Anesth Analg 2007;104:528-531
© 2007 International Anesthesia Research Society
doi: 10.1213/01.ane.0000253035.96490.22
PEDIATRIC ANESTHESIA
Minimum Alveolar Concentration of Sevoflurane for Laryngeal Mask Airway Removal in Anesthetized Children
Jeong-Rim Lee, MD,
Seong-Deok Kim, MD, PhD,
Chong-Sung Kim, MD, PhD,
Tae-Gyoon Yoon, MD, PhD, and
Hee-Soo Kim, MD, PhD
From the Department of Anesthesiology, Seoul National University College of Medicine, Seoul, Korea.
Address correspondence and reprint requests to Hee-Soo Kim, MD, PhD, Department of Anesthesiology, Seoul National University College of Medicine, Seoul, Korea. Address e-mail to dami0605{at}snu.ac.kr.
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Abstract
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BACKGROUND: In children, it is preferable to remove the laryngeal mask airway (LMA) when the patient is still anesthetized. We sought to determine the optimal minimum alveolar concentration of sevoflurane that would allow removal of the LMA in children without airway complications.
METHODS: We studied 25 unpremedicated children between 7 mo and 10 yr of age, ASA Status I, undergoing urologic or plastic surgery. General anesthesia was induced with sevoflurane and oxygen given via mask. The LMA was inserted and anesthesia was maintained with sevoflurane in oxygen. The LMA was removed at the end of surgery when the end-tidal sevoflurane concentration had reduced to a predetermined level, determined by the up-and-down method, with 0.2% as a step size. A removal accomplished without coughing, teeth clenching, gross purposeful movement, breath holding or laryngospasm, during or within 1 min after removal, was considered to be successful.
RESULTS: The minimum alveolar concentration of sevoflurane at which 50% of LMA removals were successful was 1.84% (95% confidence limits, 1.45%–1.96%), and the 95% effective dose for successful removal was 2.17% (95% confidence limits, 2.02%–3.48%).
CONCLUSIONS: LMA removal may be accomplished without coughing, moving, or any other airway complication at 1.84% end-tidal sevoflurane concentration in 50% of anesthetized children.
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Introduction
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Removal of the laryngeal mask airway (LMA) can be performed while patients are deeply anesthetized or awake. In children, removal of a LMA with the patient in a deeply anesthetized state is preferred in some selected conditions, such as asthma, as it reduces the incidence of airway complications such as coughing, biting, hypersalivation, and hypoxia (1,2), although this carries the disadvantage of active laryngeal reflexes remaining suppressed due to deep anesthesia (3) in which upper airway obstruction may occur (4). However, recovery from sevoflurane allows a brisk return of airway reflexes after LMA removal during deep anesthesia (5). Therefore, sevoflurane is a good candidate for LMA removal during anesthesia.
A previous study quantified the depth of sevoflurane anesthesia required to perform LMA removal in adults (6). However, there has been no study on the optimal sevoflurane concentration for LMA removal in children. We therefore attempted to determine the end-tidal concentration of sevoflurane required. This study, using a Dixon’s up-and-down method, was designed to determine the ED50 and ED95 of sevoflurane for removal of LMAs in children.
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METHODS
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Twenty-five children undergoing elective minor plastic (polydactyly operations, or skin graft) or urologic surgery (hydrocelectomy, orchiopexy, or circumcision) for <2 h under general anesthesia were studied. Written informed consent was obtained from the parent or guardian of each participant, and our clinical investigation committee approved the study protocol. The children were between 7 mo and 10 yr of age, ASA Status I. Patients with an abnormal airway, reactive airway disease, or a history of an upper respiratory tract infection (URI) in the preceding 3 wk were excluded.
The children were not premedicated. Upon arrival at the operating room, the patients were monitored with electrocardiography, pulse oximetry, capnography, inhaled and exhaled end-tidal sevoflurane concentrations, and noninvasive arterial blood pressure. Anesthesia was induced using an inhaled technique with 8 vol % of sevoflurane in oxygen via a pediatric circle system. After loss of consciousness, sevoflurane was adjusted to 2%–3% according to the vital signs and the same concentration of sevoflurane was maintained for several minutes until adequate jaw relaxation was attained for a LMA. The LMA size was determined by the manufacturer’s guidelines, which suggests size 2 for 6.5–20 kg, size 2.5 for 20–30 kg, and size 3 for 30–50 kg. After induction, anesthesia was maintained with sevoflurane in approximately 50% oxygen in air with a total inflow of 2 L/min, and the concentration of sevoflurane was adjusted in response to clinical signs. Spontaneous ventilation was maintained in most patients, and simultaneous intermittent mandatory ventilation rate and pressure support were adjusted according to the patient’s weight and end-tidal carbon dioxide concentration. At the end of surgery, the oropharynx was gently suctioned. The inhaled sevoflurane concentration was adjusted to the predetermined concentration, and we waited until the same concentration of exhaled end-tidal sevoflurane concentration was attained. The predetermined concentration was then maintained for at least 10 min to allow equilibrium between the alveolar and brain concentrations and the LMA was removed.
The sevoflurane target concentration for a particular patient was determined by the response of the previous patient to a larger or smaller concentration (with 0.2% as a step size) by using Dixon’s up–and-down sequential method (7), starting at 2.0% end-tidal concentration (8). The LMA was removed with cuff inflated, and jaw lifted and a facemask was routinely applied with 100% oxygen for 5 min for each child. If breath holding persisted for more than 30 s or if tidal volumes were <6 mL/kg, ventilation was manually assisted. Unsuccessful LMA removal was defined as the development of coughing, clenching, or gross purposeful movement during or within 1 min of LMA removal, development of breath holding, laryngospasm, or desaturation to Spo2 <90%. After such an unsuccessful removal, the sevoflurane concentration for LMA removal was increased by 0.2% in the next patient. Conversely, if LMA removal was successful (i.e., complication-free), the sevoflurane concentration was decreased by 0.2% in the next patient. The LMA was removed at a predetermined concentration and the result of failure-success was recorded. The patients were managed by an anesthesiologist and transferred to the recovery room when fully awake. At the recovery room, pain was controlled with nonsteroidal analgesics and emergence delirium was controlled with 0.1 mg/kg of nalbuphine IV injection and close observation.
Demographic data were collected and presented as mean ± sd. The Dixon’s up-and-down method needs six pairs of failure-success for statistical analysis (7,9), and sample size came from the basis of Dixon’s method. The up-and-down sequences were also analyzed by the probit test, which enabled us to derive the sevoflurane concentration for LMA removal, with 95% confidence limits of the mean. We also analyzed our data by a logistic regression test to obtain the probability of no movement versus end-tidal sevoflurane concentration, the maximum likelihood estimators of the model variables, and a goodness of fit.
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RESULTS
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Demographic data and the duration of the LMA insertion are presented in Table 1. The sequences of successful and unsuccessful LMA removal are shown in Figure 1. The required concentration of sevoflurane for successful LMA removal was 1.87% ± 0.11% by Dixon’s method. A logistic regression curve of the probability of no movement is shown in Figure 2 and is illustrated by the dose–response curve constructed from the probit test data. This shows that the 50% effective dose for smooth removal of a LMA was 1.84% sevoflurane (95% confidence limits, 1.45%–1.96%), and that the 95% effective dose was 2.17% (95% confidence limits, 2.02%–3.48%). The ratios of ED50 and ED95 for LMA removal to minimum alveolar concentration (MAC) for sevoflurane were 0.74 and 0.87. Maximum likelihood estimators of the logistic regression model variables in this group showed a P = 0.649 and goodness of fit x2 = 0.866.
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Figure 1. The end-tidal sevoflurane concentration in the 25 consecutive patients in whom laryngeal mask airway (LMA) removal was attempted. Each patient’s data are represented with a circle; a blanked circle means successful removal and a black circle means unsuccessful removal of the LMA. The end-tidal concentration of sevoflurane at which a successful removal is possible in 50% of pediatric patients was 1.87% ± 0.11%.
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Figure 2. Dose–response curve for sevoflurane plotted from the probit analyses of individual end-tidal concentrations and the respective patient reactions to the removal of the laryngeal mask airway (LMA). The concentrations at which there were 50% and 95% probabilities of successful LMA removal were 1.84% and 2.17%, respectively.
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LMA removal was unsuccessful in 12 patients, and we observed movement in 8 patients (2 at 1.6%, 4 at 1.8% and 2 at 2.0%) and clenching in 4 patients (1 at 1.6% and 3 at 1.8%). Laryngospasm occurred in 3 patients (1 at 1.6% and 2 at 1.8%) and breath holding in 2 patients (1.6 and 1.8%) who were easily treated with continuous positive airway pressure and assisted positive pressure ventilation with 100% oxygen. There was one patient with coughing at 1.6% who did not require any management. Desaturation <90% was not noticed in any of the patients. Other complications in the failure criteria resolved spontaneously without specific management. Within 5 min of jaw lifting, all patients were able to maintain upper airway patency by themselves, and there were no additional complications.
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DISCUSSION
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We have shown that the ED50 of sevoflurane for successful LMA removal is 1.84% and that the ED95 was 2.17%; the ED50 to MAC ratio was 0.74.
In adults, the ED50 of sevoflurane is 0.99%, ED50/MAC ratio 0.57, smaller than in children (6). From the results of our study, the LMA may be tolerated less well during light anesthesia in children compared with adults (10).
The ED50 of enflurane for LMA removal in anesthetized children is 1.02% and ED50/MAC ratio is 0.7, similar to our sevoflurane value of 0.74. (11) Enflurane and sevoflurane are both relatively nonirritating. However, it should be borne in mind that there are differences in blood: gas partition coefficients of enflurane and sevoflurane (1.9 and 0.65, respectively), and thus, the two anesthetics will require different times for equilibrium (12).
It has been recommended, in adults, that the LMA should not be removed until the patient is fully awake because of the possibility of complications (10,13). However, as characteristics of children are different from those of adults and some children do not respond to verbal commands, it is difficult to judge whether the child is awake or lightly anesthetized. Moreover, although the LMA is well tolerated in adults during light anesthesia, this is not always the case in children; and in some patient, such as those with asthma, removal may be safer during deeper anesthesia to reduce coughing, laryngospasm, biting, and hypoxia. Therefore, this study suggests that the LMA can be removed without serious complications in children. This does not mean that it should always be removed during deep anesthesia, because even though such removal can be recommended, fast return of the airway reflex is critical in operations that irritate the upper airway or increase secretions; therefore, these operations are an exception (14,15).
When an airway device is removed in a deeply anesthetized state, prolonged upper airway obstruction and delayed return of protective reflexes are the main concerns. An adequate concentration of sevoflurane, with its rapid recovery profile, is useful in this regard. In this study, the sevoflurane concentrations for successful LMA removal were between 1.8% and 2.2%. All patients could preserve their airway patency with slight support, such as chin or jaw lift, at these concentration and they could maintain their airway patency spontaneously within 5 min.
There are some limitations in this study. First, we excluded patients who had an URI within the preceding 3 wk. Because a decrease of airway conductance might persist for up to 6 wk after an URI (16), enrolled patients might have had airway hyperrectivity even though they were clinically symptom-free. Therefore, patients with a lower airway involvement in the preceding 3 wk might have affected the results. Another question would be airway reactivity related to age. A recent study reported that bronchial reactivity changed with age in children with asthma (17). That study reported that bronchial reactivity changes from 1 to 6 yr and these age-related changes reflected early changes in airway smooth muscle maturity and later changes in airway wall rigidity. However, another study performed in 2–12-yr-old children, reported that during anesthesia with 3% sevoflurane, airway resistance was not changed regardless of endotracheal tube or mask in the patients without asthma, and no apparent clinical adverse event was observed in the children with or without asthma (18). As airway reactivity could be blunted by inhaled anesthetics, it might not have affected the results of this study, even though the age range was wide. Also in this study, the patients with airway complications were distributed, regardless of age, when the same concentration was used. However, age range and concentration of sevoflurane in this study were different from those in previous studies, and therefore, we could not be absolutely certain that airway reactivity related to age did not affect our results.
In conclusion, we found that LMA removal in 50% and 95% of anesthetized children, aged from 7 mo to 10 yr, can be safely accomplished at 1.84% and 2.17% end-tidal concentration of sevoflurane. Thus, if children require removal of a LMA during anesthesia, safe LMA removal for those receiving sevoflurane may be possible at approximately 0.87 MAC, which is around 2.2% of sevoflurane.
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Footnotes
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Accepted for publication October 27, 2006.
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A Comparison of the End-Tidal Sevoflurane Concentration for Removal of the Laryngeal Mask Airway and Laryngeal Tube in Anesthetized Children
Anesth. Analg.,
April 1, 2008;
106(4):
1122 - 1125.
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Abstract
Introduction
