The choice of anaesthetic agent can influence the frequency of adverse events [1-3] and emergence times [4,5] during spontaneous breathing anaesthesia with the laryngeal mask airway. Pressure support ventilation is a more effective mode of ventilation with the laryngeal mask airway , but there are no published data about the influence of anaesthesia agents on cardiovascular variables, adverse events and emergence times using this mode of ventilation. In addition, there are no published data about the feasibility of pressure support ventilation with the ProSeal® (LMA North America Inc., San Diego, CA, USA) laryngeal mask airway. In the following study, we compare propofol, sevoflurane or isoflurane for maintenance of anaesthesia with the ProSeal® laryngeal mask airway during pressure support ventilation.
Sixty consecutive adult patients (ASA Grades I-II, aged 19-65 yr) undergoing peripheral musculo-skeletal surgery in which the ProSeal® laryngeal mask airway was considered suitable were studied. Ethical committee approval and written informed consent were obtained. Midazolam 7.5 mg premedication was given 1-h preoperatively. Anaesthesia was conducted with the patient in the supine position with the head on a standard pillow 5 cm in height. A standard anaesthesia protocol was followed and routine clinical monitoring applied. Fentanyl 2 μg kg−1 was administered. Patients were preoxygenated for 3 min. Anaesthesia was induced with propofol 2-3 mg kg−1 given over 30 s and the ProSeal® laryngeal mask airway inserted when there was no response to jaw thrust . Additional boluses of propofol 0.5 mg kg−1 were given as required until an adequate level of anaesthesia was achieved for placement of the airway. A single individual, who was experienced in the use of the ProSeal® laryngeal mask airway (>400 uses), inserted/fixed this airway (size 4, females; size 5, males) according to the manufacturer's instructions. Once an effective airway was obtained, the intracuff pressure was set at 60 cmH2O using a digital manometer (Mallinckrodt Medical, Athlone, Ireland) and the oropharyngeal leak pressure was determined . Any gastric insufflation was noted during such testing by listening over the epigastrium with a stethoscope . Correct anatomical position was determined by passing a fibreoptic endoscope to a position just proximal to the end of the airway tube and scoring the view. A well-lubricated gastric tube 14-FG was inserted through the drainage tube if there was no air leak up the drainage tube. Correct gastric tube placement was assessed by suction of fluid or detection of injected air by epigastric stethoscopy.
Patients were randomly allocated into three groups by opening a sealed envelope. Their lungs were ventilated using a modified Evita 4 lung ventilator and vaporizer (Dräger Medizintechnik GmbH, Lübeck, Germany). In group 1 (n = 20), maintenance was with sevoflurane 2% end-tidal (1 MAC). In group 2 (n = 20), maintenance was with isoflurane 1.1% endtidal (1 MAC), and in group 3 (n = 20) maintenance was with propofol 6 mg kg−1 h−1. Inflation of the lungs was manually assisted using 30% O2 and air until spontaneous ventilation resumed. Patients then underwent pressure support ventilation. Pressure support ventilation comprised positive end-expiratory pressure (PEEP) set at 5 cmH2O and the pressure support was set at 5 cmH2O above PEEP. Pressure support was initiated when inspiration produced a 2 cmH2O reduction in airway pressure. Anaesthesia was continued until the completion of surgery; intraoperative analgesia was provided using fentanyl intravenously. The gastric tube was removed before anaesthesia was discontinued. The removal time (open mouth to command) of the ProSeal® laryngeal mask airway was determined by repeatedly (every 30 s) questioning the patient. The following cardiorespiratory data were recorded by a blinded observer every minute during each ventilatory phase and the average reading taken: end-tidal CO2, oxygen saturation (SPO2), expired tidal volume, expired minute volume, respiratory rate, non-invasive mean arterial pressure, heart rate and airway occlusion pressure (P0.1). Any ingestion of air via the drainage tube was detected by observing movement of a column of lubricant placed in the drainage tube before and after gastric tube insertion. In addition, any adverse events were documented.
Sample size was selected to detect a projected difference of 25% between the two inhalation anaesthetic groups with respect to the emergence time for a type I error of 0.05 and a power of 0.9. The power analysis was based on data from a previous study of 82 patients in which emergence times were measured with the Classic laryngeal mask airway during sevoflurane and isoflurane anaesthesia . The distribution of data was determined using Kolmogorov-Smirnov analysis . Statistical analysis was with one-way analysis of variance with post hoc Bonferroni's test. Significance was taken as P < 0.05.
Demographic, surgical and anaesthetic variables are presented in Table 1. The only difference among groups was that the manually assisted breathing and emergence phases were slightly longer for the propofol group compared with the sevoflurane or isoflurane groups. Haemodynamic and respiratory variables are presented in Table 2. The only difference was that respiratory rate and minute volume was lower and end-tidal PCO2 higher in the propofol group compared to the sevoflurane or isoflurane groups. There was no ingestion of air and there were no adverse events.
We found that ventilation was less effective and emergence times were longer with propofol than sevoflurane or isoflurane during pressure support ventilation with the ProSeal® laryngeal mask airway. However, these differences are of doubtful clinical importance since gas exchange was still satisfactory and emergence times only increased by 3 min. Lung ventilation was less effective due to a reduction in respiratory rate. We also found that the time to commencement of spontaneous breathing was longer with propofol. This suggests that propofol causes a greater reduction in respiratory drive than sevoflurane and isoflurane at these doses. To our knowledge, there are no previous studies comparing different anaesthesia agents for pressure support ventilation with any airway device. Previous studies of spontaneously breathing patients with the Classic laryngeal mask airway have failed to detect any differences in cardiorespiratory variables between propofol and enflurane , and sevoflurane and isoflurane [10,13]. A previous study of spontaneously breathing patients with the Classic laryngeal mask airway has shown that emergence times are longer with propofol than sevoflurane . Interestingly, Stix and colleagues  noted ingestion of air via the drain tube in a patient who was spontaneously breathing and partially obstructed. We noted no ingestion of air during pressure support ventilation.
We conclude that lung ventilation is less effective and emergence times are longer with propofol than sevoflurane or isoflurane for maintenance of anaesthesia during pressure support ventilation with the ProSeal® laryngeal mask airway. However, these differences are small and of doubtful clinical importance.
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