Anesth Analg 2001;92:72-75
© 2001 International Anesthesia Research Society
PEDIATRIC ANESTHESIA
The Minimum Alveolar Concentration of Enflurane for Laryngeal Mask Airway Extubation in Deeply Anesthetized Children
Wenjing Xiao, MD, and
Xiaoming Deng, MD
Department of Anesthesiology, Plastic Surgery Hospital, The Chinese Academy of Medical Sciences, Peking Union Medical College, Beijing, China
Address correspondence and reprint requests to Wenjing Xiao, Department of Anesthesiology, Plastic Surgery Hospital, The Chinese Academy of Medical Sciences, Peking Union Medical College, Beijing, China, 100041. Address e-mail to xiao-wenjing{at}263.net
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Abstract
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The end-tidal anesthetic gas concentration required to prevent the anesthetized patient from coughing or moving during or immediately after the laryngeal mask airway (LMA) extubation is not known. We sought to determine the minimum alveolar concentration of enflurane required for the removal of the LMA in children. We studied 21 nonpremedicated children between 4 and 11 yr of age, ASA physical status I, undergoing procedures below the umbilicus. General anesthesia was induced with a mask by using sevoflurane, nitrous oxide, and oxygen, and the LMA was inserted. Anesthesia was maintained with enflurane, nitrous oxide, and oxygen. At the end of surgery, a predetermined end-tidal enflurane concentration was achieved, and the LMA was removed. Each concentration at which the LMA extubation was attempted was predetermined by the up-and-down method (with 0.1% as a step size). When LMA removal was accomplished without coughing, clenching teeth, or gross purposeful muscular movements during or within 1 min after removal, it was considered a successful LMA removal. Removal was considered to be unsuccessful in patients who developed breath holding or laryngospasm during or immediately after LMA removal. The minimum alveolar concentration of enflurane at which 50% of children had a successful LMA removal was found to be 1.02% (95% CL, 0.95%–1.11%), and the 95% effective dose for successful extubation was 1.14% (95% CL, 1.07%–1.66%). In conclusion, the LMA removal may be accomplished without coughing or moving at 1.02% end-tidal enflurane concentration in 50% of anesthetized children aged 4–11 yr.
Implications: There may be fewer problems associated with the laryngeal mask airway extubation when patients are deeply anesthetized. The purpose of this study was to determine the minimum concentration of enflurane for successful removal of the laryngeal mask in children.
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Introduction
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The laryngeal mask airway (LMA) extubation can be performed while patients are deeply anesthetized or when they are awake. LMA extubation in anesthetized patients may be advantageous in certain clinical situations. Laffon et al. (1) and Gataure et al. (2) reported that problems associated with removal of the LMA occurred more often when patients were awake. There are many reports about the minimum alveolar anesthetic concentration for endotracheal extubation but no data about the minimum alveolar anesthetic concentration for the LMA extubation. Therefore, we attempted to determine the minimum alveolar anesthetic concentration of enflurane for the LMA extubation. We defined MACex as the minimum alveolar concentration of enflurane at which satisfactory LMA extubation is possible in 50% of children.
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Methods
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Twenty-one children undergoing elective plastic or reconstructive surgery below the umbilicus during general anesthesia were included in the study. Informed consent was obtained from the parent or guardian of each participant. The study protocol was approved by our clinical investigation committee. The children were between ages 4 and 11 yr (6.6 ± 1.8 yr, mean ± SD) and were ASA status I. Patients with an abnormal airway, gastroesophageal reflux, or a history of a respiratory tract infection in the preceding 4 wk were excluded.
There was no medication given before surgery. All patients were required to fast appropriately according to our institutional guidelines. General anesthesia was induced via mask by using sevoflurane in oxygen and 60% nitrous oxide. Scopolamine 0.01 mg/kg was administrated IV after IV access was obtained for infusion of lactated Ringer’s solution. When the depth of anesthesia was considered satisfactory, the LMA was inserted with the opening of the LMA facing posteriorly, followed by rotation through 180°. The LMA size was determined by manufacturer’s guidelines by using size 2 for children 6.5–20 kg and size 2.5 for children 20–30 kg. All LMA cuffs were deflated and K-YTM brand sterile lubricating jelly (plain lubricant) (Johnson & Johnson, New Brunswick, NJ) was applied to the back of the mask before placement. Sevoflurane was discontinued after induction, and anesthesia was maintained in all patients with enflurane in approximately 60% nitrous oxide in oxygen with a total inflow of 5 L/min, and the concentration of enflurane was adjusted in response to clinical signs. Spontaneous ventilation was maintained in all patients during anesthesia. No systemic analgesics were administered. We used a modified Jackson Rees system for children weighing <20 kg and a pediatric circle system for those weighing 
20 kg. ETCO2 and end-tidal concentration of enflurane were measured continuously at the elbow of the breathing circuit with a precalibrated gas monitor ( AS/3TM ; Detex, Helsinki, Finland). Standard monitoring included electrocardiography, pulse oximetry, capnography, and nitrous oxide and noninvasive blood pressure monitoring. Nitrous oxide was discontinued before the end of surgery. After the surgery, a predetermined end-tidal enflurane concentration was achieved and a steady state maintained for at least 10 min to allow equilibration between the alveolar and brain concentrations. Immediately after surgery, while the depth of anesthesia was not decreased, the oropharyngeal secretion content was suctioned gently. The concentration a particular patient received was determined by the response of the previous patient to a larger or smaller concentration (with 0.1% as a step size) by using Dixon’s up-and-down sequential method (3). At the time of the LMA extubation, no residual nitrous oxide >3% was detected in the end-tidal sample. The LMA was removed, and jaw lift and face mask were applied with 100% oxygen for 5 min routinely for all children. Patients who developed coughing, teeth clenching, or gross purposeful muscular movements during or within 1 min after removal, or patients who developed breath holding, laryngospasm, or desaturation to SpO2 < 90% during or immediately after LMA removal were regarded as not having been removed successfully. A result defined as unsuccessful extubation directed an increase by 0.1% of enflurane for the next patient. The patient without the above event was considered a satisfactory LMA extubation.
Demographic data were collected and are presented as mean ± SD. The up-and-down sequences were analyzed by probit test ( SPSS for Windows 10.0; SPSS Inc., Chicago, IL), which enabled MACex with 95% confidence limits of the mean to be derived. We also analyzed our data by a logistic regression test to obtain the probability of no movement versus end-tidal enflurane concentration, the maximum likelihood estimators of the model variables, and a goodness of fit.
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Results
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Table 1 gives the details of the demographic data and duration of procedure. The sequences of successful and unsuccessful LMA extubation are shown in Figure 1. MACex of enflurane was 1.02% (95% CL, 0.95%–1.11%) with the probit test, and the 95% effective dose for smooth extubation was 1.14% (95% CL, 1.07%–1.66%). Maximal likelihood estimators of the logistic regression model variables in this group showed a P value of 0.906 and goodness of fit 
2 = 0.588. Logistic regression curves of the probability of no movement are shown in Figure 2.
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Figure 1. The responses of the 21 consecutive patients in whom laryngeal mask extubation was attempted and the end-tidal concentrations of enflurane in oxygen. Each patient’s data are represented with a circle. The minimum alveolar concentration of enflurane for laryngeal mask extubation at which a smooth tracheal extubation is possible in 50% of children was 1.02% (95% CL, 0.79%–1.15%).
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Figure 2. Dose-response curve for enflurane plotted from probit analyses of individual end-tidal concentrations and the respective reactions to tracheal extubation in this patient population. The concentrations at which there was a 50% and 95% probability of smooth laryngeal mask airway extubation were 1.02% and 1.14%, respectively.
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Two patients had laryngospasm and were easily treated without any sequelae. In three children, breath-holding occurred after extubation, and one of them experienced coughing and desaturated to below 90%. The most unsatisfactory LMA extubation involved coughing or clenching of teeth with difficult removal of LMA. After 5 min of jaw lifting all patients could maintain clear airways spontaneously.
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Discussion
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Removal of the LMA when consciousness and protective upper airway reflexes have returned is recommended by the manufacturers (4) and many authors (5–7). Other reports1 (1,2,8,9) suggest that it might be safer to remove the LMA while the patients are deeply anesthetized. The LMA is well tolerated at light levels of anesthesia in adults but not always in children, especially when inhaled anesthetics are used to maintain anesthesia. Laffon et al. (1) reported a twofold increase of complications after removal of the LMA in awake compared with deeply anesthetized pediatric patients. The LMA extubation in the deeply anesthetized patient is used in a variety of settings and after certain surgical procedures, e.g., after intraocular surgery, to prevent a sudden increase in intraocular pressure. Gataure et al. (2) and Varughese et al.1 found that deep anesthesia LMA extubation could decrease the incidence of coughing, laryngospasm, desaturation, excess salivation, biting, retching, vomiting, and airway obstruction during the emergence.
Upper airway reflexes are of primary importance to anesthesia. Their activation during anesthesia can lead to apnea and laryngospasm which, besides being a minor inconvenience, may also be life-threatening. Therefore, adequate suppression of airway reflexes is necessary for safe anesthesia, but their rapid return in the postoperative period is essential to protect the lungs from aspiration. Varughese et al.1 extubated the LMA at a relatively deep level of anesthesia, with an end-tidal concentration of halothane more than 2 MAC. This would leave the patient’s airway unprotected for a relatively long time without the patient’s airway reflexes returning. It may be better to reserve the advantage of deep extubation and reduce the interval between the LMA extubation and the return of consciousness and protective reflexes to as short a time as possible. We have defined the MACex as the alveolar concentration of inhalational anesthetics at which 50% of patients undergoing the LMA extubation without coughing or moving within one minute of extubation or without developing breath holding or laryngospasm immediately after extubation. The results of our study show that the MACex for enflurane was 1.02% and that in 95% of children 4–11 years of age, LMA extubation may be accomplished successfully at 1.14% end-tidal concentration of enflurane.
A potential source of error in our study could have been our use of nitrous oxide during the surgery. There was a significant but small end-tidal concentration of nitrous oxide at the time of extubation, but the value was always <3%. This residual nitrous oxide is unlikely to have changed the MACex value for enflurane by >0.05%.
Placing the sampling catheter between the LMA and the breathing tube potentially contaminates the end-tidal gas with fresh gas (9). More accurate measurement of end-tidal gas therefore requires sampling from the distal end of the LMA.
In conclusion, we determined that the LMA extubation in 50% and 95% of anesthetized children, aged from 4 to 11 years old, may be accomplished without coughing or moving at 1.02% and 1.14% end-tidal enflurane concentrations, respectively.
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Acknowledgments
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We would like to thank Dr. Kunling Xu for her valuable assistance with this study, Dr. Jun Xu for his comments, and Shaomei Han for her help in the statistical analysis of the data.
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Footnotes
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1 Varughese A, Culloch M, Lewis M, Stokes M. The removal of the laryngeal mask airway in children: awake or deep [abstract]? Anesthesiology 1994;81:A1321. 
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Dixon WJ. Quantal response to variable experimentation: the up-and-down method. In: McArthur JM, Colton T, eds. Statistics in endocrinology. Cambridge, MA: MIT Press, 1967: 251–64.
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Accepted for publication September 19, 2000.
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Abstract
Introduction
