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Correspondence  |   September 2002
A Reconsideration of Three Axes Alignment Theory and Sniffing Position
Author Notes
  • Hopital Avicenne, Bobigny, France.
Article Information
Correspondence
Correspondence   |   September 2002
A Reconsideration of Three Axes Alignment Theory and Sniffing Position
Anesthesiology 9 2002, Vol.97, 754. doi:
Anesthesiology 9 2002, Vol.97, 754. doi:
We appreciate the interest shown in our article, as evidenced by the letters from Drs. Daley and Norman, Benumof, Chen, Khorasani et al.  , Chou and Wu, and Stasiuk regarding our randomized study comparing the sniffing position versus  simple head extension. 1 
We agree with Drs. Daley and Norman that the fact that four of the investigators were coauthors of the previous experimental study regarding the sniffing position may be interpreted as a potential bias. Since it is difficult (or impossible) to perform a double-blind study of laryngoscopy, and since we wished to determine whether our previous experimental work might have clinical relevance, we performed this study. Of course, we would welcome the efforts of any “independent” researchers to perform a complementary study. The fact that NMBA influences glottic exposure is obvious. We did not use NMBA in this study because a portion of patients scheduled for elective surgery did not require neuromuscular blockade for surgery. Standard care for such patients in our service is to perform intubation using topical anesthesia without paralysis. The advantage of this procedure is the performance of two laryngoscopies; thus, each patient serves as his/her own control for glottic evaluation. Nevertheless, we cannot speculate on the possible influence of NMBAs. The power of our study was calculated to test the hypothesis of the superiority of the sniffing position. Of course, analysis of a subgroup is always limited by a smaller subsample size. We would strongly agree with the desirability of another study designed to examine the sniffing position in patients with predicted difficult intubations.
We agree with Dr. Benumof that the compressibility of the cushion changes critical angles involved in laryngoscopy. Once again, we only evaluated the potential benefit of the sniffing position as the position was used in our routine practice  , rather than performing a rigorous experimental evaluation of a more “extreme” sniffing position (i.e.  , greater elevation of the occiput). It is possible that a more exaggerated sniffing position might have some utility, but that is entirely speculative; our study clearly demonstrated that the systematic use of up to 7 cm of occipital elevation (and this might clearly vary with the weight of the patients head) had no influence. We do agree with Dr. Benumof that the most critical point is that simple head extension on a flat surface leads to neck flexion on the chest and facilitates direct laryngoscopy. We did not modify our protocol for obese patients.
It is well established that the intensity of lifting force during laryngoscopy, as advanced by Dr. Chen, modifies the glottic visualization. We did not quantitate lifting forces. However, the Intubation Difficulty Scale (IDS) indirectly takes this parameter into account, and we found no significant difference between the two positions in the distribution of IDS scores. 2 Rather than focusing the study on one parameter (lifting force) that can interfere with the quality of intubation, we chose to measure the global complexity of intubation, characterized by the IDS score. We thank Dr. Chen for correcting a typographical error concerning the number of patients in figure 2. For IDS = 0, the numbers of patients in the SNIFF and EXT groups were 131 and 108, respectively.
We thank Dr. Khorasani et al.  for their interesting statistical comments. The our study was not powered to demonstrate the superiority of the sniffing position in patients with a body mass index greater than 30 and/or those with head extension limitations. The sample size (and power) was calculated to test our hypothesis in a general surgical population at our institution. To specifically examine the effects of position on laryngoscopy in such selected groups of patients would require a very different series of sample size/power calculations—which we never attempted to perform. We chose our sample size based on previous studies demonstrating superiority of a simple physical maneuver to improve glottic visualization under direct laryngoscopy. As we have already noted, there are no comparative studies evaluating the sniffing position, and hence, we were forced to base our estimate on the reported effects of a different but reasonably similar intervention. Evaluation of laryngeal manipulation on glottic exposure has been widely studied, and we hypothesized that the sniffing position would have the same rate of improvement as laryngeal manipulation. This hypothesis may be discussed and criticized. The analysis of subgroups of patients with predictive factors of intubation difficulty was not the end point of our study. We have observed—not demonstrated  —that obese patients and those with head limitation might benefit from the sniffing position. Another randomized study, including only patients with predictive factors of intubation difficulty, is warranted for demonstrating this observation. On the other hand, it is well established that the incidence of obesity in European countries is smaller that observed in the US. Current prevalence data from individual national studies suggests that the range of obesity prevalence in European countries is from 10 to 20% for men and 10 to 25% for women (France: average of 12%). 1The calculation errors noted were due to an error in transcribing data from our statistical program to the manuscript. The sex ratios (M:F) in groups A and B were 126:99 and 141:90, respectively.
We agree with Drs. Chou and Wu that the mechanism of direct laryngoscopy warrants further experimental studies in order to find the best “patient/laryngoscope adjustment.” Dr. Stasiuk reinterprets our findings, and actually, if the systematic use of a cushion improves the comfort of the patient, there is really no reason to remove it in terms of optimization of the laryngoscopic procedure.
In conclusion, our study should be considered as a preliminary study of the place of the sniffing position in intubation among scheduled surgical patients. Additional larger studies, perhaps evaluating selected patient subgroups, might determine whether the sniffing position may be considered as contributory in the management of difficult intubations or whether it should be used as a standard head position before general anesthesia and intubation attempts.
References
Adnet F, Baillard C, Borron SW, Denantes C, Lefebvre L, Galinski M, Martinez C, Cupa M, Lapostolle F: Randomized study comparing the “sniffing position” with simple head extension for laryngoscopic view in elective surgery patients. A nesthesiology 2001; 95: 836–41Adnet, F Baillard, C Borron, SW Denantes, C Lefebvre, L Galinski, M Martinez, C Cupa, M Lapostolle, F
Adnet F, Borron SW, Racine SX, Clemessy JL, Fournier JL, Plaisance P, Lapandry C: The Intubation Difficulty Scale (IDS): Proposal and evaluation of a new score characterizing the complexity of endotracheal intubation. A nesthesiology 1997; 87: 1290–7Adnet, F Borron, SW Racine, SX Clemessy, JL Fournier, JL Plaisance, P Lapandry, C