Risk of pulmonary aspiration with laryngeal mask airway and tracheal tube during positive pressure ventilation

Abstract

It was with great interest that I read Bernardini and Natalini’s [1] study comparing the risk of aspiration during positive pressure ventilation via laryngeal mask airways (LMAs) and tracheal tubes. The authors rightly pointed out that fortunately this is an infrequent event and as such would require an enormous number of patients to be entered into a randomised controlled trial for meaningful conclusions to be drawn. They reported their institute’s observational data over a 10.5-year period, and concluded that their study increased the level of current evidence and that the risk of pulmonary aspiration in selected patients did not increase when a LMA was used instead of a tracheal tube. Observational studies are, however, liable to selection bias. Although rightly declared by the authors, I feel that this may not have been adequately accounted for. They chose to use propensity scores to adjust for the likelihood of patients to have received a tracheal tube over a LMA and then adding this score as a covariate to calculate an adjusted odds ratio. They highlighted this selection bias by stating that the odds ratio for aspiration in patients in whom the LMA was used was increased when the propensity score was considered in the multivariate analysis. This would imply large differences between the patients in the groups that had been assigned to have a tracheal tube or LMA. In such circumstances D’Agostino [2] recommends using the propensity score for case matching or stratification of the groups. A review of the methodology reveals particularly stringent contra-indications for LMA use at their institute, in particular, a fasting period of > 12 h required in unplanned surgery. At all of the institutes that I have worked in to date, a period of 6 h was deemed adequate for LMA use in planned and unplanned surgery as long as there were no other contra-indications. Adherence to this length of fasting may have placed more patients in the tracheal tube group undergoing unplanned surgery. As unplanned surgery was shown be associated with increased risk of pulmonary aspiration, the reason why there were so few incidences of pulmonary aspiration in the LMA group may have been because there were proportionately fewer patients in this group who underwent unplanned surgery. There was mention of the use of a default ventilator setting of 8–10 ml.kg−1actual body weight in the subjects requiring ventilation with respiratory rate adjustment based on PEco2 (or Paco2 if available). Was there any information regarding airway pressure levels or the use of neuromuscular blocking drugs in the LMA group? Both adequate neuromuscular blockade and the limiting of peak airway pressure to 20 cmH2O are recommended in the instructions for LMA use in patients undergoing positive pressure ventilation [3]. I would also be interested to know whether any information was available as to when aspiration events occurred, i.e. were they at induction, intra-operatively or postoperatively? The finding of pulmonary aspiration in seven cases of positive pressure ventilation with use of a tracheal tube as the airway device is somewhat concerning, particularly as it is widely regarded as the gold standard for airway protection, although it is reassuring that the authors of the study found no cases of mortality attributable to aspiration alone. Careful patient assessment and case selection are the mainstays of safe anaesthesia and have been previously advocated in conjunction with the use of LMA in the patient undergoing positive pressure ventilation [4]. This has been further reinforced by the findings of Bernardini and Natalini.