Figure 4.

Respiratory pacing above resting frequency causes compensatory decreases in tidal volume in conscious rats. A, Conscious quiet rat breathing pure oxygen. Prolonged respiratory pacing (3 × 5 ms pulses at 20 Hz; train frequency, 87 and 95 trains · min⁻¹) caused a gradual reduction of the tidal volume, V_T, to the resting (prestimulation) level, and a parallel decline in total ventilation, MV. After the end of the stimulus, tidal volume and frequency were transiently reduced, reflecting the hypocapnia caused by an incomplete return of MV to prestimulation levels. Gray bars in flow trace indicate behavioral disturbances. B, Expanded traces from A showing the compensatory changes in V_T during long periods of entrainment (147 s at 95 trains · min⁻¹). During the early part of the stimulation (middle, “Early”), V_T was increased compared to the resting state (left). Two and a half minutes later (right, “Late”), V_T had decreased to prestimulation levels. C, Expanded trace from A (red box) illustrating the after stimulus hypoventilation that followed 147 s of pacing at the rate of 95 trains min⁻¹. fR fell to 28 breaths · min⁻¹ before returning to resting levels. D, Group data (N = 6) summarizing frequency of respiration (fR), tidal volume (V_T) and total ventilation (MV) at rest vs during the early and late part of the entrainment period (***p < 0.001, **p < 0.01, *p < 0.05; one-way ANOVA).