Checking the ability to mask ventilate before administering long-acting neuromuscular blocking drugs

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Whereas we previously reported little or no effect of neuromuscular blockade on the ability to mask ventilate the lungs [1], Szabo et al. have now reported that neuromuscular blockade improves mask ventilation [2]. Some readers may erroneously conclude from this result that it is acceptable to administer neuromuscular blocking drugs (NBDs) without first checking the ability to mask ventilate. There are in fact several related, but often confused, questions involved: (i) should we check the ability to mask ventilate before administering NBDs? (ii) should we administer a short- or long-acting NBD? and (iii) can we predict who is difficult to mask ventilate?

In relation to the first question, Broomhead et al. divided anaesthetists using a questionnaire into ‘checkers’ (who assess the ability to mask ventilate the lungs before administering NBDs) and ‘non-checkers’ [3]. Checkers claimed they checked ‘so that they could wake the patient up’ but in fact it appeared that when checkers found difficulty with mask ventilation, they nonetheless administered a (usually short-acting) NBD. Broomhead et al. did not understand this logic, citing arguments that even short-acting NBDs do not allow ‘wake-up’ in sufficient time and if neuromuscular blockade improves the airway (as in the results of Szabo et al.) then the drugs should be given as early as possible (by ‘wake-up’ they meant colloquially in their article the restoration of spontaneous ventilation).

In our own earlier study [1], checkers offered a different and more accurate argument: that if a difficult airway was predicted, mask ventilation will maintain oxygenation from the earliest opportunity while anaesthesia is deepened [3]. This logic of ‘checkers’ also makes proper distinction between short- and long-acting NBDs.

The rationale of using short-acting NBDs in difficult mask ventilation closely resembles the rationale of their use during rapid sequence induction: if tracheal intubation fails then wake-up remains an option (though not a certainty) [4]. Note that in this context, rocuronium, with sugammadex if needed, is assumed to be akin to a short-acting NBD, although some evidence suggests that it may not be [5].

In reality, NBDs can make mask ventilation both easier (e.g. when there is underlying laryngeal spasm or chest muscle rigidity) [6] or more difficult (e.g. if relaxation-induced collapse of the upper airways worsens obstruction) [7]. The real problems are that we cannot predict in which patients which of these effects will predominate, and even when mask ventilation is difficult, we cannot be certain which factor is contributing. Figure 1 illustrates the practical choices we face during anaesthetic induction, and is essentially a line diagram summary of the points raised earlier by Calder and Yentis [8].

Figure 1.

 Possible sequences of events relating to mask ventilation during anaesthetic induction. There are three broad phases delineated by vertical dotted lines: after induction, mask ventilation is checked (‘check MV’); then there is choice of three options depending upon ability to mask ventilate (‘NMB/immediate choice’); then there are several consequences of those choices. NMB, neuromuscular blockade; DMV, difficult mask ventilation; EMV, easy mask ventilation; *Failed intubation may be possible with EMV; [1] = DMV after NMB; [2] = EMV after NMB; inline image = route followed by very few anaesthetists. Note that wake-up is only an option if suxamethonium is administered. The step that could lead to a potentially dangerous situation is marked by inline image.

Thus, it is only by mask ventilating (‘checking’) routinely that we can identify two subgroups of patients: (i) those who are initially easy and (ii) those who are initially difficult (assuming an adequate depth of anaesthesia).

For those who are easy, all anaesthetists would readily administer a long-acting NBD if required. However, for those patients who are difficult, it would seem very irrational to administer a long-acting agent (not least because it is unclear how oxygenation would be maintained while the NBD works) [4]. (Interestingly, and perhaps worryingly, Broomhead et al. found that 18% of anaesthetists would follow this irrational route) [3].

Whereas giving a short-acting NBD (suxamethonium) may not guarantee that wake-up is possible as a rescue option, the contrary is certainly true: administering a long-acting NBD in difficult mask ventilation guarantees that wake-up rescue is impossible if subsequent difficulties are encountered. In other words, the reason to ‘check’ is strictly not to help ‘wake the patient up’, but rather to avoid creating a very serious situation in which it is impossible to ‘wake the patient up’. In crises, it is important to maximise (not minimise) the rescue options available.

The ability to mask ventilate must be seen as a dynamic process, its efficacy changing with anaesthetic depth and the drugs administered. Therefore, a further important reason to check is fully to document this dynamic process. It would be very helpful for future anaesthetic management to know exactly which route in Fig. 1 a patient had followed.

We do not know the relative proportions of patients following routes 1 vs 2 as marked in Fig. 1. Most patients are likely to follow route 2, but even if a small number follow route 1, Fig. 1 helps reach the following conclusions: there is a rational defence in giving suxamethonium (or rocuronium with sugammadex back-up) to those patients in whom difficult mask ventilation has been established by prior checking. There is little or no rational defence to giving long-acting NBD (that cannot be quickly reversed) to patients without first checking, since such blind administration may close the door on a possible rescue route to those few in whom mask ventilation was, in fact, always inherently difficult.

No external funding and no competing interests declared. Previously posted at the Anaesthesia Correspondence website: http://www.anaesthesiacorrespondence.com.

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