Control of breathing during acute hemorrhage in anesthetized cats

We studied in anesthetized cats the ventilatory response to acute blood loss under hyperoxic iso-capnic conditions. From a blood pressure, of approximately 150 down to approximately 70 mm Hg ventilation increased on the average by 2.5 times. The ventilatory response was characterized by two phases: there was an initial phase (down to a PB approximately 100 mm Hg) of increase in frequency due to an excitatory effect on bulbo-pontine respiratory timing which was estimated from the duration of breaths following occlusion of the airways at the end expiratory volume. This effect was mainly due to the withdrawal of inhibitory afferents from the baroreceptors of the aortic arch. To this phase corresponded the phase of vasomotor compensation. Subsequently there was a phase of excitatory effect on the respiratory output which was estimated from the rate of change of the pressure developed in the airways during occluded breaths. This effect was mainly due to afferents from carotid sinuses; a minor role was due to the decrease in inhibitory afferents from carotid baroreceptors while the greater part was likely to be due to the hypoxic stimulation of glomus cells due to reduced blood flow. Following vagotomy above the superior laryngeal and sinus denervation the excitatory effect on respiratory timing and output were reduced to about 30% of that observed in intact cats.