Fig. 1. Variables associated with fictive breathing recorded from the in vitro brainstem–spinal cord preparation at pH levels ranging from 7.4 to 8.2. (A) Fictive breath frequency; (B) episodes per minute; (C) breaths per episode; (D) integrated vagal activity; (E) fictive breath duration. The data are reported as the mean ± 1 s.e.m. *Significant difference from the value at pH 8.2; significant difference between the control and chronically hypercapnic groups. The solid bold line through the data in A is a first order regression line (control, r²=0.83; chronic hypercapnia, r²=0.96). The dotted lines are 95% confidence intervals.