Heart rate development and sensory-evoked cardiac responses in perinatal rats

Sensory-evoked changes in heart rate (HR) are often used to assess sensory function in infants, but HR measures have not been applied in studies of perinatal sensory function. Normative HR data are needed to judge whether baseline characteristics and response properties of the late fetal and newborn heart are appropriate as a dependent measure in sensory studies. We compared parameters of HR development and phasic cardiac responses to stimulation across the perinatal period in Norway rats. EKG was measured in cesarean-delivered prenatal rats (embryonic day 20-day of birth; conception = E0) and compared to both cesarean and vaginally delivered neonates (day of birth-postnatal day 2; birth = P0). Resting heart rate increased monotonically across the perinatal period, except between E21 and DOB (Experiment 1). During an hour-long test, HR was strong and stable. Variations in HR across successive measurement intervals (10 sec) were uniformly low (less than 2%) in all age groups. In Experiment 2, fetuses and neonates responded with dramatic HR decelerations (20-80 bpm) to an intraoral infusion of lemon extract. HR responses did not appear to be reflexively mediated by changes in respiration. These findings suggest that parameters of resting HR and cardiac responses to stimulation are well suited to studies of perinatal sensory function.