Developmental changes in cardio-respiratory responses to hypoxia and hypercapnia in anesthetized low-birth-weight rats

The present study compared the developmental changes in the cardio-respirat ory responses to hypoxia and hypercapnia between full-term low-birth-weight (LBW) and control rats during the postnatal period. The heart rate (HR), r espiratory frequency (fR) and amplitude (aR were measured during hypoxia (1 00% O-2 for 10 min) and hypercapnia (5% CO2 for 10 min) in rats aged 7, 14 and 21 days, anesthetized with urethane. During hypoxia, HR was not signifi cantly modified in the younger rats of both groups. In the older rats, aged 14 and 21 days, HR was markedly diminished, with a more pronounced decreas e in LBW rats. The HR recovery was never observed in the older LBW rats. Th e fR and aR showed an age-related increase in both groups: a biphasic fR pa ttern observed on day 7 was replaced by a sustained increase on days 14 and 21. In contrast to controls, LBW rats never displayed a fR recovery during reoxygenation. In controls, aR shifted from a biphasic pattern in the youn ger rats to a sustained increase in the older ones. The LBW rats only displ ayed a decrease of aR in the younger, while in the older ones, a transient and slight increase preceded this decrease. During hypercapnia, the only si gnificant difference detected between these two groups was that aR increase d in LBW rats to a greater extent than in controls on days 14 and 21. Altog ether, our results revealed a markedly attenuated cardio-respiratory respon se to hypoxia in LBW rats, but no such effect in response to hypercapnia. ( C) 2000 Elsevier Science B.V. All rights reserved.