We thank Dr. Reed for his interest in our data and insightful comments. First, we concur that the study of the difficult airway is challenged by the exclusion of patients undergoing elective awake fiberoptic intubations. As our data demonstrate, these patients often exhibit features associated with difficult intubation or mask ventilation. At our institution, it is routine practice to perform a direct laryngoscopy after an awake fiberoptic intubation to confirm or refute the patient's presumed difficult airway. In patients without a previous history of difficult intubation, this direct laryngoscopy can be used to guide future airway management and can be used in observational clinical research. Unfortunately, the study of mask ventilation does not offer this luxury.
However, we would disagree with Dr. Reed's assertion that the exclusion of high-risk patients means that the “results and conclusions are not applicable to … even the general population.” The original manuscript indicates that 113 patients with three or more grade 3 (difficult) mask ventilation risk factors underwent elective awake fiberoptic intubation and were excluded from the study.1
More importantly, table 6 of the original manuscript shows that our data did include mask ventilation attempts on 1,670 patients with three or more risk factors.1
This sample size of 1,670 high-risk patients is larger than the entire patient population analyzed by previous mask ventilation studies.2,3
As a result, we believe that the data can be used to derive conclusions regarding the predictors of grade 3 mask ventilation in all but the highest-risk patients, those diverted to an elective awake fiberoptic intubation. Unfortunately, the study of the difficult airway will always suffer from this limitation due to the provider's responsibility to deliver ethical clinical care before considering the scientific value of a given patient's inclusion in a study.
We share Dr. Reed's curiosity regarding the predictors of grade 4 (impossible) mask ventilation. As detailed in the discussion of our data, “despite our large overall sample size, we were unable to detect a large number of IMV [impossible mask ventilation] cases and struggle to provide conclusions regarding IMV risk factors. Further studies assessing incidence, predictors, and impact of IMV are essential.”1
We are unable to explain the lack of overlap between grade 3 and grade 4 mask ventilation risk factors. Because only 37 episodes were detected, we lack the statistical power to address whether distinct pathophysiology is the root cause. We look forward to further studies addressing impossible mask ventilation.
We also thank Dr. Calder for his interest in our work. We agree with Dr. Yentis in his editorial accompanying our article and Dr. Calder when they suggest that we should inform morbidly obese patients of their increased perioperative risk due their increased weight.4
Although this point was not highlighted in our article because of space constraints, our omission should not be construed as implicit approval of unhealthy lifestyles resulting in morbid obesity. Every clinician has experienced the multitude of challenges posed by the increasing prevalence of obesity throughout the world, and our data demonstrate that difficult mask ventilation is among those challenges.
Dr. Calder poses an interesting question regarding the role of neuromuscular blockade in difficult airway management. Our article did not specifically address the role of these medications in mask ventilation because of previous work that did not demonstrate a relationship.2,5
However, Dr. Calder's query regarding the use of neuromuscular blockade in a specific subset, patients in whom mask ventilation was impossible, is intriguing. As a result, we have reviewed the 37 patients with grade 4 mask ventilation in our data set. Of these, only one patient was not given any form of neuromuscular blockade before intubation. This patient was intubated successfully with a grade 1 direct laryngoscopy view. Four patients were administered a nondepolarizing neuromuscular blocking agent before the observation of grade 4 mask ventilation. Of these four patients, one patient could not be intubated and required an emergent cricothyrotomy. The remaining 32 patients received a dose of succinylcholine before the first attempt at intubation. All of these patients were intubated successfully.
The conclusions or recommendations to be drawn from these data are controversial. Some may propose that routine practice should include establishing “excellent intubating conditions,”6
especially if impossible mask ventilation is encountered. This would optimize the attempt to control the airway. Unfortunately, recent research has found that commonly used succinylcholine doses result in neuromuscular blockade ranging from 4.4 to 7.5 min in duration.7
If tracheal intubation could not be achieved in these patients, only laryngeal mask airway ventilation or surgical airway access remain an option to prevent hypoxia during these crucial minutes.8
Others may prefer to follow the conventional wisdom that if mask ventilation cannot be established, neuromuscular blockade should be withheld. If tracheal intubation is not successful, the patient should be awoken and fiberoptic intubation should be pursued. As mentioned earlier, research into management of the difficult airway remains limited by exclusion of patients requiring an elective awake fiberoptic intubation, and there are little additional data to guide our recommendations.
In summary, we appreciate Drs. Reed and Calder's insightful commentary. We believe that our data offer insight into the incidence and risk factors of grade 3 and 4 mask ventilation. Nevertheless, further investigations are clearly needed in this understudied area of airway management. In addition, we hope that the additional data presented regarding the use of neuromuscular blockade in patients with grade 4 mask ventilation ignites a thoughtful debate.
Sachin Kheterpal, M.D., M.B.A.,*
Kevin K. Tremper, Ph.D., M.D.
*University of Michigan Medical School, Ann Arbor, Michigan. firstname.lastname@example.org
1. Kheterpal S, Han R, Tremper KK, Shanks A, Tait AR, O'Reilly M, Ludwig TA: Incidence and predictors of difficult and impossible mask ventilation. Anesthesiology 2006; 105:885–91
2. Langeron O, Masso E, Huraux C, Guggiari M, Bianchi A, Coriat P, Riou B: Prediction of difficult mask ventilation. Anesthesiology 2000; 92:1229–36
3. Yildiz TS, Solak M, Toker K: The incidence and risk factors of difficult mask ventilation. J Anesth 2005; 19:7–11
4. Yentis SM: Predicting trouble in airway management. Anesthesiology 2006; 105:871–2
5. Goodwin MW, Pandit JJ, Hames K, Popat M, Yentis SM: The effect of neuromuscular blockade on the efficiency of mask ventilation of the lungs. Anaesthesia 2003; 58:60–3
6. Viby-Mogensen J, Engbaek J, Eriksson LI, Gramstad L, Jensen E, Jensen FS, Koscielniak-Nielsen Z, Skovgaard LT, Ostergaard D: Good clinical research practice (GCRP) in pharmacodynamic studies of neuromuscular blocking agents. Acta Anaesthesiol Scand 1996; 40:59–74
7. Naguib M, Samarkandi AH, El-Din ME, Abdullah K, Khaled M, Alharby SW: The dose of succinylcholine required for excellent endotracheal intubating conditions. Anesth Analg 2006; 102:151–5
8. 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
© 2007 American Society of Anesthesiologists, Inc.