Date Published: February 29, 2012
Publisher: Hindawi Publishing Corporation
Author(s): Daryl Lindsay Williams, James M. Zeng, Karl D. Alexander, David T. Andrews.
We conducted a randomised single-blind controlled trial comparing the LMA-Unique (LMAU) and the AMBU AuraOnce (AMBU) disposable laryngeal mask in spontaneously breathing adult patients undergoing general anaesthesia. Eighty-two adult patients (ASA status I–IV) were randomly allocated to receive the LMAU or AMBU and were blinded to device selection. Patients received a standardized anesthetic and all airway devices were inserted by trained anaesthetists. Size selection was guided by manufacturer recommendations. All data were collected by a single, unblinded observer. When compared with the LMAU, the AMBU produced significantly higher airway sealing pressures (AMBU 20 ± 6; LMAU 15 ± 7 cm H2O; P = 0.001). There was no statistical difference between the two devices for overall success rate, insertion time, number of adjustments, laryngeal alignment, blood-staining, and sore throat (P ≥ 0.05). The AMBU AuraOnce disposable laryngeal mask provided a higher oropharyngeal leak pressure compared to the LMA Unique in spontaneously breathing adult patients.
In 1988, Dr. Archie Brain introduced a reusable supraglottic airway device, the Laryngeal Mask Airway Classic (LMAC), a proven safe and effective device in airway management. There are several single-use disposable alternatives to the LMAC. They differ in shape, stiffness, and cuff properties to the LMAC . We conducted a randomized comparison of two disposable supraglottic airways and their seal efficacy: the Laryngeal Mask Airway-Unique (LMAU; Laryngeal Mask Company, Henley-on-Thames, United Kingdom) and the AMBU AuraOnce Disposable Laryngeal Mask (AMBU; AMBU A/S, Denmark). The LMAU is made from a medical grade PVC compound and is similar in design to the LMAC. The AMBU is constructed from a single piece PVC mould and is available in a full range of paediatric and adult sizes. It incorporates a 90-degree preformed curvature designed to better approximate airway anatomy  and lacks the aperture bars of the LMAU.
After receiving institutional review board approval (Royal Melbourne Hospital), written informed consent was obtained during the preanaesthetic assessment on 82 consecutive patients aged above 18 years (ASA I–IV)  undergoing spontaneous ventilating general anaesthesia. Patients undergoing peripheral surgery in orthopaedics and plastic surgery and patients undergoing breast (general) or urological surgery were deemed suitable for recruitment. Patients were excluded if they had contraindications to the use of a supraglottic airway (BMI > 40, interincisor distance <2.5 cm, or aspiration risk) or had a contraindication to our anaesthetic protocol. Patients were allocated to either the LMAU or AMBU groups by a pregenerated random number sequence concealed in a sealed opaque envelope. This was opened after informed consent and patients remained blinded to their group allocation. Weight-based sizing of the airway devices was used based on manufacturer recommendations (size 3: 30–50 kg; size 4: 50–70 kg; size 5 > 70 kg). All participating consultant anaesthetists had >200 previous clinical attempts with an LMAU equivalent device (LMAC). To ensure adequate familiarity with the AMBU device, all anaesthetists completed an AMBU education program consisting of interactive tutorial and successful completion of 10 insertions with the AMBU on a manufacturer recommended part-task airway trainer. Routine preinsertion tests were performed according to the manufacturers’ protocols [2, 4]. No premedication was given. Patients were preoxygenated and anaesthesia was induced with propofol (1-2 mg/kg), fentanyl (1–3 μg/kg), ± midazolam (0.025–0.05 mg/kg) at the discretion of the anaesthetist. Routine anaesthetic monitoring was instituted as per Australian and New Zealand College of Anaesthetists guidelines for general anaesthesia .
Ninety-nine patients were identified as eligible for the study, eight patients declined consent, two patients had their surgery postponed, and seven patients were excluded by the treating anaesthetist, leaving eighty-two consecutive patients who were consented for the study. In three patients, equipment (fibreoptic bronchoscope) was unavailable, resulting in seventy-nine patients being randomised. At baseline, groups were similar (Table 1). The primary endpoint of oropharyngeal leak pressure (OLP) was completed in all patients except one in the AMBU group, where an airway could not be established after three insertion attempts. This was recorded as a failed airway in the AMBU group; however postoperative data were analysed on an intention-to-treat basis. The OLP for the AMBU (20 ± 6 cm H2O) was higher than for the LMAU (15 ± 7 cm H2O; P = 0.001) (Table 2). Of the secondary endpoints, a successful airway was established in all patients of the LMAU group. One patient had a failed airway in the AMBU group. Time to insertion, successful insertions, laryngeal alignment, blood staining, and sore throat were similar between groups (Table 2). Cuff pressures were similar for both devices (LMAU 84 ± 39 mmHg; AMBU 98 ± 36 mmHg; P = 0.473). Cuff pressure in the LMAU changed slightly over time whilst the AMBU remained stable (LMAU −7 ± 8 mmHg, AMBU 0 ± 3 mmHg, P = 0.01). Respiratory complications such as hiccups (LMAU n = 1, AMBU n = 1; P = 0.766), stridor (LMAU n = 3, AMBU n = 4; P = 0.529), and oxygen desaturations below 95% (LMAU n = 0, AMBU n = 1; P = 0.513) were similar between groups.
Our study demonstrated higher oropharyngeal leak pressures in the AMBU compared to the LMAU. This may be due to the 90-degree angulation of the tube as described by Vaida  and is consistent with other recently published results of Shariffuddin and Wang  and Francksen et al. . The mean oropharyngeal leakage pressure achieved in the AMBU group (20 ± 6 cm H2O) was similar to that of Shariffuddin (19 ± 7.5 cm H2O) but was lower than that of Lopez, Francksen, Gernoth and Hagberg (32.2 ± 6.8; 21 (13–40); 25.6 ± 5.2; 24 ± 5.5 cm H2O) [9, 11–13]. Our LMAU sealing pressure was also lower than that of other studies [7, 11, 14, 15]. These differences may be accounted for by a variety of mechanisms. We only allowed a maximum airway pressure of 30 cm H2O when performing the manometric stability test. Also, the injection of a maximum volume into the cuff (and subsequent higher cuff pressures) may have increased oropharyngeal leak due to cuff overinflation [14, 16]. Size selection may also play a role in oropharyngeal leakage pressure [17–19]. We use weight-based sizing as per manufacturer’s recommendations, which resulted in 23 patients (29%) receiving size 3 masks, which is known to be associated with lower OLPs [17–19]. The differences may also be due to our large cohort of anaesthetists rather than a single experienced anaesthetist for insertion of the device .
Our study found the AMBU to provide higher oropharyngeal leakage pressure than the LMAU in spontaneously breathing patients undergoing general anaesthesia. Overall success rates, insertion times, laryngeal alignment, and postoperative sore throat were statistically similar between both devices.