VENTILATORY RESPONSES TO PROGRESSIVE HYPOXIA AND HYPERCAPNIA IN DEVELOPING SHEEP

We aimed to determine the time course of postnatal development of vent ilatory responsiveness to progressive hypoxia and hypercapnia. Eight l ambs underwent hypoxic and hypercapnic rebreathing tests at weekly int ervals from soon after birth to 6 weeks of age. Six mature ewes were a lso studied. During the tests blood samples were collected at interval s from the aorta so that arterial P-O2 (Pa-O2) and P-CO2 (Pa-CO2) coul d be related to ventilation. Hypoxic sensitivity was defined as the pe rcentage increase in minute ventilation when Pa-O2 fell from control v alues (104.7 +/- 6.9 mmHg) to 50 mmHg. When measured in this way, hypo xic sensitivity increased significantly from 64.2 +/- 19.3% (mean +/- SE) in the newborn (2.3 +/- 0.4 days) to 150.4 +/- 14.0% at 3-4 weeks (25.1 +/- 0.6 days, P = 0.05). The hypoxic sensitivity of ewes (66.3 /- 16.8%) was greatly reduced compared to 6-week-old lambs (140.3 +/- 18.9%, P< 0.05). Hypercapnic sensitivity (ml . min(-1). kg(-1.) mmHg C O2-1) did not change significantly with age. We conclude that hypoxic sensitivity increases during the first 3-4 postnatal weeks and decline s between infancy and adulthood. In contrast hypercapnic sensitivity d oes not change with age, although tidal volume and breathing frequency contributions to ventilatory responses change with advancing postnata l age.