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Re: Effect of 25˚ back-up position on laryngeal view at laryngoscopy
- Jong-Man Kang (29 June 2008)
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Effect of 25˚ back-up position on laryngeal view at laryngoscopy
- Gavin Drummond, Gareth Kessell (20 November 2007)
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Jong-Man Kang
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Editor – we read with interest the letter “Effect of 25˚ back-up position on laryngeal view at laryngoscopy". We agree that Adnet and colleagues (1) did find that the sniffing position was better for obese and head extension limited patients. It would have been better to compare the three positions (simple head extension, sniffing position, 25° back up position) in our study.(2) In addition, sniffing position combined with 25° back up state should have been included. However we chose only two positions because we wanted to know the pure effect of 25° back up in the same conditions. Mathematically the forces will be W times cosine(45°;) in the flat position, W times cosine(70°;) in the 25°; back-up position, where W is the weight of the laryngeal structures. However the difference between the average endoscopy angles(58.6°-39.8°=19.8°) of the two positions was less than 25°(=the difference between table angles).(2) We think this occurs because laryngoscopic forces are directed more caudally in the back-up position and thus more effectively displace anterior tissue structures to expose the glottis and the endoscope angle is not increased as much as the table angle. Because the force that the laryngoscopist exerts in the back-up position will be like “pushing forward” style compared with “pulling up” style in supine position, the difference of forces may be more than mathematical calculation. It is also possible that the laryngoscopist was more comfortable working in the 25°; back-up position as per the comments of Drummond and colleagues. However, because the oropharyngeal axis of the patient in the supine position is more parallel to the floor than 25° back up position, the laryngoscopist need to lower his head or bend his back or knees to see the larynx in the supine position. In 25° back up position, the oropharyngeal axis of the patient will be steeper than supine position and the line of the laryngoscopist’s vision will become more natural seeing the glottis. Drummond and colleagues were concerned that the use of an endoscope manipulated to varying depths within the oropharynx to represent best view on laryngoscopy. However we described in the article that after the blade of the laryngoscope was inserted and lifted up, the 0° rigid endoscope was inserted into the oral cavity to the optimal depth to obtain the best view of the glottis. We placed the scope at the midline of the mouth cavity from the incisors and at a depth to get a whole view of the glottic area. The depth of the rigid scope was not the same in every patient. However the depth was the same at each position in one patient.(2) In conclusion, we think it is impossible that simple maneuver like 25° back-up position make Cormack grade 3 or 4 to the lower grade. We expect 25° back-up position as an additional method improving laryngeal view. We think 25° back-up position is not one single perfect method to make a difficult airway easy but an accessory method to be helpful with other methods such as external laryngeal manipulation or using a stylet. 1. Adnet F, Baillard C, Borron SW, Denantes C, Lefebvre L, Galinski M, et al: Randomized study comparing the "sniffing position" with simple head extension for laryngoscopic view in elective surgery patients. Anesthesiology 2001; 95: 836-41. 2. Lee BJ, Kang JM, Kim DO: Laryngeal exposure during laryngoscopy is better in the 25 degrees back-up position than in the supine position. Br J Anaesth 2007; 99: 581-6. Conflict of Interest:None declared |
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Gavin Drummond , Gareth Kessell
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Editor - we read with interest the study, by Lee and colleagues (1), finding that the laryngeal view, as assessed by POGO scores, was improved significantly in the 25˚ back-up position when compared with the flat supine position. The authors say they expect that the 25˚ back-up position may be included in the difficult airway algorithm. The study was done with the head in simple extension rather than with the head in the more conventional sniffing position. We think it would have been more informative if the sniffing position had been used as this would have shown the effect of the 25˚ back-up position as compared with usual clinical practice. We agree that Adnet and colleagues (2) did not find any significant difference between the two positions. They did, however, find that the sniffing position was better for obese and head extension limited patients – precisely the type of patients more likely to be in need of the difficult airway algorithm. We are having some difficulty in understanding why the 25˚ back- up position gives a better laryngeal view than the flat supine position. As can be seen from Fig 4 (1), the relative positions of the head, neck and laryngoscope handle are the same in both the flat and 25˚ back-up position with the angle to the horizontal of the laryngoscope handle reduced from 45˚ to 20˚ to compensate for the 25˚ increase in body position. The only difference in the two positions is the change in the effect of gravity opposing the pull along the handle of the laryngoscope. In the flat position, this force will be W times cosine(45˚) , where W is the weight of the laryngeal structures. In the 25˚ back-up position the force will be W times cosine(70˚). So, the reduction in force between the two positions will be just over one third of W. We doubt that this force is large enough to have the effect the authors have observed since it can be easily compensated for by an extra pull on the laryngoscope in the flat position. If it was the case that the gravitational effect was significant then we would expect even better laryngeal views as the back-up angle increases beyond 25˚, with the best view at an impractical 135˚ where gravity would act directly down the line of the laryngoscope handle. Accepting the limitations of increased likelihood of hypotension on induction it would be interesting to know the effect of steeper angles on laryngeal view. It is also possible that the laryngoscopist was more comfortable working in the 25˚ back-up position. It is noted that the laryngoscopist was working in the sitting position with the patient supine yet was working in the more usual standing position with the patient at 25˚. The laryngoscopist may have subconsciously favoured the 25˚ back-up position or as the authors suggest maybe head extension was different in both positions. We agree with Lee and colleagues that it is difficult to see how the laryngoscopist could be blinded to bed position but would be interested to see whether these results would be replicated by other laryngoscopists that are not aware of the hypothesis being investigated and therefore not biased towards expecting a better view in the back-up position. A further concern is the use of an endoscope manipulated to varying depths within the oropharynx to represent best view on laryngoscopy. An improved view obtained by this method may not translate to an improved view using the laryngoscopist’s eye. In summary then, although we congratulate Lee and colleagues on an interesting piece of work, we are not yet convinced that they have found a real effect and before considering adopting the 25˚ back-up position in the difficult airway algorithm we would like to see a larger study using more “blinded” laryngoscopists, more angles and the patient’s head in the sniffing position. Gavin Drummond, Gareth Kessell, Department of Anaesthesia, James Cook University Hospital, Middlesbrough References: 1. Lee B J, Kang J M, Kim D O. Laryngeal exposure during laryngoscopy is better in the 25˚ back-up position than in the supine position. British Journal of Anaesthesia 2007; 99: 581-6. 2. Adnet F, Baillard C, Borron SW et al. Randomized study comparing the “sniffing position” with simple head extension for laryngoscopic view in elective surgery patients. Anesthesiology 2001; 95:836-41. Conflict of Interest:None declared |
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