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LETTER TO THE EDITOR

Neck Circumference and Difficult Intubation

Department of Anaesthesia; James Cook University Hospital; Middlesbrough, UK; adam.hassani{at}ncl.ac.uk

To the Editor:

Regarding the observation by Gonzalez et al.1 emphasizing the importance of increased neck circumference as a cause of difficult intubation in obese patients, the authors note that their findings are similar to that of Juvin et al.2 However, both of these investigations suffered from the same design drawback, i.e., no attempt to blind the intubating anesthesiologist to the aim of the study. We accept that this is difficult, but it is certainly not "impossible," as the intubating anesthesiologist need not necessarily be informed of the purpose of the study. Bias is practically inevitable, especially when the study aim was "to confirm that obese patients are more difficult to intubate than lean patients." The intubation difficulty scale used in this study may further compound the potential problem of bias, if the intubating anesthesiologists have not been blinded, as it contains several potentially subjective elements.

In addition, we note that there was a preponderance of female patients in the obese group. Although this may be representative of the typical gender distribution of obese patients, it has implications when attempting to identify a neck circumference measurement that may help to predict difficult intubation. The authors state that their results confirm the findings of Brodsky et al.3 who found that increased neck circumference is associated with difficult intubation. Interestingly, Brodsky et al. also had a very marked female preponderance in their study group. However, the median neck circumference of the "easy intubation" group in their study was 46 cm, whereas Gonzalez et al. suggest that difficult intubation should be suspected with a neck circumference of greater than 43 cm. It therefore follows that the majority of patients in Brodsky's "easy intubation" group would have been incorrectly predicted as difficult by Gonzalez et al.'s cutoff point. This is an example of the problem of experimental design highlighted by Wilson, i.e., "a test inherently performs well on the data used to create it."4 Clearly, 43 cm was chosen as it gave the best results in statistical analysis for sensitivity, specificity, and positive predictive value for this data set, but no recommendations can be made regarding the use of this cutoff point in clinical practice before it has been tested prospectively. Men generally have thicker necks3 and, therefore, it seems unlikely that one value would be applicable for both male and female patients. Consequently, a different cutoff figure for men may be required.

REFERENCES

1. Gonzalez H, Minville V, Delanoue K, Mazerolles M, Concina D, Fourcade O. The importance of increased neck circumference to intubation difficulties in obese patients. Anesth Analg 2008;106:1132–6[Abstract/Free Full Text]
2. Juvin P, Lavaut E, Dupont H, Lefevre P, Demetriou M, Dumoulin JL, Desmonts JM. Difficult tracheal intubation is more common in obese than in lean patients. Anesth Analg 2003;97:595–600[Abstract/Free Full Text]
3. Brodsky JB, Lemmens HJ, Brock-Utne JG, Vierra M, Saidman LJ. Morbid obesity and tracheal intubation. Anesth Analg 2002;94:732–6[Abstract/Free Full Text]
4. Wilson ME. Predicting difficult intubation. Br J Anaesth 1993;71:333–4[Free Full Text]

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