Depressed responsiveness of the carotid sinus reflex in conscious newborn animals

The responsiveness of the carotid sinus reflex was evaluated by comparing the effects of bilateral carotid occlusion (BCO) in conscious adult dogs and puppies on measurements of arterial pressure, cardiac output, heart rate, and calculations of total peripheral resistance (TPR). In eight adult dogs, BCO increased mean arterial pressure by 57 +/- 6%, TPR by 48 +/- 5%, and heart rate by 45 +/- 15%. In puppies, BCO induced smaller increases (P less than 0.05) in mean arterial pressure (30 +/- 5%) and TPR (29 +/- 4%), while heart rate did not change. After elimination of opposing vagal and aortic baroreceptor reflexes, the differences in responses to BCO of mean arterial pressure and TPR between adults and newborns were even greater. Thus, the carotid baroreceptor reflex appears to be depressed in the newborn when compared with the fully developed reflex in the normal, conscious adult.