FOR an anesthetist, there can be few, if any, more alarming clinical situations than the unexpected total inability to manage a patient’s airway. That this occurs rarely is of course a good thing, although its rarity itself both encourages complacency and makes it difficult to study.
The first problem is defining the problem. Historically, anesthetists have focused on tracheal intubation, perhaps because it has traditionally been an exclusive anesthetic skill and is seen as the definitive airway technique, and because difficult or failed intubation has been such an important cause of anesthetic morbidity and mortality over the years. However, difficult intubation is not easy to define, because there are degrees of difficulty and they may vary between intubators. Defining failed intubation might seem, on the face of it, easier, but there are different thresholds for declaring failure, depending on the would-be intubator, the urgency of the situation, and who or what else is available, and a second person (or even a third) may succeed when the first has failed. These difficulties, plus the rarity of failure, have led to use of the view obtained at direct laryngoscopy—albeit using different methods of grading the view—as a surrogate for difficult or failed intubation in clinical studies (reviewed recently by Shiga et al. 1). These studies generally find the same thing: that certain clinical features are more likely to be present in patients in whom laryngoscopy is difficult, but because most laryngoscopies are easy, most patients who have such features actually pose no problems.
But even if it were possible to predict difficult laryngoscopy accurately, this is not quite what we need to know. First, an endotracheal tube may be easy to place despite a poor laryngoscopic view, and even a reasonable view may be associated with difficulty passing a tube. Second, failed intubation alone may not necessarily lead to disaster, because there are alternative ways of maintaining oxygenation, the most simple of which is ventilation by facemask.
Surprisingly, predicting difficult mask ventilation has attracted little attention—perhaps, again, at least partly because of its rarity and difficulty in defining it. In this issue of the Journal, Kheterpal et al. 2have added to the work of Langeron et al. 3in this area, both groups finding that obesity, older age, snoring, and the presence of a beard stood out as risk factors for difficulty (both groups suggesting that beards that stood out too much should perhaps be removed, although they do not suggest remedial action for obesity). As with tracheal intubation, though, most people, including those with these risk factors—difficulty defining snoring or beards notwithstanding—are easy to ventilate by mask. And, as with intubation, predicting difficult mask ventilation does not give us what we really need to know: More useful would be the ability to predict which patients are at risk from difficulty with both intubation and mask ventilation, because if there is trouble with one technique, the anesthetist can simply use the other in most cases. Kheterpal et al. looked at this too, and although numbers were small, they identified obesity, snoring, limited jaw movement, and abnormal neck anatomy as risk factors.
But is this what we really need to know either? With the laryngeal mask airway now such a valued component of airway management strategies4,5and so widely and readily available, perhaps we should strive to predict in which patients there will be difficulty with intubation, mask ventilation, and ventilation with the laryngeal mask airway? Kheterpal et al. found only 37 patients out of 22,660 in whom mask ventilation was impossible; intubation was difficult in 10 of these, and surgical cricothyrotomy was required in 1. Details of other methods of airway management that were attempted in this single case are not provided, but it is difficult to imagine cricothyrotomy being performed without trying the laryngeal mask airway or equivalent first. A definitive study of this aspect would be challenging, mainly because the number of patients unfortunate enough to fulfill these criteria is tiny.
All of this brings us to the next problem, at which I have been hinting above—i.e. , the limited usefulness of predictive tests when the thing they are trying to predict is very rare. In such a situation, unless there is near 100% sensitivity and specificity, the positive and negative predictive values will be low. In the case of airway management, most patients are easy to manage and most patients predicted to be difficult are not, whereas a few who are predicted to be easy are anything but. Understanding this limitation is important if the so-called predictive tests are to be used sensibly.6
Finally, a weakness of most prediction studies is that their findings only relate to the sample from which they were derived. Kheterpal et al. should be commended for gathering data from more than 22,000 patients; their findings must now be validated in another sample, and one can only hope that the same group, or another one, is ready to take up the challenge, if not the beard trimmers.
Magill Department of Anaesthetics, Intensive Care and Pain Management, Chelsea & Westminster Hospital, London, United Kingdom. s.yentis@imperial.ac.uk
