The aim of this study was to find the optimum depth of anaesthesia for removal of the laryngeal mask airway (LMA). Although there is consensus regarding the depth of anaesthesia for LMA insertion, there is no agreement on the appropriate depth for its removal. The manufacturer recommends that the LMA be removed after the return of airway reflexes.1 The opposing argument is that it may be safer to remove the LMA while patients are deeply anaesthetised.2 Desflurane is known to have a rapid onset and offset of action, thereby making it easier for the anaesthetist to adjust the depth of anaesthesia.3 This study was designed to determine the desflurane concentration required for the smooth removal of the LMA in anaesthetised adult patients.
Approval for this study (Ethical Committee 29/13) was provided by the Ethical Committee of the Ministry of Health, Diskapi Yildirim Beyazit Research and the Training Hospital, Ankara, Turkey (President Assistant Professor Oner Odabas) on 2 April 2009. All patients included in the study were adults scheduled for urological surgery who had provided written consent. We excluded those with upper respiratory tract infection during the last 3 weeks, abnormal airway anatomy, BMI of more than 30 kg m−2 and a history of gastro-oesophageal reflux (GOR). There was no premedication. Routine monitoring with ECG, pulse oximetry and non-invasive blood pressure measurement was applied to all. End-tidal CO2 and end-tidal desflurane concentrations were continuously measured in the breathing cycle with a precalibrated gas monitor (Scio Four Oxi plus Medibus Fabius GS; Dräger Medical, Lübeck, Germany). The gas sampling flow rate was 200 ml min−1. Intravenous induction was performed with propofol 2.5 mg kg−1 and remifentanil 10 μg within 30 s. An LMA Classic appropriate for the patient's body weight was inserted once the ciliary reflex was lost. No other opioid was used for induction except for remifentanil. A semi-closed anaesthesia circuit was used to maintain fresh gas flow at 6 l min−1. Anaesthesia was maintained with 50% nitrous oxide in oxygen, and desflurane according to the patient's clinical response. Ventilation was adjusted to maintain the end-tidal CO2 at 35–45 mmHg. Nitrous oxide was turned off at the end of surgery. The inspired desflurane concentration was adjusted to achieve the predetermined end-tidal concentration, and this was maintained for 10 min to establish equilibrium between the cerebral and alveolar concentrations. The target concentration of desflurane was determined by the response of the previous patient to a larger or smaller concentration (with 0.5 step size) using Dixon's up-and-down method4,5 (Fig. 1). At the time of LMA removal, residual nitrous oxide detected in the end-tidal sample was not greater than 3%. The initial patient received 4% desflurane for LMA removal, which was regarded as unsuccessful if complications (coughing, teeth clenching, gross purposeful movements, breath holding, laryngospasm, desaturation to SpO2 <90%) occurred within 1 min following device removal. When LMA removal was unsuccessful, the desflurane concentration for the next patient was increased by 0.5%. When the LMA removal was successful (without any complications), the desflurane concentration was decreased by 0.5%. We calculated the value for minimum alveolar concentration (MAC) by analysing the midpoint concentration of all independent pairs of patients involving a cross-over (i.e. failure to success exchange). MAC was defined as the average of the cross-over midpoints in each pair. MAC is expressed as mean ± SD.
The LMA was removed with the cuff inflated. The pH of the secretions at the tip of the LMA was measured using pH paper after removal of the LMA. A pH of 3 or less was defined as possible evidence of regurgitation. All patients received patient-controlled analgesia (PCA; Pain Management Provider; Abbott, Chicago, Illinois, USA) for 24 h post-operatively. Tramadol intravenously was prescribed with 20 mg bolus PCA dose, 20 min lock period and a total of 200 mg for 4 h. All patients were monitored in the post-operative recovery room. Those with a modified Aldrete score of at least 13 were transferred to the related clinic.6
The Dixon's up-and-down method requires six pairs with failure-success for statistical analysis. The ED50 was defined as the average of the cross-over midpoints in each pair.
Statistical analysis was performed with SPSS 15.0 for Windows (SPSS Inc., Chicago, Illinois, USA). The data were summarised as mean ± SD for normally distributed variables, and as median (range) for other variables. Probit regression was used for estimates of ED50 and ED95 and 95% confidence intervals (CIs).
A total of 23 patients, (three women and 20 men) ranging from 18 to 60 years of age with American Society of Anesthesiologists (ASA) physical status class 1, were recruited.
Patients' details, LMA duration and pH values at the LMA tip are shown in Table 1. In all patients, the LMA was inserted at the first attempt. The end-tidal desflurane EC50 for smooth removal of the LMA was 2.1% (95% CI 1.1–2.9%), and the EC95 value was 3.9% (95% CI 3.1–7.9%). The end-tidal desflurane data were suitable for probit regression analysis (P = 0.299). The logistic regression curves for the probability of smooth removal are shown in Fig. 2. The MAC for the smooth removal of the LMA was 2.7 ± 0.6. The ED50, and the ED95 to the MAC ratio of desflurane were 0.4 and 0.7, respectively.
Complications are shown in Table 2. Laryngospasm and desaturation were observed simultaneously in three patients, the 12th, 13th and 14th consecutively, and they were treated with short duration positive pressure ventilation. The LMA tip pH was 3 for the 13th patient with desaturation. Other complications (coughing, teeth clenching, gross purposeful movements and breath holding) required no specific treatment.
The end-tidal desflurane concentration required for uncomplicated removal of the LMA was 2.1% for 50% of the group and 3.9% for 95%. The removal of the LMA during mild or deep anaesthesia with various inhalation agents (halothane, isoflurane, sevoflurane) has been subject to several studies. One group concluded that it was safer to perform LMA removal in deeply anaesthetised patients,7,8 whereas another group of researchers concluded that the depth of anaesthesia during LMA removal does not appear to effect the incidence of post-operative complications.9 Airway hyperactivity during light anaesthesia may cause adverse effects such as apnoea or laryngospasm. Therefore, suppression of airway reflexes is desirable, but rapid return of protective airway reflexes in the post-operative period is important for prevention of pulmonary aspiration. Thus, the shortest interval between removal of the LMA and the return of protective reflexes is preferred. Desflurane was chosen for the study because it is not very soluble in blood and allows more rapid emergence from anaesthesia.10 Because desflurane irritates the airway, we chose to use an intravenous induction.11
The incidence of airway obstruction, regurgitation and aspiration may increase in line with the depth of anaesthesia, but mild anaesthesia may cause GOR.12 In one study, problems (coughing, retching, etc.) associated with removal of the LMA occurred when the patients were awake, and the incidence of regurgitation was much higher in the awake group.2 We wished to determine the level of anaesthesia required to avoid these complications.
Despite the suggestion that removal of the LMA with the cuff deflated would decrease the trauma in the oropharynx, we removed the LMA with the cuff fully inflated to prevent the retention of secretions on the LMA, as suggested by Deakin et al.13 This allowed us to measure the pH of secretions at the tip of the LMA, anticipating that this would decrease if exposed to regurgitated acid gastric contents. Roux et al.14 confirmed that the use of LMA was associated with a higher incidence of GOR. In a study by Gataure et al.,2 the incidence of regurgitation as reflected by the pH of secretions at the tip of the LMA is much lower (pH ≤3) in the awake group than that in the anaesthetised group (pH >3). They observed an association between laryngospasm, desaturation and a lower pH. We encountered the problem of desaturation in one patient with an LMA secretion pH of 3. The pH values of other patients were more than 3. None had signs of aspiration.
In our study, complications such as laryngospasm and desaturation may have occurred because anaesthetic depth was insufficient to suppress the airway reflex for LMA removal. The up-down method depends on failures and successes, and so a proportion of complications could be expected. The irritant effect of desflurane may also have played a role.
In conclusion, the end-tidal desflurane concentration for smooth and uncomplicated LMA removal was 3.9% in 95% of patients. LMA removal in adults receiving desflurane may be possible at approximately 0.7 MAC.
The present work was supported by the Ministry of Health, Diskapi Yildirim Beyazit Research and the Training Hospital Department of Anesthesiology Ankara, Turkey. No author has a conflict of interest.
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