A review by Asai and Brimacombe3 re-emphasized the fact that LMAs are often over-inflated by anaesthetists and operating department practitioners (ODPs), despite several papers advocating inflation of the cuff with volumes less than the manufacturer's recommended maximum. Reported cuff pressures have been as high as 250 mm Hg or more.4 In order to measure such high pressures, a pressure measuring device was constructed using a metal bourdon gauge from an old sphygmomanometer, which was connected to a three-way tap with an Abbot-T-connector and a short length of green oxygen tubing (Fig. 1). The gauge was calibrated by the medical physics department using a Timeter RT 200 pressure calibration device. The three-way tap was turned off between readings (Fig. 1), to prevent leakage through the gauge.
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Some patients in Groups 2 and 3 were given a neuromuscular blocking drug, and their lungs were artificially ventilated. The age, weight and sex of all the patients was recorded, as were LMA cuff pressures at the start and end of anaesthesia. Obvious venous congestion of the neck was also noted, specifically looking for purple discoloration of the skin of the neck and/or face, with or without mottling. If there was doubt about slight congestion, it was not counted. In some cases of severe congestion, the cuff pressure was lowered to just above airway leak pressure in an attempt to reduce it.
In Group 1, very high cuff pressures were recorded (mean 132, range 41238 mm Hg) increasing to a mean of 153 (range 41241) mm Hg after nitrous oxide, oxygen, sevoflurane anaesthesia. There were 14 patients with marked venous congestion of the neck. In three patients, congestion visibly improved over a few minutes after the cuff was partially deflated to reduce the pressure. In Group 2, a lower cuff pressure was noted (mean 62 (range 7114) mm Hg, increasing to mean 88 (7131) mm Hg) by the end of anaesthesia. Only four patients were noted to have venous congestion.
In Group 3, the cuff was inflated according to the recommendations of Asai and Brimacombe3 (1015 ml initially and then another 510 ml as necessary to prevent a leak at 15 cm H2O airway pressure, or >15 cm H2O in patients at increased risk of aspiration from above the cuff). In this group, the LMA cuff pressure was lower (mean 20, range 0 to 66 mm Hg, increasing to mean 29, range 374 mm Hg) with the pressure in one LMA being atmospheric, yet still preventing an airway leak during artificial ventilation. No patient in this group became congested. In the 15 patients in Group 3 undergoing controlled ventilation, the mean cuff pressure was 21 (range 066) mm Hg, increasing to 32 (range 474) mm Hg (Table 1).
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I conclude that we should adopt the guidelines used in the patients in Group 3, inflating LMA cuffs to just above airway leak pressure. An audit of LMA cuff pressures could usefully be carried out in all anaesthetic departments. The second edition of the LMA instruction manual recommends monitoring the intra-cuff pressure to prevent postoperative throat discomfort, but this practice has not generally been adopted.7 I would suggest that every theatre should be equipped with a laryngeal mask cuff pressure gauge, but meanwhile anaesthetists could use the simple pressure gauge I have described (Fig. 1).
Portsmouth, UK
Footnotes
LMA® is the property of Intavent Ltd.
References
1 Colbert SA, O'Hanlon DM, Flanagan F, Page R, Moriarty DC. The laryngeal mask airway reduces blood flow in the common carotid artery bulb. Can J Anaesth 1998; 45: 237[Abstract]
2 Nandwani N, Fairfield MC, Krarup K, Thompson J. The effect of laryngeal mask airway insertion on the position of the internal jugular vein. Anaesthesia 1997; 52: 7783[CrossRef][ISI][Medline]
3 Asai T, Brimacombe J. Cuff volume and size selection with the laryngeal mask. Anaesthesia 2000; 55: 117984[CrossRef][ISI][Medline]
4 Margot R. Pressure exerted by the laryngeal mask airway cuff upon the pharyngeal mucosa. Br J Anaesth 1993; 70: 259[Abstract]
5 Brimacombe J, Keller C, Morris R, Mecklam D. A comparison of the disposable versus the reusable laryngeal mask airway in paralysed adult patients. Anaesth Analg 1998; 87: 9214[Abstract]
6 Keller C, Brimacombe J, Benzer A. Calculated vs measured pharyngeal mucosal pressures with the laryngeal mask airway during cuff inflation: assessment of four locations. Br J Anaesth 1999; 82: 399401
7 Brimacombe J, Holyoake L, Keller C, et al. Emergence characteristics and post-operative laryngopharyngeal morbidity with the laryngeal mask airway: a comparison of high versus low initial cuff volume. Anaesthesia 2000; 55: 33843[CrossRef][ISI][Medline]