Figure 1. The percentage change (mean +/- SE) in respiratory system resistance (Rrs) is shown after 5 and 10 min of maintenance anesthesia with 0.25 mg [center dot] kg sup -1 [center dot] min sup -1 thiopental plus 50% nitrous oxide (n = 7); 1.1 MAC isoflurane (n = 20); 1.1 MAC halothane (n =20); or 1.1 MAC sevoflurane (ne = 19). All three volatile anesthetics decreased Rrssignificantly below the Rrspresent after intubation and below the response to thiopental/nitrous oxide (P = 0.05). Sevoflurane decreased Rrsmore than either halothane and isoflurane at 5 and 10 min (P < 0.05), although the difference between halothane and sevoflurane was no longer significant when a single outlier in the halothane group was removed. The differences between halothane and isoflurane were not significant. The 10-min response was significantly lower (p < 0.05 by paired t test) than the 5-min response for halothane and isoflurane, but not for sevoflurane.