Relationships between breathing activity and heart rate in fetal baboons.

Coordination between breathing and heart rate (HR) is known to occur in many species and is commonly called respiratory sinus arrhythmia (RSA). In this report, we provide evidence that a homologous phenomenon can be demonstrated in the fetuses of nonhuman primates. In chronically instrumented fetal baboons we found cyclical variations in HR that were coincident with fetal breathing movements. We found clear instances of fetal RSA in each of three baboon fetuses. The average change in HR with each fetal breath was approximately 2 beats/min. Fetal breaths were associated with an acceleration/deceleration cycle in HR, and the onset of accelerations was coincident with the onset of inspiratory activity. These studies indicate that in baboons integration of respiratory and cardiovascular control mechanisms develop in utero.