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PEDIATRIC ANESTHESIA

Factors Associated with Successful Tracheal Intubation of Children with Sevoflurane and No Muscle Relaxant

George D. Politis, MD MPH^*, Michael J. Frankland, MD, Robert L. James, MS, Jacland F. ReVille, MD, Michael P. Rieker, CRNA, and Betty C. Petree, CRNA

*Department of Anesthesiology, University of Virginia Health System, Charlottesville, Virginia; and Department of Anesthesiology, Wake Forest University School of Medicine, Winston-Salem, North Carolina

Address correspondence and reprint requests to Dr. Politis, Department of Anesthesiology, University of Virginia Health System, PO Box 800710, Charlottesville, VA 22908. Address e-mail to gdp8a{at}virginia.edu

	Abstract

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Better definition of end points required to achieve successful tracheal intubation after induction with sevoflurane could improve patient care. The authors therefore designed a study that could determine, with meaningful confidence intervals, the time required to successfully intubate 80% of children by using 8% inspired sevoflurane and no muscle relaxant. We hypothesized that the time required could vary by age or body mass index. One-hundred fifty-three ASA physical status I or II patients received induction with 8% sevoflurane in 60% nitrous oxide with discontinuation of nitrous oxide 1 min after the start of the induction. The time until laryngoscopy remained close to the time required to achieve 80% successful intubation by varying induction time according to the success rate in each group of five patients. A probit model of induction time and age found that both were predictive of successful intubation (P values of 0.006 and 0.02, respectively). The induction times needed to achieve 80% successful intubation were 137 s (95% confidence interval, 94.6–159 s) and 187 s (153–230 s) for ages 1–4 yr and 4–8 yr, respectively. The persistence of spontaneous ventilation at the time of laryngoscopy, despite attempts to control ventilation, was associated with poor intubation conditions (P < 0.001).

IMPLICATIONS: To successfully intubate 80% of children by using sevoflurane and no muscle relaxant, induction times of 137 and 187 s were needed in children of 1–4 yr and 4–8 yr, respectively.

	Introduction

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Tracheal intubation of healthy pediatric patients by using an inhaled anesthetic without a muscle relaxant is common practice among members of the Society for Pediatric Anesthesia, and the introduction of sevoflurane may have increased the use of that practice (1). When intubating with this technique, deciding when to perform laryngoscopy can be based on induction time (2), end-tidal (ET) anesthetic concentration (3), physical examination (4), or changes in blood pressure (BP), heart rate (HR), or respiratory pattern. Although the presence of physical examination end points, such as constricted centralized pupils, may frequently result in successful tracheal intubation (4), such end points are considerably more subjective than induction time, and many patients may be ready for laryngoscopy before clinical criteria are met. Hemodynamic markers of readiness may not be useful with sevoflurane, because HR and BP change minimally (5) and may even increase during sevoflurane induction (6). Some studies have determined the ET sevoflurane concentration required for achieving 50% successful intubation of children, waiting for at least 15 min before intubation to allow for equilibration of sevoflurane concentrations (7,8). Other studies have used rapid methods for tracheal intubation to determine clinically useful data for the sevoflurane ET concentration and induction time required for achieving 50% successful intubation of children (2,3). Although such studies can generate narrow 95% confidence intervals (CIs) around end points determined to achieve 50% success, we would consider a 50% failure rate unacceptably frequent for this procedure. Those authors’ attempts to extrapolate results and approximate end points for achieving larger percentages of success resulted in large and sometimes meaningless CIs. Additionally, their studies were performed with sevoflurane vaporizers with a maximum dialed concentration of 5%, rather than the 8% setting widely available and used in the United States. This study was designed to determine the amount of induction time required to achieve satisfactory tracheal intubation conditions in 80% of healthy patients aged 12 mo to 8 yr. Although clinicians might prefer to know the induction time required to achieve 90% or 95% success, a study designed to obtain narrow 95% confidence limits around those induction times would require very large patient enrollment. Our study sought to minimize the induction time, and therefore used nitrous oxide (N₂O) (5,9) and 8% inhaled sevoflurane from the start of induction (10,11), with rapid control of ventilation to speed the increase in alveolar gas concentration. Because other drugs are known to influence the amount of sevoflurane required during tracheal intubation (7,12–14) and because we wanted to generate data that were applicable to our clinical practice, we followed our routine of midazolam preanesthetic sedation and use of N₂O at the beginning of induction. On the basis of data by Lerman et al. (15), we hypothesized that the induction time required for satisfactory intubation conditions would vary by patient age or size.

	Methods

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After receiving IRB approval and informed consent from guardians, we recruited ASA physical status I or II patients aged 12–96 mo. All patients were scheduled to receive an endotracheal tube (ETT) during an elective procedure, and none had stigmata of a difficult airway. Patients were immediately removed from the study if there was any violation of study protocol. Because we hypothesized that induction time would vary by age, subjects were prospectively separated into two age groups, 12–48 mo or 48–96 mo, with a division point chosen that we believed would result in similar group sizes. We prospectively separated subjects by age to allow age group-specific induction times to vary (according to the algorithm below) more widely than would have been possible with all patients grouped together.

All patients received premedication with 0.6 mg/kg of midazolam, followed by induction 15–45 min later with 8% dialed sevoflurane and 60% N₂O. N₂O was discontinued 1 min after the start of induction. Midazolam, sevoflurane, and N₂O were the only medications given before laryngoscopy. Anesthesia was administered by using primed pediatric circle systems and appropriately sized pediatric masks (Vital Signs, Inc., Totowa, NJ), with total flows maintained at 10 L/min throughout induction. Airways were managed by attending anesthesiologists, certified registered nurse anesthetists, or experienced anesthesia residents. To ensure experienced airway management, residents were excluded if they had not completed more than 1 yr of clinical anesthesia training or if they had not completed their pediatric anesthesia rotation. Because minute ventilation during induction was likely to affect the induction time required to achieve good intubating conditions, we sought a means to make ventilation as uniform as possible. To achieve that goal, in each case ventilation was assisted and then controlled as quickly as possible, and instructions were given to hyperventilate without administering airway pressures in excess of 20 cm H₂O. In cases in which spontaneous ventilation persisted despite attempted control of ventilation, assisted ventilation was maintained. If airway obstruction occurred, an oral airway was immediately inserted. ETCO₂ measurements were made just before laryngoscopy and immediately after intubation in an effort to assess ventilation. Attempts were made to obtain venous access before laryngoscopy. Laryngoscope blade and ETT cuffing and size were at the discretion of the patient’s attending anesthesiologist. However, the appropriateness of the ETT size was always checked by an investigator [by using the formula (age + 16)/4], and stylets were placed in all ETTs to provide the laryngoscopist with the best opportunity to successfully place the ETT on the single laryngoscopy attempt allowed.

Tracheal intubation was attempted after a predetermined induction time. Induction time was defined as the time between the initiation of induction and the start of laryngoscopy. Intubation conditions were assessed by the laryngoscopist, including jaw relaxation, vocal cord position, and visualization of glottic structures, by using a standard 1–4 grading system (16). The induction time for the initial five patients was 4 min. Subsequent times were allocated for each succeeding group of five patients by using a modification of the allocation scheme used by Dixon and Mood (17). The algorithm for changing the induction time was designed to cluster our sample points around the induction time that would provide successful intubation conditions in 80% of patients. Rather than following Dixon and Mood’s method of increasing or decreasing the induction time after each patient, we made changes in the induction time after each group of five consecutive patients, depending on whether an 80% success rate was reached. Therefore, success in exactly four of five patients dictated no change in the succeeding group’s induction time, whereas more or lesser success dictated a 20-s decrease or increase, respectively. Adequacy of intubation conditions was in each case determined by the individual managing the airway and by one of the investigators. Inadequate intubation conditions were declared if a patient moved during laryngoscopy, if the ETT could not be placed for any reason other than the presence of an unexpected difficult airway, if oxygen saturation decreased to <90% (in which case laryngoscopy was terminated), or if persistent coughing (>5 s), vigorous movement, or prolonged movement (>5 s) occurred after ETT placement. Only a single laryngoscopy attempt was allowed. Small, brief movements of extremities occurring after ETT placement did not lead to a classification of inadequate conditions.

HR and a noninvasive BP measurement were obtained at baseline (after midazolam and just before induction) and again just before laryngoscopy. ET gases were recorded (Capnomac; Datex, Helsinki, Finland) immediately before laryngoscopy and, when an ETT was successfully placed, immediately after ETT placement. The gas sampling port was located in a standard 90° elbow placed adjacent to the ETT. Large volumes were given for breaths used to determine ET gas concentrations, in an effort to obtain true ET concentrations uncontaminated by fresh gas flow. Postintubation ET gases were obtained by using a self-reinflating bag with a dedicated source of 5 L/min oxygen flow, to eliminate the possibility of altering values by the administration of gases remaining in the circle system.

A probit analysis was used to determine the induction time and sevoflurane ET concentration required to achieve 50%, 80%, 90%, and 95% success rates during intubation. The probit models were fit to the data, both with and without covariates for age (1 age < 4 yr vs 4 age < 8 yr), body mass index (kg/m²), transient airway obstruction, and spontaneous ventilation. Probit models were fit by using log-transformed induction time and sevoflurane concentrations. Simple linear regression tested the effect of induction time on BP and HR. Spearman rank correlation was used to test for an association between ETCO₂ and age. Pre- and postintubation mean ET gas values are given with SE in parentheses. Induction times and the ET sevoflurane concentration required for 80% successful tracheal intubation are given with their 95% CIs in parentheses. Power analysis determined that 150 patients would be sufficient to estimate the 80% success point with a coefficient of variation of <5% and a power of 90%.

	Results

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One-hundred-sixty-two patients were enrolled in the study. Nine protocol violations occurred, rendering a total of 153 patients studied. Surgeries received included tonsillectomy and/or adenoidectomy (51%), strabismus repair (7%), various plastics procedures (7%), ureteral reimplantation (5%), tympanoplasty (5%), and miscellaneous other procedures (25%). Ventilation was controlled by the time of laryngoscopy in 124 subjects (81%), with all but 4 of those subjects remaining apneic during intubation. The mean preintubation ETCO₂ was 25 mm Hg (±7.7). A vein was successfully cannulated by the time of laryngoscopy in 79% of subjects. Airway obstruction during induction occurred in 18 patients and was immediately relieved in each case by placement of an oral airway. Inadequate intubation conditions were present in 24 patients. The majority were related to either movement during laryngoscopy (12 of 24) or to excessive movement or prolonged cough after ETT placement (6 of 24). Of the 129 patients successfully intubated, vocal cords were completely open in 85%, and a Grade 1 view was present in 86%. There was no coughing or movement in 82% of patients successfully intubated. The mean preintubation ET N₂O concentration was 5.8% (±6.1), whereas the mean ET gas concentrations of N₂O and CO₂ immediately after successful tracheal intubation were 6.0% (±3.9) and 37 mm Hg (±5.7), respectively. There was no correlation between age and postintubation ETCO₂ concentration (r = 0.109, P = 0.28).

A probit model that included both induction time and age group found both to be predictive of successful intubation, with P values of 0.006 and 0.02, respectively. The induction time to achieve 80% successful intubation was 137 s (94.6–159 s) and 187 s (153–230 s) for ages 1–4 yr and 4–8 yr, respectively. A separate model included both sevoflurane ET concentration and age group but found that only sevoflurane ET concentration could predict successful intubation (P = 0.04). Sevoflurane ET concentration was therefore placed in a univariate model, and the concentration needed to achieve 80% successful intubation was 4.85% (2.83%–5.53%). The induction times and sevoflurane ET concentrations required for 50%, 80%, 90%, and 95% success rates are given in Table 1, and our probit model for induction time and age is illustrated in Figure 1. Continued spontaneous ventilation at laryngoscopy despite attempts to control ventilation was found to be strongly associated with intubation failure (P < 0.001). We could find no association between inadequate intubation conditions and either body mass index or transient airway obstruction during induction.

View larger version (17K): [in this window] [in a new window]   Figure 1. Probability of successful intubation versus induction time with probit models shown for ages 1–4 yr and 4–8 yr; 95% confidence limits are shown around the 80% and 90% success points. ED = effective dose.

	Discussion

Top Abstract Introduction Methods Results Discussion References

 
Our study was designed to determine the induction time for achieving 80% success during tracheal intubation of children, by using 8% inhaled sevoflurane and no muscle relaxant, under conditions that mimic our own clinical practice. We found that the time required to achieve successful intubation conditions did vary by age and that our method of rapid induction led to 80% success at 137 and 187 seconds for ages one to four and four to eight years, respectively. Successful prediction of good intubation conditions may avoid early initiation of laryngoscopy, when stimulation might lead to laryngospasm, bronchospasm, or other complications. Proper timing of ETT placement may also avoid unnecessarily excessive induction time and anesthetic depth. Our results pertain to children one to eight years old who have received midazolam preanesthetic sedation and induction that initially uses N₂O. We included oral midazolam in our protocol because of its widespread use in the United States (18), because we use it in our daily practice, and because evidence in adults indicates that premedication with midazolam can decrease time to acceptable intubation conditions (14). We induced anesthesia with 60% N₂O and 8% dialed sevoflurane, rather than with incrementally increasing sevoflurane, because we desired the more rapid induction that the former method offers (5,9–11) and the decreased excitement provided by N₂O (5,9). N₂O was discontinued during induction, per our routine practice, because we desired close to 100% oxygen at laryngoscopy. N₂O may have influenced our results by speeding induction and by the additive effect it exerts on the amount of sevoflurane required for tracheal intubation without a relaxant (12). In our patients, the additive effect should have been small, given the minimal amount of N₂O present at the time of laryngoscopy.

The induction time and ET sevoflurane concentration for achieving 50% successful intubation conditions have been previously determined (2,3). We believe that those end points are of limited clinical value because 50% failure is excessively frequent for this procedure. Although clinicians might prefer to know the induction time required to achieve 90% or 95% success, targeting 80% success may be reasonable considering our apparent small incidence of complications, even when failures occurred. We had only 1 case of laryngospasm out of 24 patients who had poor intubation conditions, and that laryngospasm produced no sequelae. Notably, the laryngospasm did appear to occur as a result of laryngoscopy in an inadequately anesthetized patient. The studies cited previously, which targeted the 50% success point, used statistical models to extrapolate the induction time or sevoflurane ET concentration needed to achieve 95% success. Those extrapolations resulted in wide, clinically meaningless 95% CIs, which included impractically long induction times and concentrations of sevoflurane well beyond those delivered by standard vaporizers. Additionally, extrapolated numbers are of less value because they could be erroneous if the mathematical models on which they were based are wrong. Despite our considerably larger patient enrollment than in those previous studies, our study was also unable to use mathematical modeling to extrapolate a clinically meaningful CI at points away from where data were collected. Our methodology focused data collection at the 80% success point, did not require extrapolation to predict 80% success, and generated a clinically meaningful CI around the induction time and the ET sevoflurane concentration for achieving 80% success.

Both induction time and sevoflurane ET concentration may be useful predictors of intubation success. Clearly, these two predictors are interrelated. However, we designed our study to determine the induction time because we believe that it is a more useful clinical predictor during a time when inspired and alveolar sevoflurane concentrations are far from equilibrium. Without equilibrium, sevoflurane ET concentration measurement may become spurious if fresh gas dilutes the alveolar gas. Because a face mask introduces a large amount of volume relative to the patient’s expired volume, especially in children, a significant artifact in the measured ET gas concentration can occur because of dilution of expired gas with fresh gas present in the mask. We attempted to compensate for this dilution artifact by administering large inspiratory volumes during breaths used to make ET gas measurements. Still, we found postintubation ET measurements of CO₂ to be substantially higher than preintubation values, suggesting a persistent dilutional effect. Variation in mask volumes from one manufacturer to another might lead to differences in the degree of artifact. Therefore, the ET sevoflurane concentration found to be effective in our study may not be useful to practitioners with slightly different equipment. Better data regarding the preintubation ET gas concentration could have been obtained by sampling from a catheter positioned in the pharynx. We elected not to do so because we believe such sampling would not be practical in a clinical setting.

The induction time required should depend on the minute ventilation during induction. Creating a study design that standardized the amount of ventilation during induction might have improved the reproducibility and utility of our results. However, maintaining a uniform amount of ventilation during inhaled induction is difficult, and we have alternatively reported postintubation ETCO₂ values as a crude measure of minute ventilation. We recognize that ETCO₂ values are also dependent on the patient’s metabolic rate and cardiac output. It is interesting to note that despite being instructed to hyperventilate patients, those individuals who managed the airway on average ventilated to normocarbia, reflected by a postintubation mean ETCO₂ of 37 mm Hg. In our study, different individuals with differing pediatric airway skills performed airway management. None of those individuals were novices with pediatric airways. We acknowledge that induction times may be longer or shorter than those we are reporting when very skilled individuals or novices, respectively, are managing airways.

Younger children required less induction time for successful intubation compared with those in the older age group, according to our probit analysis. That finding agrees with Lerman et al. (15), who found that the times from application of the face mask to the loss of eyelash reflex and intubation increased with increasing age. Our study did not determine reasons for this age-related difference. Possibilities include an increased ability to rapidly control ventilation or higher minute ventilation per weight in smaller children. Of interest was the strong association between poor intubating conditions and continued spontaneous ventilation despite an attempt to control ventilation. Because of the strength of that association, we suggest that when using our technique for induction, if control of ventilation has not been possible by the targeted induction time, then induction should continue until ventilation can be controlled.

Minimal negative hemodynamic effect occurred during our sevoflurane inductions. Mean HR before laryngoscopy increased by 20%, whereas mean arterial blood pressure decreased by 3%–8%. These HR findings coincide with those of Kern et al. (6). Because increasing anesthetic depth correlates poorly with decreasing hemodynamic variables in the age group studied, it is difficult to use alterations in HR or BP to decide when a patient has reached adequate depth for laryngoscopy. Of note is the fact that such a benign hemodynamic profile does not exist in the age group younger than one year (15), which was not studied here. Although there are no data to determine the comparative safety of tracheal intubation with and without muscle relaxants, complications were rare in our 153 patients, rendering some support to the safety of this technique in the age group studied.

In conclusion, our study was designed to determine the induction time for achieving 80% success during tracheal intubation of children one to eight years old, by using 8% inhaled sevoflurane and no muscle relaxant, under conditions that mimic our own clinical practice. Eighty percent success can be obtained with approximately 137 and 187 seconds for ages one to four years and four to eight years, respectively. Although diminution in hemodynamic variables does not appear to be a useful predictor, other indicators, such as preintubation ET sevoflurane concentration and ability to control ventilation, may predict successful intubation conditions.

	Acknowledgments

 
Funded by the Department of Anesthesiology, Wake Forest University School of Medicine, Winston-Salem, NC.

	Footnotes

 
Presented in part at the Society of Pediatric Anesthesia Winter Meeting, San Diego, CA, February 2001.

	References

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This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a colleague Similar articles in this journal Similar articles in ISI Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (7) Citing Articles via Google Scholar Google Scholar Articles by Politis, G. D. Articles by Petree, B. C. Search for Related Content PubMed PubMed Citation Articles by Politis, G. D. Articles by Petree, B. C. Related Collections Anesthetic Techniques Pediatrics Airway

Anesthesia & Analgesia® is published for the International Anesthesia Research Society® by Lippincott Williams & Wilkins with the assistance of Stanford University Libraries' HighWire Press®. Copyright 2006 by the International Anesthesia Research Society. Online ISSN: 1526-7598   Print ISSN: 0003-2999