Role of hypotension induced by PGI 2 on the control of depth and frequency of breathing

Abstract

We studied the respiratory response to infusion of prostacyclin (PGI 2)in 7 anaesthetized pigs in an attempt to understand whether the changes in pattern of Breathing were due to a direct action of the substance or to the concomitant hypotensive effect. The depth and frequency of breathing were analyzed in terms of the threshold-inhibition curve for termination of inspiration ( V T T I relationship) and of bulbo-pontine rhythm, estimated from inspiratory and expiratory time during occlusion of the airways at the end expiratory level ( T I T I relationship). This provide the central respiratory rhythm when the phasic lung volume receives no input from pulmonary stretch receptors. The hypotension induced by PGI 2 increased pulmonary ventilation mainly through a change in V T and caused a slight rightward displacement of V T T I relationship without modifying the slope of the curve. This effect seems to be vagally mediated. PGI 2 also changed the bulbo-pontine rhythm. Our results show that PGI 2 modifies the vagal discharge and bulbo-pontine rhythm by two opposite mechanisms: hypotension and an apparent direct action on carotid and aortic baroreceptors.