Figure 2. Summary data from protocol 1. (A) Effect of halothane concentration on peak phrenic activity (PPA) for the hyperoxic hypercapnic control states (solid bars) and for the corresponding hypoxic runs (hatched bars). All PPA values were normalized to the hyperoxic control PPA at 1 MAC. Asterisks, Significant increases due to hypoxia compared with the respective hyperoxic control. ***P < 0.001. (section), Significant halothane dose-dependent differences for the hyperoxic control conditions compared with the 1 MAC dose. (section, section, section)P <0.001. (B) Dose-dependent depression of the hypoxia-induced component of PPA or the net hypoxic response (difference of PPA[hypoxia] and PPA[hyperoxia]) by halothane. All net responses were greater than zero (P < 0.005). Asterisks, Differences in the net response compared with the 1 MAC dose. *P < 0.05. ***P < 0.001. (C) Plots of PPA as a function of Pa sub O²for the four halothane doses indicated show that increasing the halothane dose reduces the slope (oxygen sensitivity) and produces a downward baseline shift.