Reflux during positive pressure ventilation via the laryngeal mask airway?

Sir,—We read with interest the pilot study by Valentine, Stakes and Bellamy [1] comparing lower oesophageal pH changes in patients managed with either the laryngeal mask airway (LMA) or tracheal tube during intermittent positive pressure ventilation (IPPV). However, we suggest that the following factors in their study design may have had an influence on their results. (1) The authors probably used the No. 3 LMA in 60% of the LMA cases. The No. 3 LMA is a paediatric size and should not normally be used for adult patients > 50 kg undergoing IPPV. The No. 5 LMA was introduced recently to provide a better mask fit during IPPV and reduce the incidence of leak. Where available, it is commonly used in 20 % of adult patients, with the No. 4 used in all other patients exceeding 50 kg in weight. Inadequate mask surface area may have been responsible for malpositioning and oesophageal insufflation. Recording of LMA position by fiberoptic visualization might have provided useful information in assessing mask fit. (2) The results of the study implied that reflux occurred most frequently at antagonism of neuromuscular block and the authors suggested that this was caused by return of tone to the abdominal musculature and an increase in intragastric pressure at this stage. Regurgitation and aspiration with the LMA, as with other forms of airways, are commonly associated with periods of light anaesthesia [2]. The fact that the LMA is tolerated at lighter planes of anaesthesia [3] may have led the investigator to administer a reversal agent at a lighter level of anaesthesia than subjects with a tracheal tube. It would have been useful to know the relationship between depth of anaesthesia and neuromuscular block at the time the pH measurements were obtained. The principles of recovery with the LMA and IPPV differ from those following spontaneous breathing or in patients with an intubated trachea. When using the LMA with IPPV, antagonism of neuromuscular block is best carried out under a continued level of anaesthesia. There is then a smooth transition to spontaneous ventilation which avoids the problem of a semi-reversed patient struggling against partial antagonism. Antagonism of neuromuscular block coincidentally with discontinuation of anaesthesia can cause unco-ordinated reflex responses and airway obstruction. If the airway is obstructed secondary to malposition or reflex cord closure, significant negative intrathoracic pressures may develop during inspiration and promote gastro-oesophageal reflux. Wang and colleagues have shown in animal studies that partial airway obstruction causes large increases in thoraco-abdominal pressure gradients which may overcome the antireflux barrier even though there is no alteration in LOS tone [4]. Work conducted by O'Mullane in the 1950s found similar results in human subjects [5]. The high incidence of lower oesophageal reflux during antagonism (8 of 10 patients) in the study by Valentine, Stakes and Bellamy implies that anaesthesia should not be discontinued before or coincidentally with antagonism of neuromuscular block when using the LMA with IPPV.