Properties of phrenic nerve activity during experimental periodic breathing in cats.

Abstract

Periodic breathing was evoked in Nembutal-anesthetized cats by graded hemorrhage and common carotid artery occlusion. The cats were vagotomized, paralyzed, and artificially ventilated, and the integrated phrenic neurogram (IPN) was recorded. In all cases, the periodic breathing was associated with sustained oscillation of blood pressure (Mayer waves). Twelve episodes of periodic breathing were analyzed. End-inspiratory IPN voltage (Vm) showed characteristic quasi-sinusoidal oscillations with a 30-s average period, accompanied in all 12 cases by a synchronous oscillation of IPN slope. Both oscillations persisted after abolishing the blood pressure waves with an α-adrenergic blocking agent, showing that they were not dependent on phasic baroreceptor input. In four cases there were oscillations of inspiratory and total respiratory cycle duration (and in three of these, expiratory duration also) in synchrony with the Vm and slope waves. In two further cases, expiratory duration oscillated 180° out of phase to Vm and slope, while inspiratory duration records showed only irregular variations. In the remaining six cases, inspiratory, expiratory, and total respiratory cycle duration remained relatively constant during the Vm and slope oscillation. The observed modulation of total respiratory cycle duration indicates that the mechanism generating periodic breathing was acting on the brainstem respiratory oscillator, where respiratory cycle duration is set. The slope modulation is also likely to result from an action at the brainstem oscillator level, although a spinal site of action cannot be entirely excluded.