The incidence of the OCR is presented in Table 2. Baseline HR was higher in the sevoflurane group (P = 0.002). The OCR occurred less often in the sevoflurane group (S 38%, H 79%;P = 0.009). Halothane was found to have the lower mean lowest HR during OCR. There was no difference in the baseline systolic blood pressure (S 102 ± 12 mm Hg, H 98 ± 12 mm Hg). Dysrhythmias were seen after traction on the ocular muscles and were more common in the halothane group (S 4%, H 42%;P = 0.004). One child in the sevoflurane group experienced nodal rhythm. In the halothane group, five children had nodal rhythm, one had irregular sinus rhythm, one had a transient sinus arrest, that is, a missing P wave with a delayed QRS by 0.4 s, and three had ventricular extrasystoles. One of the children in the halothane group had severe ventricular extrasystoles on traction of the ocular muscles and was switched to isoflurane for the maintenance anesthetic.
The ORR and respiratory parameters are summarized in Table 3. The baseline RR was higher in the sevoflurane group (S 38.2 ± 6, H 31.7 ± 8.2;P = 0.005). The baseline VT was similar in both groups, as was the baseline PETCO2. Significant changes in VT accompanied the OCR in both groups (P < 0.01). There were no significant differences in VT and PETCO2 changes between groups. During the OCR, the RR remained higher in the sevoflurane group than in the halothane group (P = 0.016). Overall there was a significant decline in the RR from baseline values (P = 0.03).
Three children had evidence of hypoventilation after LMA insertion (S 2, H 1) on high end-tidal vapor concentrations, and ventilation was briefly assisted until spontaneous respiration returned. Another 11 children, most of whom had received halothane (S 3, H 8), required an increase in FIO2 and/or assisted ventilation to correct hypoventilation during maintenance anesthesia. These children included three, all receiving halothane, who had irritable airways with laryngospasm and coughing, four children who had SpO2 values < 95% (S 2, H 2), and four who had PETCO2 > 60 mm Hg (S 1, H 3). One child receiving halothane had a persistent irritable airway and could not be settled, even after deepening the level of anesthesia, and was switched to IV propofol for maintenance. This child was excluded from the analysis of the OCR and ORR, but included in the results on respiratory complications.
In spontaneously breathing children undergoing strabismus correction, the OCR occurred significantly less often with 1.3 MAC sevoflurane in N2O than with halothane. Baseline HR was higher in children receiving sevoflurane. These findings have also been observed in similarly aged children during manual ventilation with 1 and 2 MAC sevoflurane and halothane in 100% oxygen (6). In that study, the authors postulated that the higher HR with sevoflurane was possibly caused by the differing effects of sevoflurane and halothane on the baroreflex. The vagal nerve is involved not only in the baroreflex but also in the OCR. Because we found that the OCR occurred less often in the sevoflurane group, a comparatively greater depression of the vagal activity by sevoflurane could theoretically lead to a less pronounced bradycardia on stimulation of the OCR.
We also observed considerably fewer dysrhythmias in children receiving sevoflurane than in those receiving halothane. This has also been observed in other studies in children (7) and may be attributed to the lack of effect of sevoflurane on myocardial conduction as compared with halothane (11).
We found that at 1.3 MAC sevoflurane or halothane the RR was higher with sevoflurane. We also observed higher minute volumes with sevoflurane, but the PETCO2 was similar to that of halothane. One possible explanation for this apparent discrepancy is a lower cardiac output with halothane when compared with an equivalent MAC of sevoflurane (12). Data from the literature on sevoflurane are inconclusive regarding whether the RR increases or decreases with increasing MAC. Doi et al. (13) and Komatsu et al. (14) showed a dose-dependent tachypnea. In contrast, Yamakage et al. (15) found a progressive decrease in RR, although in that study, halothane interestingly produced no change in RR. Johannesson et al. (16) found that the RR and PETCO2 were similar with both anesthetics during surgery. These findings may be explained by their definition of equipotency, which was based on clinical signs, and the fact that the authors used larger halothane concentrations than in our study. Brown et al. (5), in a group of younger children aged 6–24 months, also found no difference in VT or PETCO2 between sevoflurane and halothane in 60% N2O, but observed a higher RR with halothane. As higher halothane MAC values were used (S 1.4 MAC, H 1.8 MAC), and halothane is known to cause a dose-related tachypnea (17), this might explain the difference in RR observed by the authors.
Three children in our study, all anesthetized with halothane, had irritable airways and required assisted ventilation. Attempts to settle the children by deepening the level of anesthesia only resulted in apnea, suggesting that these respiratory problems were not caused by inadequate levels of anesthesia.
In our study, during positive OCRs, a decrease in VT and PETCO2 occurred, and this was similar in both groups. The observed decrease in minute ventilation during the OCR was not accompanied by an increase in PETCO2 but by a transient decrease. This may have been caused by the decrease in cardiac output during the OCR bradycardia.
A proportion of children had evidence of hypoventilation with spontaneous respiration through a LMA. Three had problems after LMA insertion, presumably because of high vapor concentrations, and this may be considered acceptable as a temporary consequence of this technique. However, 22% of the total number of children, of whom most had received halothane, required an increase in FIO2 and/or assisted ventilation to maintain the criteria of adequate ventilation during maintenance.
The crucial issue in this study is whether the compared alveolar vapor concentrations were equipotent. Various authors have commented on this problem (5).
The principle of overpressure induction consists of the creation of the steepest possible gradient between inspired and alveolar vapor concentrations to hasten the achievement of a specified endpoint. The endpoint chosen determines the MAC reached and will be similar to the accepted MAC for that endpoint. The inspired vapor must then be immediately reduced to prevent overdosing. If this is not done, this technique may create potentially dangerous large tissue vapor concentrations, similar to the circumstances seen with intubation under deep volatile anesthesia.
The endpoint in this study was readiness for LMA insertion which was between two and three minutes after beginning the induction for both anesthetics. Therefore, both were equipotent at the time that the vaporizer was switched off and the mask removed for LMA insertion.
We chose to use the overpressure technique with sevoflurane and halothane as it had been shown to be associated with minor complications (18). Interestingly, in that study, the authors were of the opinion that the lack of respiratory problems was the result of the use of preoperative oral atropine. We found that there were more respiratory complications with halothane than with sevoflurane, and this has also been observed in two other studies in children—one with incremental vapor induction (19) and one on ease of LMA insertion (20).
As halothane has a slower uptake and washout than sevoflurane, the time to the achievement of maintenance alveolar concentrations may have differed and have led to discrepancies. An average of 10 minutes passed from the time that the vaporizer concentration was reduced to the time surgery began. During this time, the remaining monitors were applied and the operative field was prepared. End tidal and inspired vapor concentrations had then reached equilibrium. The OCR and ORR were determined only after surgery started.
In determining equipotent maintenance concentrations, the term “MAC multiple” has been used, although it is not known what distortions arise from this concept. Moreover, the use of N2O creates further difficulties in calculating equipotency. When N2O and halothane are used together in children, it has been stated that MAC multiples of halothane and N2O can be simply added when the total MAC is 1 (9). From the only two existing studies in children on this subject (8,21), the evidence suggests that MAC multiples of sevoflurane and N2O are less than purely additive, and the hypothesis is that they act at a common site of action or mechanism. Furthermore, the amount of the MAC-reducing effect of N2O on sevoflurane is different for MAC skin incision, as it is for MAC tracheal intubation. The MAC multiples we used were 1.21 for the sevoflurane group and 1.24 for the halothane group. Considering the many points where inaccuracies can enter in the concept of MAC multiples, we feel that the comparison of the two anesthetics in our study was acceptable.
In conclusion, children undergoing outpatient strabismus surgery with spontaneous breathing must be intensively monitored for bradycardia and hypoventilation. We found that sevoflurane was associated with a lower incidence of the OCR, airway irritability, and fewer ventilatory interventions compared with halothane. Sevoflurane may be the better choice of inhaled anesthetic for this procedure.
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© 2000 International Anesthesia Research Society
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