Vagally mediated volume-dependent modulation of inspiratory duration in the neonatal lamb

Inspiratory mechanical loads elicit a reflex volume-timing response in human infants and experimental animals. In adult animals, this reflex has been shown to be mediated by vagal afferents. It was hypothesized that the volume-related regulation of inspiratory duration would also be vagally mediated in the newborn. Single-breath graded inspiratory loads were presented to 4-day-old anesthetized neonatal lambs breathing through a facemask, after tracheostomy, and after bilateral cervical vagotomy. Inspired volume decreased and inspiratory duration (TI) increased with increasing load magnitude. The increase in TI was greater with facemask loads than with loads presented after tracheostomy for equivalent changes in inspired volume. The volume-related prolongation of TI after tracheostomy was abolished after cervical bilateral vagotomy. The results demonstrate that the volume-timing reflex in neonatal lambs after tracheostomy is mediated by vagal afferents. However, in the intact lamb, there is also a significant contribution to this reflex by upper airway afferents.