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Correspondence  |   March 2006
Predicting Difficult Intubation
Author Affiliations & Notes
  • Toshiya Shiga, M.D., Ph.D.
    *
  • *Nippon Medical School, Chiba Hokusoh Hospital, Chiba, Japan.
Article Information
Correspondence
Correspondence   |   March 2006
Predicting Difficult Intubation
Anesthesiology 3 2006, Vol.104, 618-619. doi:
Anesthesiology 3 2006, Vol.104, 618-619. doi:
In Reply:—
We thank Drs. Collins and Rincón for their interest in our study.1 Both doctors emphasized that difficult intubation is not synonymous with difficult laryngoscopy. We used the term difficult intubation  because most studies use a Cormack-Lehane grade of 3 or more to define difficult intubation. The American Society of Anesthesiologists Task Force on Difficult Airway Management2 defines difficult tracheal intubation as that requiring multiple attempts, in the presence or absence of tracheal pathological features, whereas difficult laryngoscopy is defined as being impossible “to visualize any portion of the vocal cords after multiple attempts at conventional laryngoscopy.” We could have altered the title to a more appropriate one such as “Predicting Difficult Laryngoscopy in Apparently Normal Patients,” rather than “Predicting Difficult Intubation in Apparently Normal Patients.” We agree with both doctors in that, strictly speaking, our findings are validated only in cases of difficult laryngoscopy, not in cases of difficult intubation or difficult airway. Nevertheless, both words were often confused in many of the studies and reviews we cited.
Dr. Collins pointed out that we incorrectly cited his results regarding the incidence of difficult laryngoscopy in an obese population. Misinterpretation of the data in the process of data extraction for a meta-analysis is possible unless additional information is requested from every author cited, which is unduly challenging. We recalculated the incidence of difficult intubation (more precisely, difficult laryngoscopy) in obese patients according to corrected data provided by Dr. Collins. Our revised analysis showed the incidence of difficult laryngoscopy in obese patients to be 12.7% (95% confidence interval, 11.5–14.0%), which was 15.8% (95% confidence interval, 14.3–17.5%) in our original data and is still more than twice as high as that in nonobese patients. This suggests that difficult laryngoscopy is more likely to occur in obese patients than in nonobese patients. More than a decade ago, Wilson showed obesity to be a risk factor for difficult intubation or difficult laryngoscopy,3 but whether it is indeed a risk factor remains controversial. Further discussion on this topic is needed.
We think the average anesthesiologist is not as skilled in dealing with airways of obese patients as are those who experience a high volume of these cases, such as those at Dr. Collins’ bariatric surgical center. We believe that the head-elevated laryngoscopy position is very useful in working with obese patients, but further randomized controlled trials are required.
Dr. Rincón noted that our analysis excluded both the upper lip bite test and indirect laryngoscopy. In searching MEDLINE and the Cochrane Central Register (1980 through May 2004), we could find only one or two reports on these methods; furthermore, these tests are not as popular or generally used as are the Mallampati classification or Wilson risk score. Therefore, we did not include these tests. Dr. Rincón also said that our conclusions are based on the big assumptions that sensitivity, specificity, and likelihood ratios are not modified by incidence. However, the general understanding is that positive and negative predictive values depend on the prevalence of abnormality in the study sample, but sensitivity, specificity and likelihood ratios are independent of prevalence.4 We may not be able to answer adequately the latter question from Dr. Rincón because we are not statistical experts, but we believe that generalization of the test results to other sample populations is possible whether the tests are based on physical examination or laboratory testing. We think that it is not the characteristic of the test, but prevalence of abnormality, that matters.
We did not take into account the interobserver agreement because it was not specified in most of the studies included in our meta-analysis. Yet, we agree on that this is an important factor influencing the diagnostic accuracy of bedside screening tests.
*Nippon Medical School, Chiba Hokusoh Hospital, Chiba, Japan.
References
Shiga T, Wajima Z, Inoue T, Sakamoto A: Predicting difficult intubation in apparently normal patients: A meta-analysis of bedside screening test performance. Anesthesiology 2005; 103:429–37Shiga, T Wajima, Z Inoue, T Sakamoto, A
American Society of Anesthesiologists Task Force on Management of the Difficult Airway. Practice guidelines for management of the difficult airway: An updated report by the American Society of Anesthesiologists Task Force on Management of the Difficult Airway. Anesthesiology 2003;98:1269–77.American Society of Anesthesiologists Task Force on Management of the Difficult Airway,
Wilson ME, Spiegelhalter D, Robertson JA, Lesser P: Predicting difficult intubation. Br J Anaesth 1988; 61:211–6Wilson, ME Spiegelhalter, D Robertson, JA Lesser, P
Tatsioni A, Zarin DA, Aronson N, Samson DJ, Flamm CR, Schmid C, Lau J: Challenges in systematic reviews of diagnostic technologies. Ann Intern Med 2005; 142:1048–55Tatsioni, A Zarin, DA Aronson, N Samson, DJ Flamm, CR Schmid, C Lau, J