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Optimal End-Tidal Sevoflurane Concentration for the Removal of the Laryngeal Mask Airway in Anesthetized Adults

Shim, Yon Hee, MD; Shin, Cheung Soo, MD; Chang, Chul Ho, MD; Shin, Yang-Sik, MD

doi: 10.1213/01.ane.0000166977.17442.63
Anesthetic Pharmacology: Research Report
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Sevoflurane provides smooth and rapid emergence from anesthesia and can be used when the removal of a laryngeal mask airway (LMA) is required in anesthetized patients. We sought to determine the optimal end-tidal concentrations of sevoflurane required for the removal of LMA in anesthetized adults. We studied 35 adults, aged 22–64 years old with an ASA physical status I or II, who were undergoing perineal surgery. General anesthesia was induced with thiopental, and the LMA was then inserted. Anesthesia was maintained with sevoflurane, oxygen, and air. After the surgery, the target concentration was maintained for at least 10 min, and then the LMA was removed. Each target concentration at the time of removal was predetermined by the Dixon up-down method (with 0.1% as a step size) starting at 1.7% end-tidal concentra-tion of sevoflurane. The LMA removal was considered successful when there was no coughing, clenching of teeth, or gross purposeful movements during or within 1 min after removal and also if there was no breath holding, laryngospasm, or desaturation after removal. The end-tidal concentration of sevoflurane to achieve successful LMA removal in 50% of adults was 0.99% ± 0.09% (mean ± sd) and in 95% of adults was 1.18% (95% confidence limits, 1.07%–1.79%). In conclusion, we have determined that LMA removal in 50% and 95% of anesthetized adults can be safely accomplished without coughing, moving, or any other airway complications at 0.99% and 1.18% end-tidal concentrations of sevoflurane.

IMPLICATIONS: Because the removal of the laryngeal mask airway (LMA) in the anesthetized state is required in some clinical situations, we sought to determine the end-tidal concentration of sevoflurane to safely remove the LMA in anesthetized adults.

Department of Anesthesiology and Pain Medicine and Anesthesia and Pain Research Institute, Yonsei University College of Medicine, Seoul, Korea

Accepted for publication March 1, 2005.

Address correspondence and reprint requests to Yang-Sik Shin, MD, Department of Anesthesiology and Pain Medicine and Anesthesia and Pain Research Institute, Yonsei University College of Medicine, CPO Box 1217, Seoul 120-752, Korea. Address e-mail to ysshin@yumc.yonsei.ac.kr.

The removal of the laryngeal mask airway (LMA; Laryngeal Mask Company, Henley-on-Thames, UK) in anesthetized patients is performed when the patient is breathing spontaneously and when airway reflexes are still depressed (1). This technique may provide for a smoother emergence from anesthesia, and it may reduce cough and other airway complications such as bronchospasm(2–4). Leaving the patient with an airway unprotected from secretions or obstruction is the primary disadvantage during the removal in the anesthetized state (5,6). Techniques that reduce the time from LMA removal to the return of protective airway reflexes would minimize the risk of aspiration, contamination, or airway obstruction. Sevoflurane is a nonpungent volatile anesthetic with a rapid recovery profile and is suitable for LMA removal in the anesthetized state(2,7–9).

No previous studies have quantified the depth of anesthesia (this is expressed as the end-tidal gas concentration), which is required to perform the LMA removal in anesthetized adults. Therefore, we attempted to determine the end-tidal concentrations of sevoflurane that are required for the removal of the LMA in anesthetized adults.

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Methods

We studied 39 adults, aged 22–64 years old with an ASA physical status I or II, who were scheduled for perineal surgery. The institutional clinical investigation committee approved our study, and each patient gave informed consent. Those adults having reactive airway disease or signs of upper respiratory infection were excluded from the study. We also excluded patients who had an abnormal airway, were obese (>50% of the ideal body weight), had gastroesophageal reflux, and for whom LMA was contraindicated.

After insertion of an IV cannula, glycopyrrolate 0.003 mg/kg was administered IV 10–15 min before the anesthetic induction. The patients were monitored with electrocardiography, pulse oximetry, and a noninvasive arterial blood pressure monitor. The end-tidal concentration of CO2 and sevoflurane was continuously measured at the elbow of breathing circuit with a precalibrated gas monitor (Smart Anesthesia Multi-Gas (SAM)™; GE Marquette Medical Systems Company, Milwaukee, WI) at a sampling flow rate of 250 mL/min.

General anesthesia was induced with 5.5 mg/kg of IV thiopental sodium. All patients had the LMA inserted as soon as the eyelash reflex was suppressed after the induction of anesthesia. The LMA size chosen was 4 for the men and 3 for the women. Anesthesia was maintained with sevoflurane in oxygen 1 L/min and air 1 L/min without any neuromuscular blockers. The concentration of sevoflurane was adjusted in response to clinical signs. Spontaneous ventilation was maintained in all patients during anesthesia through a circle system. After the completion of the surgery, the oropharyngeal secretion was gently suctioned. When the end-tidal sevoflurane concentration reached a predetermined value, the ratio of the end-tidal to inspiratory sevoflurane concentration was kept at 0.9–1.0 for at least 10 min to establish equilibration among the cerebral, arterial blood, and alveolar gas tensions before LMA removal was attempted. The LMA was removed with the cuff inflated and jaw lifted, and a face mask was routinely applied with the patient spontaneously breathing 100% oxygen for 10 min. No narcotic or benzodiazepine was administered, and no local anesthetic was applied to the airway.

The target concentration was determined by the response of the previously tested patient to a larger or smaller sevoflurane concentration (with 0.1% as a step size) by the Dixon up-down method (10) starting at 1.7% end-tidal concentration of sevoflurane (1 minimum alveolar anesthetic concentration [MAC] in adults) (11). Patients who developed coughing, clenched teeth, or gross purposeful movements during or within 1 min after LMA removal or those patients who developed breath holding, laryngospasm, or desaturation to a Spo 2 < 90% during or immediately after LMA removal were regarded as unsuccessful trials. A result defined as an unsuccessful removal resulted in an increase by 0.1% of sevoflurane for the next patient. The patient showing none of the above events was considered as a successful removal trial, and this resulted in a decrease by 0.1% of sevoflurane for the next patient.

Demographic data were collected and are presented as mean ± sd. We analyzed the values for the 50% effective dose (ED50) obtained by calculating the midpoint concentration of all independent pairs of patients involving a crossover (i.e., not successful to successful LMA removal). The ED50 was defined as the average of the crossover midpoints in each pair. The up-and-down sequences were also analyzed by a probit test (SAS proprietary software; SAS Institute Inc, Cary, NC), which enabled us to derive the mean of the end-tidal concentration with 95% confidence limits. The maximal likelihood estimators of the model variables were performed using a probit test (SAS) that furnished the best-fitting sigmoid curve.

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Results

We excluded four patients from our analysis who received additional thiopental sodium or neuromuscular blockers during the insertion of LMA. After successful insertion of LMA, there were no airway problems during the maintenance of anesthesia. The patients' demographic data and the duration of the LMA insertion are presented in Table 1. The ED50 for LMA removal at which a successful removal is possible in 50% of adults was 0.99% ± 0.09% (Fig. 1). The dose-response curve constructed by using a probit test on the patients' data (Fig. 2) revealed that the end-tidal concentration of sevoflurane at which 95% of adults had a successful LMA removal (ED95) was 1.18% (95% confidence limits, 1.07%–1.79%).

Table 1

Table 1

Figure 1.

Figure 1.

Figure 2.

Figure 2.

The ED50 and ED95 to the MAC ratio of sevoflurane for LMA removal was 0.57 and 0.69, respectively, in adults.

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Discussion

Although the manufactures' instructions for removal of the LMA recommend waiting until protective reflexes have returned (12), there is controversy regarding the optimal time for LMA removal (2–6,13–18). A more frequent incidence of airway hyperreactivity and complications has been reported by some studies (2–4) when the LMA was removed in the awake state versus the anesthetized state. The LMA is generally well tolerated in waking adults, but we noticed that it is often clamped by biting in waking adults during recovery, and damage to the LMA can occur. Such damage is an important consideration, both for patient safety and also in economic terms. This technique also prevents damage to the LMA by clenched teeth (19).

The main disadvantage of LMA removal in the anesthetized state is that protective airway reflexes have not recovered. Sevoflurane, with its rapid recovery profile, is effective for the quick return of airway reflexes.

The results of our study indicate that the end-tidal concentration of sevoflurane at which 50% of adults had a successful LMA removal (ED50) was 0.99%. Based on published MAC values for sevoflurane of 1.71 in adults (11), the ED50/MAC ratio was 0.57. This was similar to MACEX/MAC ratio (0.63) (7), where MACEX is the MAC for tracheal extubation. The ED95 for LMA removal was 1.18%, and the ED95/MAC ratio was 0.69, which is quite different from the ED95 of the extubation/MAC ratio (≈ 1 MAC) (7). The ED50/MAC ratio and the ED95/MAC ratio of sevoflurane for LMA removal are similar to those for enflurane, although those values for enflurane were determined in children (10).

During their emergence from anesthesia after LMA removal, none of the patients suffered from any serious airway-related complications such as breath holding, laryngospasm, or desaturation to a Spo2 < 90%. The most unsuccessful removal involved some coughing or gross purposeful movements. This was probably related to appropriate depth of anesthesia to blunt airway reflex.

We removed the LMA with the cuff inflated to minimize the retention of secretions, although the manufactures of the LMA recommend its removal with cuff deflated to reduce trauma to the pharynx and to the LMA cuff. Although we suctioned the oropharynx immediately before LMA removal, some secretions might have remained, which could cause coughing or laryngospasm. If aspirated, this may lead to pneumonia. LMA removal with the cuff inflated removes more secretions than when it is deflated (20), and this procedure does not affect early postoperative laryngopharyngeal morbidity (21). Therefore, LMA removal with the cuff inflated may be beneficial in reducing postoperative morbidity if airway reflexes have not recovered. We used a small LMA (size 4 in men and size 3 in women) to reduce the risk of a sore throat that sometimes develops when a large LMA is used (22).

In conclusion, we have determined that LMA removal in 95% of anesthetized adults can be safely accomplished without coughing, patient movement, or any other airway complications at 1.18% end-tidal concentration of sevoflurane. Therefore, we would expect that LMA removal should be possible at approximately 0.7 MAC of sevoflurane when the removal of LMA in the anesthetized state is highly desirable in situations, such as after laryngeal and intraocular surgery.

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