PREMATURITY ALTERS HYPOXIC AND HYPERCAPNIC VENTILATORY RESPONSES IN DEVELOPING LAMBS

We have determined the effects of preterm birth on the postnatal devel opment of ventilatory responses to progressive hypoxia and hypercapnia in awake lambs. Hypoxic and hypercapnic rebreathing tests were perfor med at weekly intervals in 5 preterm (born at 135 +/- 0.5 d) and 5 ter m (born at 146 +/- 0.2 d) lambs up to 6-7 weeks after birth. Term lamb s were also studied at 25 weeks after birth. During rebreathing tests, we measured arterial P-O2 and P-CO2 and related them to minute ventil ation (Vover dotI). Owing to variability in resting Pa-O2, hypoxic sen sitivity was defined as the percentage increase in Vover dotI when Pa- O2 fell to 60% of resting values. Hypoxic sensitivities of preterm lam bs did not change with age (68.9 +/- 24.4%), whereas values for term l ambs more than doubled over the first 6 weeks (day 2, 73.9 +/- 15.8%; week 6, 227.4 +/- 24.9%) but returned to early postnatal values by wee k 25 (87.0 +/- 21.2%). Hypercapnic sensitivities (ml min(-1) kg(-1) mm Hg CO2-1) of preterm lambs were lower than those of term lambs between day 2 and week 2, but reached values in term lambs thereafter. We con clude that preterm birth abolishes the normal postnatal maturation of hypoxic ventilatory sensitivity, and temporarily depresses hypercapnic sensitivity.