Figure 3. Effect of halothane dose on peak phrenic nerve activity (PPA) in 11 animals (protocol 1). (A) Peak phrenic nerve activity during the hypercapnic hyperoxic control state (solid bars) and during the carbon dioxide infusion (cross-hatched bars). The PPA values were normalized to the hyperoxic control PPA at 1 MAC. (section), Significant differences of the hyperoxic control period PPA values compared with the 1 MAC control level. (section, section, section)P < 0.001. Asterisks indicate significant increases due to the test carbon dioxide infusions relative to the respective hyperoxic control levels (**P < 0.01, ***P < 0.001). 1.0 EC, 1 MAC end control. (B) Dose-dependent depression of the carbon dioxide bolus-induced component of PPA or net PPA response (difference of PPA(bolus) and PPA (control)) by halothane. All net responses were greater than zero. (section)Difference in net response compared to the 1 MAC dose (section, section)P < 0.01, (section, section, section)P < 0.001). (C) Plots of PPA as a function of the carbon dioxide stimulus for the four halothane doses illustrate that increasing halothane dose reduces the slope (peripheral carbon dioxide sensitivity) and causes a downward baseline shift.