Determination of Minimum alveolar concentration of desflurane for maintaining BIS below 50 in children and the effect of caudal analgesia on it

Bispectral index (BIS) is an EEG derived parameter. Anesthetic agents are titrated to a specific BIS value during general anaesthesia to allow adjustment of the amount of anaesthetic agent required for adequate depth of anesthesia thus resulting in a more rapid emergence. Further, BIS monitoring reduces the incidence of intraoperative awareness under general anaesthesia in adults.

Anesthetic agents have a comparable effect on EEG throughout the life [1]. Though conceived as an adult tool, use of BIS for titrating the effect of anaesthesia in children has been supported in several studies [2-6].  Further, as in adults, BIS values in pediatric patients inversely correlate with end tidal concentration of sevoflurane and isoflurane [7, 8]. The minimum alveolar concentration of sevoflurane for maintaining BIS below 50 (MACBIS50) has been determined to be 2.83+/-0.16% in children [4]. The minimum alveolar concentration (MAC) of sevoflurane required to prevent purposeful movement in response to noxious stimuli in 50% of patients has been reported to be 2.4% in children [9]. Thus, the ratio of MACBIS50/MAC is 1.18 in pediatric patients. However, there have been no reports investigating these values for desflurane.

Regional techniques have been shown to decrease the requirement of anaesthetics agents. There was 34% reduction in MACBIS50 of sevoflurane after administration of epidural lignocaine in adults [10]. No study has evaluated the effect of caudal anaesthesia on MACBIS50 of inhalational agent in children

We hypothesized that caudal block will decrease the MACBIS50 of desflurane. Primary outcome of the study was to determine MACBIS50 of desflurane and effect of caudal block on it. Success rate of LMA removal, incidence of coughing and airway complications were the secondary outcomes studied.

Results

Parentsof 42 patients consented to participate in the study. Study protocol could notbe followed in three patients. (Figure1) These three children were excludedbefore the inclusion of next patient. Table 1 shows the baseline patientcharacteristics of children in two groups. Fig. 2 shows the sequences ofindividual patients in both groups using up-and-down method. The MACBIS50values calculated with the Dixon’s up-and-down method was significantly higher (p=0.0007)in Group D [5.75%(0.288)} than Group C [4.5%(0.381)]. Mean endtidal CO2 wascomparable in both the groups.

A logistic regression curve of theprobability of attaining BIS50 is shown in Figure 3 and 4 and is illustrated bythe dose–response curve constructed for both groups using probit test data TheMACBIS50 values estimated by probit analysis were 5.57% (95%CI 5.22 to 5.95) inGroup D and 4.32%(95% CI 3.12 to 5.08) in Group C. Values were similar to thoseobtained with Dixon up down method. Corresponding ED95 values were 6.158 (5.85to 8.04) for Group D and 5.18 (4.7 to 6.6) for Group C.