Adaptation to reflex effects of prolonged lung inflation

Adaptation to the reflex effects of sustained changes in lung volume on inspiratory duration (TI), expiratory duration (TE), and the phrenic neurogram was examined. Test inflations in gallamine-paralyzed dogs anesthetized with pentobarbital sodium were made during a 6-min trial while the animal was not ventilated: 2 min at functional residual capacity (FRC), 2 min at elevated airway pressure, and 2 min back at FRC. The dogs were hyperoxygenated and arterial PCO2 was kept constant by an infusion of tris (hydroxymethyl) aminomethane. The maintained inflations produced minimal changes in TI. On return to FRC, TI was prolonged in proportion to the magnitude of the prior inflation. In contrast, inflation produced marked prolongation of TE, which then adapted back toward preinflation values. On return to FRC, TE shortened initially to values below control. This shortening increased with greater prior lung inflations. The times to reestablish steady-state values upon return to FRC differed for TI (14.8 +/- 4.6 s) and TE (33.8 +/- 12.7 s). The magnitude of the phrenic neurogram at a fixed time from onset of inspiration and its slope were unchanged with inflation. These results indicate that respiratory phase durations are influenced not only by pulmonary afferent input within each respiratory cycle but also by prior vagal afferent activity that engages central processes with long, although different, time constants. Afferent input to the slow central process controlling TI is not gated to only one phase of the respiratory cycle.