Department of Anaesthesia, Ysbyty Gwynedd, Bangor LL57 2PW, UK
This work has been presented to the Liverpool Society of Anaesthetists at the Registrars Prize Competition 2002. It was awarded first prize.
Accepted for publication: June 24, 2002
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Abstract |
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Methods. Twenty-six subjects with varying experience of intubation were photographed during laryngoscopy of an intubation training mannequin. Posture was measured from the photographs and the data were analysed with the MannWhitney U-test.
Results. The less experienced group had shallower lines of sight, levered more, and stood with their face closer to the mannequin (P=0.037, 0.018 and 0.06 respectively).
Conclusions. Novice anaesthetists should be given explicit instructions on correct trolley height and should be taught to intubate with a straight back.
Br J Anaesth 2002; 89: 7724
Keywords: anaesthetic techniques, anaesthetist, posture; education, junior staff; equipment, mannequin
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Introduction |
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Method and results |
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The following measurements were taken from the photographs (Fig. 1): (A) the angle between the line of sight and the horizontal; (B) the angle between the line of sight and the handle of the laryngoscope (upper edge); (C) the angle between the handle of the laryngoscope and the horizontal; and (D) the distance between the eye and the heel of the laryngoscope. In all cases the base of the training mannequin was taken as horizontal.
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Twenty-six subjects took part in the study. One subject was removed from the study on the grounds that he was a significant outlier: he was more than two standard deviations below the mean eye-to-laryngoscope distance and more than three standard deviations below the mean of the experienced group, to which he was allocated. The remainder were allocated to a more experienced or less experienced group (Table 1). All participants were able to see the glottis. The less experienced group had significantly shallower lines of sight (A) [experienced group, mean (SD) 41.2 (4.8)°; less experienced group, 37.9 (4.2)°; P<0.05; overall range 3151°]. The more experienced group levered less, with significantly lower laryngoscope handle angles (C) [experienced group, mean 36.6 (5.5)°; less experienced group, 44.1 (7.0)°; P<0.05; overall range 2957°]. Eye-to-laryngoscope distances (D) were greater in the more experienced group, but the difference did not reach significance [experienced group, 35.6 (9.8) cm; less experienced group, 27.4 (8.7) cm; P>0.05; overall range 1957 cm]. The angle of the line of sight to the laryngoscope handle was similar in the two groups, and showed a smaller overall range than other measurements [100.4 (3.7)°, range 92107°]. There was no correlation between line of sight and eye-to-laryngoscope distance.
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Comment |
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One of the most interesting aspects of this study is the limited range of the angle between the laryngoscope handle and the line of sight, which is readily explained by the design of the laryngoscope. Juniors are told not to lever and are warned of broken teeth. These data demonstrate that by levering they also lower their line of sight, hence making it more difficult to see.
A lower line of sight caused by levering in turn requires a reduction in height on the part of the anaesthetist. The less experienced group compensated with the upper body, by stooping and bringing their face closer to the patient. This may reduce binocular vision, as noted by Matthews and Johnson.1 The more experienced group compensated for height with the lower body, by bending their knees or using a similar manoeuvre. One experienced subject compensated by bending his torso laterally, something the author had not seen before.
The poor correlation between low line of sight and low eye-to-laryngoscope distance was disappointing, because the lower sight-line should lead to stooping. However, a major confounding factor was the fixed height of the trolley, which was necessary because of the fixed position of the camera.
Correct trolley height is another factor on which very little is known. Otto2 suggests having the patients head between the anaesthetists xiphisternum and umbilicus. The author suggests having the trolley surface at the same level as the anaesthetists anterior superior iliac spines, which results in a similar position.
This study has other defects, apart from the fixed trolley height. The Laerdal Airway Management Trainer is a popular training mannequin, but is not the same as a real subject. It is much less compliant and is more difficult to intubate. Its stiffness means that it is very difficult to intubate on two pillows, as most anaesthetists normally would. However, it gave a standard intubation model, which was identical for all subjects.
Presbyopia was not considered in the study design. However, most of the participants were quite young, so eyesight should not confound the data too much. The decision to remove one of the participants is controversial, but justified on the grounds that a study as small as this would be extremely sensitive to outliers. The criticism that a forced dichotomy has been created between more and less experienced groups is possibly valid; a better design would have been to study consultants against junior SHOs (senior house officers). However, the limited numbers of participants prevented this.
In spite of the shortfalls, some conclusions are justified. All anaesthetists should place the trolley at the correct height. Juniors should intubate with their back straight and should be taught to manipulate the line of sight by levering less instead of chasing the sight-line by stooping. In situations where the trolley height is fixed, compensation for height should occur with the lower body.
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References |
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2 Otto CW. Tracheal intubation. In: Nunn JF, Utting JE, Brown BR, eds. General Anaesthesia. London: Butterworths, 1989; 51239