1 Department of Anaesthesiology, Kansai Medical University, 10-15 Fumizono-cho, Moriguchi City, Osaka 570-8507, Japan. 2 Department of Anaesthesia, Matsue Red Cross Hospital, Matsue City, Shimane 690-8506, Japan *Corresponding author
LMA® is the property of Intavent Limited.
Accepted for publication: May 12, 2002
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Abstract |
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Method. In a randomized, crossover design, the laryngeal tube and laryngeal mask were inserted in turn after induction of anaesthesia and neuromuscular block. The cuffs were inflated until the intracuff pressure reached 60 cm H2O. We measured adequacy of ventilation and the minimum airway pressure at which gas leaked around the cuff. The presence or absence of gastric insufflation was studied at an inflation pressure of 20 cm H2O.
Results. It was possible to ventilate through the laryngeal tube in 21 patients and through the laryngeal mask in 21 patients. The mean leak pressure for the laryngeal tube (26 (SD 5) cm H2O) was significantly greater than that for the laryngeal mask (19 (4) cm H2O) (P<0.01; 95% confidence intervals for mean difference: 5.310.2 cm H2O). Gastric insufflation did not occur when the laryngeal tube was used and was noted in three patients when the laryngeal mask was used.
Conclusion. The laryngeal tube provides a better seal in the oropharynx than the laryngeal mask.
Br J Anaesth 2002; 89: 72932
Keywords: equipment, masks laryngeal; equipment, tubes laryngeal
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Introduction |
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Patients and methods |
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In the anaesthetic room, an electrocardiograph, a pulse oximeter and an arterial pressure cuff were attached and an i.v. cannula was inserted. A firm pad (7 cm in height) was placed under the patients occiput. After the patient had breathed oxygen through a facemask for a minimum of 3 min, anaesthesia was induced with a sleep dose of propofol 2.03.0 mg kg1 i.v., supplemented with fentanyl 2 µg kg1. Neuromuscular block was obtained with vecuronium and was confirmed using a peripheral nerve stimulator. Anaesthesia was maintained with a continuous infusion of propofol while the patient was breathing oxygen (but not nitrous oxide) until the end of the study.
In a randomized, crossover design, the laryngeal tube and laryngeal mask were inserted in turn. The order was randomized by tossing a coin. Three anaesthetists were involved in insertion of these devices. All of them had used the laryngeal mask routinely for between 2 and 12 yr. One anaesthetist had used the laryngeal tube more than 100 times, whereas the other two had used it fewer than 10 times before the start of the study.
The sizes of laryngeal tube1 and laryngeal mask5 according to the patients height are shown in Table 1. The laryngeal tube was inserted into the oropharynx by the following method. Before insertion, the cuffs were deflated and a water-soluble lubricant (KY jelly) was applied to the cuffs. The patients neck was extended (sniffing position). The tip of the laryngeal tube was placed against the hard palate behind the upper incisors and the device was slid down in the centre of the mouth until a resistance was felt or the second bold black line on the tube (Fig. 1) had just passed between the upper and lower teeth. The cuffs were inflated using a cuff inflator (VBM, Medizintechnik, Germany) until the intracuff pressure reached approximately 60 cm H2O.1 The laryngeal mask was inserted using the technique described in the manufacturers instruction manual, and its cuff inflated using the cuff inflator until the intracuff pressure reached approximately 60 cm H2O.6
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If ventilation was possible, the device was fixed to the patient by the following methods: for the laryngeal tube, the bite block provided was inserted, the laryngeal tube snagged into its wedge and both were fixed using sticky tape (Fig. 1). For the laryngeal mask, a wad of gauze was inserted into the patients mouth and both were fixed using sticky tape. After fixation of the device, the airway pressure was initially maintained at 18 cm H2O by squeezing the reservoir bag for 3 s. The inflation pressure was measured using an airway pressure gauge. If there was no gas leak, airway pressure was increased at each breath by 2 cm H2O, whereas if there was gas leak, airway pressure was decreased at each breath by 2 cm H2O. The airway pressure at which gas leaked around the test device was recorded. The adequacy of ventilation was scored in the five categories shown in Table 2.
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Our main interest was to compare the leak pressures between the laryngeal tube and laryngeal mask, and thus the number of patients required was calculated on the basis of this factor. The mean airway pressure at which gas leaks around the laryngeal mask has been reported to range from 18 to 20 cm H2O (SD approximately 5 cm H2O).9 We considered that an increase in the leak pressure to 2526 cm H2O (i.e. a 30% increase) would be a clinically important difference. Twenty-two patients would be required to detect this difference with a power of 80% (if comparison was made between two independent groups). For gastric insufflation, we did not plan to apply any hypothesis for comparing the groups, since the number of patients we planned to study could be too small to detect a difference, and because the clinical significance of this observation during such a short period of time at a fixed airway pressure (20 cm H2O) was not certain.
Plots of normal scores showed that the data for the differences in the leak pressures between the laryngeal tube and laryngeal mask were normally distributed, and thus a paired t-test was used for comparison. P<0.05 was considered significant. The 95% confidence intervals (CI) for the difference in the mean leak pressure between the laryngeal tube and the laryngeal mask were also calculated.
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Results |
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It was possible to insert and ventilate through the laryngeal tube in 21 of 22 patients (without changing its position after inflation of the cuffs). Ventilation was adequate at the first attempt in 19 patients and at the second attempt in another two patients. Adequate ventilation was not possible after two attempts in the remaining one patient. It was possible to insert and ventilate through the laryngeal mask in 21 of 22 patients. Ventilation was adequate at the first attempt in 20 patients and at the second attempt in another one patient. Adequate ventilation was not possible in the remaining one patient.
The mean volume of air placed in the cuff to give the intracuff pressure of 60 cm H2O was 75 (SD 8) ml for the laryngeal tube and 19 (4) ml for the laryngeal mask. The minimum mean airway pressure at which gas leaked around the cuff of the laryngeal tube was 26 (5) cm H2O, which was significantly greater than that of the laryngeal mask (19 (4) cm H2O) (P<0.01; 95% CI for mean difference: 5.3 10.2 cm H2O) (Fig. 3). When the laryngeal tube was used, gas did not leak around the cuff at an airway pressure of 30 cm H2O in 12 of 22 patients (55%), whereas when the laryngeal mask was used gas always leaked around the cuff with an airway pressure of less than 30 cm H2O (Table 2).
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Discussion |
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The laryngeal tube provided a good airtight seal in most patients (Table 2), and often there was no gas leak around the cuff at an airway pressure of 30 cm H2O. The leak pressure for the laryngeal tube was significantly higher than that for the laryngeal mask (Fig. 3).
Compared with the laryngeal mask, the laryngeal tube provided a significantly better seal to the oropharynx. In addition, gastric insufflation did not occur in any of 22 patients when the laryngeal tube was used, whereas it occurred in three patients when the laryngeal mask was used. Therefore, the laryngeal tube may be more suitable than the laryngeal mask during intermittent positive-pressure ventilation. However, studies of the laryngeal tube are few, with only a few preliminary reports1012 of its efficacy during the entire course of anaesthesia and of postoperative complications such as sore throat or lingual nerve palsy. Therefore, caution is required with the results obtained in this study until the safety of the routine use of the laryngeal tube during anaesthesia is known.
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Acknowledgement |
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References |
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