask to prevent aspiration of gastric contents. British Journal of Anaesthesia 1994; 72: 52-54. Sir,—Thank you for the opportunity to reply to the comments made on the Oesophageal Vent-Laryngeal Mask. The device described is only a prototype that requires refinement to allow better conformation to the oropharyngeal anatomy which could make placement no more difficult than the standard laryngeal mask airway (LMA). I note Dr Brain's comments but remember him describing a new modification of the LMA that has a tube attached to the dorsum of the LMA (not penetrating the oesophagus) for the drainage of regurgitated contents, at a conference on " The Use of Laryngcal Mask for Resuscitation" in London, January 13, 1994. Hiccups under general anaesthesia may increase the incidence of regurgitation but two patients that hiccuped in my study did not regurgitate. The presence of the LMA anterior to the oesophageal tube encourages it to slide along the posterior wall of the pharynx and the oesophagus, making trachcal intubation most unlikely. Search for a better design of the LMA (and various other techniques) to prevent regurgitation however, continues. T. AKHTAR King's College Hospital London
Ferrous distortion during MRI Sir,—The presence of ferromagnetic material in the proximity of magnets during magnetic resonance imaging (MRI) may produce unwanted interference and degradation of image quality [1]. Standard tracheal tubes may become kinked in patients undergoing head and neck procedures or in association with changes in patient position. Reinforced tracheal tubes are used in these situations to prevent intraoperative hypoxaemia and increases in airway pressure. We report a case of image distortion caused by a stainless steel reinforced tracheal rube. A 49-yr-old female presented for stereotactic biopsy of a right temporal space-occupying lesion. Anaesthesia was induced with fentanyl 2 |ig kg"1 and thiopentone 4 mg kg"1, and vecuronium 0.1 mg kg"1 was administered to facilitate tracheal intubation with a 7.5-mm cuffed reinforced tracheal tube. Anaesthesia was maintained with nitrous oxide and isoflurane in oxygen and normocapnic controlled ventilation. A stereotactic frame was applied and imaging commenced. However, marked image distortion was observed. A biopsy specimen of the targeted lesion was reported as normal brain tissue. An open brain biopsy was required to obtain histological specimens which revealed glioblastoma multiforma. This is the first reported case of a reinforced tracheal tube producing image distortion during an MRI procedure. Image distortion associated with armoured tracheal tubes has major clinical implications. We suggest the use of a nylon reinforced tube when a non-kinkable tube is indicated during MRI procedures. Extra anaesthetic vigilance is warranted because such tubes may be more prone to kinking. This case report confirms previous experience that any metal object, if sufficiently close to the region to be scanned, may cause image distortion [2]. M. CARROLL M. ELJAMEL A. J. CUNNINGHAM Departments of Anaesthesia and Neurosurgery Beaumont Hospital, Dublin
FIG. 1. One of the patented designs of the laryngeal mask airway.
1. Menon DK, Peden CJ, Hall AS, Sargentoni J, Whirwam JG. Magnetic resonance for the anaesthetist. Part I: physical
Downloaded from http://bja.oxfordjournals.org/ at New York University on May 30, 2015
1. Akhtar TM. Oesophageal Vent-Laryngeal Mask to prevent aspiration of gastric contents. British Journal of Anaesthesia 1994; 72: 52-54. 2. Asai T, Barclay K, Power I, Vaughan RS. Cricoid pressure impedes placement of the laryngeal mask airway and subsequent txachcal intubation through the mask. British Journal of Anaesthesia 1994; 72: 47-51. 3. Ansermino JM, Blogg CE. Cricoid pressure may prevent insertion of the laryngeal mask airway. British Journal of Anaesthesia 1992; 69: 465-^67. 4. Gajraj NM, Pace NA, Pennant JH, Victory RA, Johnson ER, White PF. Use of the laryngeal mask airway following unintentional oesophageal intubation. Anesthesia and Analgesia 1993; 76: S112.
but there is a further advantage in that the mask serves to shield the glottis from the tube as the latter is passed downwards, guiding it into the upper oesophageal sphincter. A characteristic pressure is felt as the tube encounters and passes through the sphincter, so that it is not difficult to judge the level of tube placement.