The Airway Management Device (AMD) (Nagor, Douglas, Isle of Man; manufactured by Biosil, Cumbernauld, UK) is a supraglottic device designed for use with spontaneous respiration and positive pressure ventilation. It is a single lumen tube with a double cuff and double aperture design which facilitates the introduction of an orogastric tube to aspirate gastric contents (Fig. 1). The first generation AMD had several problems which included airway obstruction, congested tongue and regurgitation [1,2]. In 2002, additional sizes were added and modifications were made to increase its oropharyngeal leak pressure and decrease cuff pressure. Trials evaluating the modified AMD have been limited [3,4].
The Proseal laryngeal mask airway (PLMA) (Intravent, Orthofix, Maidenhead, UK) has shown to provide a good oropharyngeal leak pressure and has been effectively used for positive pressure ventilation [5-7] (Fig. 2).
There is currently no study in the literature comparing the modified AMD and the PLMA. In this prospective randomized trial, we planned to compare the modified AMD with the PLMA to assess the following: (i) first time insertion success rates, (ii) time to effective airway placement, (iii) oropharyngeal leak pressure and (iv) complications during anaesthesia for major gynaecological procedures.
After Ethics Committee approval and informed consent, we recruited 82 ASA I and II patients who were admitted for elective major gynaecological operations. We assessed the patients for features of difficult intubation using the modified Mallampati classification (Class 1: soft palate, uvula, fauces and pillars visible; Class 2: soft palate, fauces and pillars visible; Class 3: only soft palate visible; Class 4: soft palate not visible) and recruited only patients with Mallampati classification Grades 1 and 2 . Patients at risk of aspiration (morbid obesity, hiatus hernia and gastro-oesophageal reflux) or evidence of a potentially difficult airway (neck pathology or upper respiratory tract pathology) were excluded.
The selection of the size of the airway device was based on manufacturer's recommendation . However, based on the patients height, body mass index (BMI) and mouth opening, the attending anaesthetists were allowed to select a more appropriate size at their discretion .
The two investigators (L.L.P. and Y.L.) with more than 5 yr of anaesthetic experience inserted the airway device for all patients. Both investigators were proficient with the classic LMA but have limited experience with the PLMA and the modified AMD. The instruction manuals of the airway devices were studied and each investigator had five successful insertions with the PLMA and the modified AMD before the start of the trial.
Electrocardiogram, pulse oximetry, capnography and non-invasive blood pressure measurements were attached to the patient and anaesthesia was induced with intravenous propofol 2 mg kg−1, fentanyl 2 μg kg−1 and atracurium 0.5 mg kg−1. The patients were ventilated for 3 min using sevoflurane 2-3% and 100% O2 and when muscle paralysis was achieved, the AMD or the PLMA was inserted according to the manufacturers' recommendations.
The patients were randomized into two groups. In Group A (n = 41), the distal cuff of the AMD was inflated and lubricated. The device was inserted until resistance was met when the distal cuff reached the hypopharynx. The proximal cuff was then inflated with air (incrementally by 10 mL each time) until an airtight seal was achieved. The volume of air needed was recorded and the cuff pressure was measured.
In Group P (n = 41), the cuff of the PLMA was fully deflated, lubricated and the device inserted using the digital technique. When resistance was met, the cuff was inflated with air until a cuff pressure of 60 cmH2O was achieved and the volume of air needed was recorded.
Successful placement was confirmed via auscultation and presence of end-tidal carbon dioxide on the capnogram. The number of attempts at insertion (we defined each attempt as insertion and complete withdrawal of the device from the patient's oral airway) and the time to effective airway placement (defined as from the time the device was picked up to the time the first end-tidal carbon dioxide waveform appeared) was recorded. A # 14 orogastric tube was inserted via the gastric drainage aperture after successful airway placement and failure to pass the orogastric tube was recorded. The oropharyngeal leak pressure was recorded by closing the adjustable pressure limiting valve and insufflating the closed breathing system with 3L min−1 of fresh gas flow.
The investigator could use additional manoeuvres (chin lift, jaw thrust, head extension) to achieve airway patency and was allowed up to three attempts to position the airway device. When we failed to position the airway successfully after three attempts, that was recorded as a failure and the alternative airway device was inserted. An endotracheal tube was inserted under direct laryngoscopy when we failed to secure the airway after three attempts with the alternative airway device.
The tidal volume was set at between 8 and 12 mL kg−1 and respiratory rate ranged from 8 to 14 breaths min−1 to maintain end-tidal carbon dioxide of between 35 and 45 mmHg. The airway device would be reposition with additional manoeuvres or reinserted (if the manoeuvres failed) if inadequate ventilation or desaturation (SPO2 < 91%) occurred during anaesthesia. The peak airway pressure registered for each patient was also recorded.
Anaesthesia was maintained with 1.5-2% sevoflurane and 66% nitrous in oxygen for all patients. During the maintenance of anaesthesia, complications such as loss of airway, desaturation, inadequate ventilation, bleeding and congested tongue were recorded. The investigator could use additional manoeuvres (chin lift, jaw thrust, head extension) or reposition/replaced the airway device if necessary.
At the end of the surgery, the muscle paralysis was reversed and the airway device removed after regular spontaneous respiration had returned. The patients were monitored in the post anaesthetic care unit and the presence of sore throat was assessed prior to discharge.
The primary aim of the study was to compare the first time insertion success rates. A sample size of 80 was needed to detect a 20% difference between the PLMA and the AMD with a power of 80% and a significance level of 0.05. We used the t-test and U-test to analyse parametric and non-parametric data, respectively.
We analysed the 82 patients recruited. We found no differences in patient characteristics data for both groups (Table 1). All patients in Group P and 34 patients in Group A (100% vs. 83%, P = 0.012) had an effective airway after the first attempt. Six patients in Group A needed a second attempt before airway was established. One patient in Group A failed to achieve an effective airway after three attempts and the PLMA was successfully inserted at the first attempt for this patient.
Time to effective airway was shorter for Group P than Group A (mean 21.9 ± 7.8 s vs. 40.2 ± 48.0 s, P < 0.001). The oropharyngeal leak pressure was also significantly higher for Group P than Group A (mean 31.2 ± 5.7 cmH2O vs. 24.2 ± 8.3 cmH2O, P < 0.001) (Table 2).
During anaesthesia, 20 patients in Group A and six patients in Group P had at least one complication (P = 0.001). The total number of complications in Group A was also higher than in Group P (39 vs. 7, P = 0.008). In Group A there were eight patients and one patient who had one episode and two episodes respectively of loss of airway during surgery. Two patients in Group A had episodes of desaturation (Table 3). Eight of these patients recovered after adjustment of the airway but three patients needed to have the AMD removed and replaced by the PLMA for maintenance of anaesthesia for the rest of the surgery.
In this study, we demonstrated that the first time insertion success rates were lower using the modified AMD. The AMD also needed a longer time to achieve effective airway placement when compared with the PLMA. Even though the investigators might have similar levels of experience with the PLMA and the modified AMD, past experience with the classic LMA might have contributed to the ease of use of the PLMA for this study. A study comparing the modified AMD with the classic LMA showed that the modified AMD had 75% first time success rate at establishing a clear airway compared to the 90% for the classic LMA . This is comparable with our study which showed an 85% first time success rate for the modified AMD. In another study evaluating the modified AMD, the airway was secured on the first attempt in 70%. The lower first attempt success rate could be attributed to the difference in the characteristic profile . In this study, male patients had a 33% failure rate and female patients demonstrated only 6% failure rate. Since our study was undertaken only in female patients this could have contributed to our higher first time insertion success rate.
Chiu and colleagues in an initial evaluation of the modified AMD showed a 100% successful insertion and adequate ventilation in all their patients. That case series was not randomized and had no control group. Furthermore, the sample size was small and cuff pressures were not measured .
Despite the modifications made to the AMD to improve its efficacy and safety, its mean oropharyngeal leak pressure was 24 cmH2O and there was a significant number of episodes of loss of airway during maintenance of anaesthesia. Loss of airway was the primary complication (22%) during the maintenance phase of our study. A previous study evaluating the modified AMD also showed loss of airway occurring in 19% of the patients . In the modified AMD, the cuff volumes used ranged from 15 to 70 mL and cuff pressures from 30 to 110 cmH2O. Although the manufacturers of the AMD recommend that air inflating the proximal cuff is injected until a gas-tight seal is maintained, our study demonstrated that the cuff pressures and volume of air used to achieve this varies greatly. This could have contributed to the inconsistent performance of the airway. As the duration of the surgery progresses, the cuff could get over inflated as nitrous is diffused into the cuff, contributing to the loss of airway .
There were more laparoscopic surgery cases in the PLMA group but this did not reach statistical significance and the mean surgical duration in both groups was similar (Table 1). Laparoscopic ovarian cystectomy requires penumoperitoneum and trendelenburg position, both factors contribute to a higher peak airway pressure during ventilation. Despite this, we managed to demonstrate a better ventilatory profile in the PLMA group. For safe application of an airway device, it is important that it is correctly positioned and provides ventilation and oxygenation reliably for the whole duration of the surgery. The PLMA demonstrated a good seal with no incidence of obstruction, loss of airway or desaturation in all our patients. However, we like to emphasize that the correct positioning of the PLMA is important to achieve this and if in doubt, fiberoptic bronchoscopic vision should be available to ascertain its correct placement.
The initial evaluations of the first generation AMD showed inability to establish an airway in 2-10%, airway obstruction incidence of 12%, tongue congestion of 24%, conversion to another airway device in 10% [1,2]. The modified airway does not seem to have addressed these issues. Our study may suffer from the pitfalls of the investigators having limited experience with the airway device and a study population restricted to female patients. However, in view of the results we feel that the modified AMD needs further evaluation before its reliability and safety could be assessed for further use in anaesthesia.
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