1 Department of Anaesthesiology, Kansai Medical University, 10-15 Fumizono-cho, Moriguchi City, Osaka 570-8507, Japan. 2 Ishikiri-Seiki Hospital, Yayoi, Higashi-Osaka 579-8026, Japan
Corresponding author. E-mail: asait@takii.kmu.ac.jp
Accepted for publication: November 27, 2002
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Abstract |
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Method. We studied 300 patients to compare the ease of laryngoscopy with each type of Macintosh laryngoscope blade. In a random crossover design, after induction of anaesthesia and neuromuscular block, the two blades were inserted in turn, and the views of the glottis at laryngoscopy (Cormack and Lehane scores) were compared.
Results. There was a difference in the view of the glottis in 80 patients. Among these patients, the view was better for the English blade for 63 patients and the standard blade was better for 17 patients. Laryngoscopy was difficult (grade 3 or 4) for at least one blade in 42 of 300 patients (14%). In these 42 patients, there was a difference in the score between the blades in 28 patients; the view was better for the English blade in 25 patients (60%) and for the standard blade in three patients (7%). The view was significantly better for the English blade than for the standard blade (P<0.001; 95% confidence interval 4574%).
Conclusions. In patients in whom laryngoscopy was unexpectedly difficult, the English blade provided a better glottic view significantly more frequently than the standard blade.
Br J Anaesth 2003: 90: 45760
Keywords: equipment, laryngoscopes; intubation tracheal, difficult
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Introduction |
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Several different types of Macintosh laryngoscope have been developed, and there are currently two major types available: the standard and the English type (E type) laryngoscope blades.3 4 The English blade differs from the standard blade in a few aspects (Fig. 1). Compared with the standard blade, the English blade is longer, its curve is more continuous across the entire length of the blade, and the height of the blade flange is shorter. In addition, the flange of the English blade continues close to the blade tip, whereas the flange of the standard blade ends more abruptly and further away from the tip. Contrary to Macintoshs opinion, we felt that the English blade might be better than the standard one in facilitating laryngoscopy. There have been no reports assessing this claim, so we compare the ease of laryngoscopy with the two types of Macintosh blades.
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Methods |
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The view of the oropharynx was classified before anaesthesia according to Mallampati and colleagues5 and Samsoon and Young.6 If the faucial pillars, soft palate and uvula could not be seen (score 4), the patient was excluded from the study. The distance between the thyroid notch and the thyromental distance was recorded. In the anaesthetic room, a firm pad (7 cm in height) was placed under the patients occiput, but not under the neck. After preoxygenation of the patient, anaesthesia was induced with propofol i.v. and neuromuscular block was obtained with vecuronium. Neuromuscular block was confirmed using a peripheral nerve stimulator. Anaesthesia was maintained with either propofol or sevoflurane in oxygen during the study period and analgesic agents were given according to the anaesthetists preference. If ventilation through face mask was inadequate, the patient was withdrawn from the study.
We used the standard and English Macintosh blades, both of which were manufactured by Welch Allyn (NY, USA). Both blades were made of metal and contained a fibrelight. In a random crossover fashion, the standard and English blades were used in turn. The order was randomized by tossing a coin. A blade of size 3 or 4 was selected in accordance with the anaesthetists preference, but the same size was used for the two blades. The view of the glottis at laryngoscopy was graded according to Cormack and Lehane7 for each blade. No attempt was made to improve the view of the glottis by applying pressure on the neck. The trachea was intubated at the second attempt at laryngoscopy. The anaesthetist was asked whether one blade was better or worse than, or similar to, the other blade in exposing the glottis.
The view of the oropharynx before general anaesthesia was defined as difficult when the score was 3.5 6 Laryngoscopy was defined as difficult when the view of the glottis was grade 3 or 4.711
Our main interest was to compare the ease of viewing the glottis between the two laryngoscope blades. We considered that there would be a clinically important difference if the view of the glottis was grade 3 or 4 using one blade but grade 1 or 2 using the other blade. A 2x2 table (grade 1 or 2 vs grade 3 or 4 for the two laryngoscopes) was made and McNemars test (a test of paired proportions) was used to compare the two blades.
We also used the 2 test for trend to test the relationship between the ease of the view of the glottis and the quality of preoperative view of the oropharynx, in terms of the view of the oropharynx (classes 1 and 2 vs class 3). Grades 3 and 4 of the view of the glottis were also pooled to avoid expected frequencies of less than 5.12 P<0.05 was considered significant. We also calculated the 95% confidence interval for the possibility of one blade being better than the other in patients with difficult laryngoscopy.
The reported incidence of difficult laryngoscopy (grade 3 or 4) is 313%.711 In our preliminary study of 30 patients13 (who were not included in the present study), the view of the glottis was graded 3 or 4 in four patients, in all of whom the English blade gave a better view than the standard blade. Therefore, the difference in the incidence of difficult laryngoscopy between the two blades in this preliminary study was 13% (four of 30 patients). We expected that the incidence of difficult laryngoscopy using the English blade could be up to 5% in the formal study, and therefore decided to calculate the number of patients required to detect a difference of 8% (135%). Three hundred patients would be required to detect this difference with a power of 90% and a one-sided 95% confidence interval.12
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Results |
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Discussion |
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It may not be surprising that Macintosh felt that the precise shape or curve of the blade does not seem to matter much provided the tip does not go beyond the epiglottis,1 as he did not compare different blades in each patient, and in our crossover study there was no clinically important difference between the two blades (i.e. grade 1 or 2 for one blade and grade 3 or 4 for the other) in 272 patients (91%). Nevertheless, the incidence of difficult laryngoscopy can be markedly different with different shapes of Macintosh laryngoscope. We found that the incidence was 13% (39 of 300 patients) for the standard blade and 6% (18 patients) for the English blade. Therefore, it seems reasonable to conclude that, although both the standard and the English blade can be used for most patients, it is worth trying another type of blade when laryngoscopy is difficult.
Our study has some limitations. First, we did not assess the ease of tracheal intubation because the study was done at several hospitals, where tracheal tubes of the same specification were not available. In addition, we felt that it would be unethical to compare the ease of tracheal intubation between two blades as a crossover design. Therefore, it is not known if tracheal intubation using the English blade is easier than using the standard type, as the ease of laryngoscopy may not reflect the ease of tracheal intubation. Secondly, we did not compare the ease of laryngoscopy with each blade in patients whose tracheas were judged to be difficult to intubate. Lastly, the ease of use might have been different using blades made by a different manufacturer.
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References |
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