EEG entropy values during isoflurane, sevoflurane and halothane anesthesia with and without nitrous oxide

We hypothesized that like bispectral index, entropy may be anesthetic agent specific. We carried out a study to assess the entropy values of different anesthetics at equi-minimal alveolar concentrations (MACs) with air and nitrous oxide as carrier gases. Thirty adult patients undergoing spine surgery were randomized to receive halothane, isoflurane, or sevoflurane, in 2 stages, (a) with air/oxygen mixture (2:1) and (b) in nitrous oxide/oxygen (2:1). Heart rate, mean arterial blood pressure, response entropy (RE), and state entropy (SE) were noted at 1.0 and 1.5 MACs for each agent. Statistical analysis was done using the 2-way analysis of variance followed by Bonferroni correction and Student t test for paired data. P value of less than 0.05 were considered significant. The demographics and baseline values of heart rate, mean arterial blood pressure, RE, and SE were comparable. Changing from air/oxygen as carrier gas to 66% nitrous oxide in oxygen resulted in significant increase in both RE and SE at 1.0 MAC for all the agents (P<0.05). Among the agents, it was found that both RE and SE values were significantly higher during halothane anesthesia as compared with sevoflurane and isoflurane (P<0.05). At 1.5 MAC for all agents, after addition of nitrous oxide, there was an insignificant reduction in both RE and SE (P>0.05). Again the values of RE and SE remained high for halothane as compared with isoflurane and sevoflurane. In conclusion, our data suggest a possibility of misinterpretation of anesthetic hypnosis when entropy values increase with addition of nitrous oxide to 1 MAC isoflurane and sevoflurane.