Figure 4. Index of static rate of regulation (sROR) during BASELINE (fentanyl + nitrous oxide), LOW DOSE (0.5 MAC for volatile anesthetic, 100 micro gram *symbol* kg sup -1 *symbol* min sup -1 for propofol), and HIGH DOSE (1.5 MAC for volatile anesthetic, 200 micro gram *symbol* kg sup -1 *symbol* min sup -1 for propofol) anesthetics. Values are mean plus/minus SE. *P < 0.05 versus baseline. **P < 0.001 versus baseline. Values of sROR ranged between 85% and 95% during baseline fentanyl/nitrous oxide anesthesia, and there were no intergroup differences. Low-dose desflurane resulted in a slight decrease in sROR, whereas in high doses of both isoflurane and desflurane significantly decreased sROR.