Postnatal maturation of the respiratory response to O2 in awake newborn lambs.

The present study was designed to analyze maturation of the respiratory response to hypoxia and hyperoxia in 41 newborn lambs aged 2-90 days. The O2 respiratory response was tested by progressive isocarbic hypoxia while minute ventilation (VE), tidal volume (VT), respiratory frequency (f), mean inspiratory flow (VT/TI), were measured. The occlusion pressure (P0.1) was also determined as an index of the neuromuscular output of the drive to breathe. The strength of the peripheral chemoreceptors was evaluated by the transient O2 100% inhalation test and by the early change in VE (delta VE) to cyanide (KCN) infusion. In all of the 2-, 10-, 30-, and 90-day groups of lambs, hypoxia induced a significant increase in P0.1 and in delta VE, delta VT, f, and VT/TI. The 2-day-old lambs showed a significantly lower response to hypoxia than did the older groups. Similarly, during the hyperoxia test and KCN infusion. Delta VE was smaller in the 2-day lambs than in the other lambs. We conclude that the postnatal maturation of the ventilatory response to O2 occurs in the first 10 days of life in newborn lambs and it is largely due to an increase in sensitivity of the O2 chemoreceptors.