A randomised, blinded, controlled trial of silicone disposable laryngeal masks during anaesthesia in spontaneously breathing adult patients

The intervention groups will receive either the Meditech Systems Limited (MSL) or ProAct (PA) L aryngeal Mask Airway (LMA) for airway maintenance. We hypothesise that there will be no difference (equivalence) between the three silicone laryngeal masks studied. The ease of insertion, time to achieve effective airway, success rate on first or subsequent attempts, cuff seal pressure, and incidence of oro-pharyngeal trauma will be measured and we believe they will be similar in all groups. The laryngeal mask airway is a reusable supraglottic airway device that has been available since 1998 and is widely used during elective general anaesthesia where endotracheal intubation is not thought necessary. Newer single-use disposable supraglottic airway devices are now available, which potentially reduce the risk of infection transfer. The laryngeal mask is the most commonly used supraglottic device; its silicone construction ensures that it is easy to insert and minimises airway trauma. The Therapeutics Goods Administration in Australia has recently assessed a number of silicone disposable LMAs, and they are now available for use in Australia. The LMAC is easy to insert, creates an effective airway seal but provides only limited protection against aspiration. The LMAC is predominantly silicone construction with a PVC cuff. The major differences between the LMAC and the disposable devices are that the MSL and PA LMA have no aperture bars on the inferior surface of the laryngeal cuff. Anaesthetic technique Routine anaesthetic monitoring, as per Australian and New Zealand College of Anaesthetists guidelines for general anaesthesia, will be instituted [6]. A standardised general anaesthetic technique will be employed. Patients will not be pre-medicated. Induction of anaesthesia will utilise propofol 1-2 mg/kg, fentanyl (1-3 mg/kg), and midazolam (0.025-0.05 mg/kg), at the discretion of the anaesthetist concerned, to produce a loss of lash reflex. Following induction of anaesthesia patients undergo controlled ventilation using an anaesthetic facemask and 100% oxygen with sevoflurane (end tidal concentration at discretion of the anaesthetist). Following insertion of supraglottic airway device and return of spontaneous ventilation, maintenance of anaesthesia is to be achieved using sevoflurane (end tidal concentration at discretion of the anaesthetist) and a fresh gas flow with an inspired oxygen concentration of 70-100%. No nitrous oxide will be used. Small doses of metaraminol or phenylephrine can be administered to control systemic arterial blood pressure. Maintenance of general anaesthesia will be with sevoflurane in an oxygen:air mixture to achieve an inspired oxygen level of 70-100%. Insertion of supraglottic airway device A practitioner experienced in the use of LMA devices will carry out insertion. Routine recommended preinsertion tests of the control and study device will be performed [7]. The airway device will be inserted when loss of lash reflex and jaw tone allows. Successful device placement will be confirmed by: · resistance to further downward movement of the device · cuff inflation with the appropriate volume of gas to achieve an intracuff pressure of 60cmH20. · effective ventilation as defined by assisted ventilation with chest wall movement, observance of a square wave capnograph trace and assisted ventilation without a leak at airway pressure of at least 10cmH20. At the conclusion of surgery the anaesthetic gases will be replaced by 100% oxygen to allow patient recovery. When protective reflexes are noted to have returned to normal, the airway device will be removed after deflation of the cuff. The device will be inspected for secretions and blood.