Fig. 1. Concentration–response curves of halothane (n = 5) and enflurane (n = 5) on spontaneous sinus cycle duration (RR, ms) and on perfusion pressure (PP, mmHg). Halothane (open circles) induced a concentration-dependent increase in RR interval, whereas enflurane (filled circles) did not. The perfusion pressure decreased with both agents, without concentration-dependence. # P < 0.001 versus baseline;‡ P < 0.01 versus baseline;† P < 0.05 versus baseline;$ P < 0.05 versus 1%;* P < 0.05 versus 3%.