Comparison of two Macintosh laryngoscope blades in 300 patients

T. Asai1, S. Matsumoto1, K. Fujise2, S. Johmura1 and K. Shingu1

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


    Abstract
 Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
Background. There are two forms of Macintosh laryngoscope blade. Compared with the standard blade, the English blade is longer, its curve is more continuous across the entire length of the blade, the flange of the blade continues much closer to the blade tip, and the height of the flange is shorter.

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 45–74%).

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: 457–60

Keywords: equipment, laryngoscopes; intubation tracheal, difficult


    Introduction
 Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
Since Macintosh described a new laryngoscope in 1943,1 it has been used most widely for tracheal intubation. In his first report,1 Macintosh stated that the ‘precise shape or curve of the blade does not seem to matter much provided the tip does not go beyond the epiglottis’. During the next 2 yr he used several types of blade, ranging from those with a well-marked curve to a perfectly straight type, and found that ‘provided the laryngoscope is used properly and the blade the right length there is so little difference in the performances of the various blades that I had some difficulty in deciding which was the best for all-round use’.2

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 Macintosh’s 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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Fig 1 The English type and standard Macintosh laryngoscope blades.

 

    Methods
 Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
After obtaining approval from the local research ethics committee and written informed consent from the patients, we studied 300 patients (ASA I or II, aged 18–75 yr) undergoing elective surgery and in whom tracheal intubation was indicated. Patients were excluded if they had any pathology of the neck, upper respiratory or alimentary tracts, or were at risk of pulmonary aspiration of gastric contents. Fifteen anaesthetists were recruited to undertake the study to reduce possible observer bias.

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 patient’s 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 anaesthetist’s 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 anaesthetist’s 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 McNemar’s test (a test of paired proportions) was used to compare the two blades.

We also used the {chi}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 3–13%.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% (13–5%). Three hundred patients would be required to detect this difference with a power of 90% and a one-sided 95% confidence interval.12


    Results
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 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
The patients’ characteristics are shown in Table 1. The mean thyromental distance was 6.5 (SD 0.9) cm (Table 1).


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Table 1 Patients’ characteristics. Data are mean (SD) [range]
 
There was a significant association between the preoperative view of the oropharynx (Mallampati score) and the view of the glottis at laryngoscopy for both the standard blade (P<0.001) and the English blade (P<0.001) (Table 2).


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Table 2 Relationship between preoperative view of the oropharynx (Mallampati score) and the view of the glottis (Cormack and Lehane score) for the standard and English Macintosh laryngoscope blades. Data are number (%) of patients
 
There was no difference in the view of the glottis (Cormack and Lehane score) between the two laryngoscope blades in 220 of 300 patients, whereas there was a difference in the remaining 80 patients (Table 3). Among these 80 patients, the view was better for the English blade for 63 patients whereas the standard blade was better for the remaining 17 patients (Table 3).


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Table 3 View of the glottis (Cormack and Lehane score) with the standard and English laryngoscopes
 
Laryngoscopy was difficult (grade 3 or 4) with at least one blade in 42 of 300 patients (14%) (Table 4). Among these 42 patients, there was a difference in the score between the two blades for 28 patients, in whom the view was better for the English blade for 25 patients, whereas the standard blade was better in the remaining three patients (Table 3). McNemar’s test showed that, in patients in whom the laryngoscopy was difficult, the view of the glottis was significantly better for the English blade than for the standard blade (P<0.001). The 95% confidence interval for the possibility of the English blade being better than the standard one in patients with difficult laryngoscopy [25 of 42 patients (60%)] was 45–74%.


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Table 4 Proportions of easy (Cormack and Lehane score 1 or 2) and difficult (Cormack and Lehane score 3 or 4) laryngoscopy with the standard and English laryngoscope blades
 
Anaesthetists judged that the view of the glottis using the English blade was better than that using the standard blade in 103 of 300 patients (34%) and worse in 34 patients (11%), and that there was no difference between the two laryngoscopes in the remaining 163 patients (54%). In the 42 patients in whom laryngoscopy was difficult (grade 3 or 4) with at least one blade, the anaesthetists judged that the view of the glottis using the English blade was better than the view with the standard blade in 25 of 42 patients (60%) and worse in three patients (7%), and that there was no difference between the two laryngoscope blades in the remaining 14 patients (33%).


    Discussion
 Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
We found that, compared with the standard laryngoscope blade, the English blade provided a better view of the glottis significantly more frequently in patients in whom no difficulty in laryngoscopy had been predicted. We believe that this difference is clinically important, as in patients with possible difficult laryngoscopy (glottic score 3 or 4) the English blade allowed a significantly better glottic view than the standard blade in 60% (25 of 42 patients) of patients, with the 95% confidence interval of 45–74%.

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.


    References
 Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
1 Macintosh RR. A new laryngoscope. Lancet 1943; i: 205

2 Macintosh RR. Laryngoscope blades. Lancet 1944; i: 485

3 Jephcott A. The Macintosh laryngoscope. A historical note on its clinical and commercial development. Anaesthesia 1984; 39: 474–9[ISI][Medline]

4 Dorsh JA, Dorsh SE. Laryngoscopes. In: Understanding Anesthesia Equipment, 4th Edn. Baltimore: Williams and Wilkins, 1998; 505–56

5 Mallampati SR, Gatt SP, Gugino LD, et al. A clinical sign to predict difficult tracheal intubation: a prospective study. Can Anaesth Soc J 1985; 32: 429–34[ISI][Medline]

6 Samsoon GLT, Young JRB. Difficult tracheal intubation: a retrospective study. Anaesthesia 1987; 42: 487–90[ISI][Medline]

7 Cormack RS, Lehane J. Difficult tracheal intubation in obstetrics. Anaesthesia 1984; 39: 1105–10[ISI][Medline]

8 Williams KN, Carli F, Cormack RS. Unexpected, difficult laryngoscopy: a prospective survey in routine general surgery. Br J Anaesth 1991; 66: 38–44[Abstract]

9 Tse JC, Rimm EB, Hussain A. Predicting difficult endotracheal intubation in surgical patients scheduled for general anesthesia: a prospective blind study. Anesth Analg 1995; 81: 254–8[Abstract]

10 Wilson ME, Spiegelhalter D, Robertson JA, Lesser P. Predicting difficult intubation. Br J Anaesth 1988; 61: 215–16

11 Asai T, Koga K, Vaughan RS. Respiratory complications associated with tracheal intubation and extubation. Br J Anaesth 1998; 80: 767–75[Abstract/Free Full Text]

12 Altman DG. Comparing groups—categorical data. In: Altman DG, ed. Practical Statistics for Medical Research. London: Chapman and Hall, 1991: 229–76

13 Asai T, Matsumoto S, Shingu K. Two different types of Macintosh laryngoscopes. Anaesthesia 2003; 57: 1229[CrossRef]





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