LONG-LASTING EFFECT OF PROLONGED HYPOXEMIA AFTER BIRTH ON THE IMMEDIATE VENTILATORY RESPONSE TO CHANGES IN ARTERIAL PARTIAL-PRESSURE OF OXYGEN IN YOUNG LAMBS

The effect of prolonged hypoxemia (H) after birth on the evolution of the ventilatory response to changes in arterial partial pressure of 02 was determined in unanesthetized, awake lambs. H was induced for 12 d after birth in seven lambs through exposure to 0.10 fraction of inspi red O2 (FiO2). Five control (C) lambs were kept in 0.21 FiO2. The vent ilatory response (percent increase from baseline) to acute hypoxia was tested with 0.14 FiO2 and 0.10 FiO2. The tonic activity of the periph eral chemoreceptors was assessed by the transient pure oxygen inhalati on test (Dejours' test). The occlusion technique was used to measure t he baseline neuromuscular drive of breathing. A markedly decreased ear ly ventilatory response to acute hypoxia persisted in the H lambs for at least 5 wk after termination of H compared with the C group. The se cond phase of the response was significantly lower only at 12 d (the e nd of H) and was thereafter comparable to that in the C lambs. The ven tilatory response to hyperoxia was significantly lower in the H lambs only at the end of hypoxemia at 12 d and rapidly normalized after retu rn to normoxia. H did not significantly affect resting neuromuscular d rive. These results show that postnatal maturation of the ventilatory response to changes in arterial partial pressure of O2 can be delayed by prolonged postnatal hypoxemia. The effect on the response to hypero xia is transient, whereas the response to acute hypoxia is affected fo r an extended time. This study illustrates the importance of an adequa te postnatal arterial partial pressure of O2 for the development of th e ventilatory response to acute hypoxia.