Repeated acute hypoxia temporarily attenuates the ventilatory respiratory response to hypoxia in conscious newborn rats

We asked whether repeated hypoxic exposures during the early neonatal perio ds could affect the ventilatory control, such as the lung volume-dependent ventilatory inhibition (HBR), pulmonary ventilation ((V) over dot E), and C O2 production ((V) over dot CO2). Within each litter of rats, one group of pups (experimental group H) was exposed to 6% O-2 (30-min duration twice a day from postnatal d 1 to 4). The other group (control group C) was exposed to air. At 5 d after birth, the HER was triggered by lung inflation via ne gative body surface pressure (10 cm H2O). Measurements of (V) over dot E an d (V) over dot CO2 were done by plethysmography and the inflow-outflow CO2 difference, respectively. At 2 wk of age, (V) over dot E and (V) over dot C O2 measurements were repeated by the barometric technique and the inflow-ou tflow CO2 difference, respectively. Each conscious pup was breathing normox ia (21% O-2) and then hypoxia (10% O-2). Results were as follows: 1) during normoxia, HER was stronger and both (V) over dot E and (V) over dot CO2 we re higher in H pups than in C pups; 2) during hypoxia, the HER of C was as in normoxia, whereas that of ii was increased above the normoxic value; 3) during hypoxia, C maintained (V) over dot E, whereas H decreased it; 4) in hypoxia, (V) over dot CO2 was reduced significantly in both groups; 5) at 2 wk of age, (V) over dot E and (V) over dot CO2 did not differ between ii a nd C during normoxia or in response to 10% hypoxia. We conclude that in rat pups, repeated hypoxic episodes can modify the NBR and, at least temporari ly, reduce the (V) over dot E response to hypoxia with a decrease in (V) ov er dot CO2. The findings are in agreement with the view that repeated hypox ic exposures in the neonatal. period could interfere with the development o f respiratory control and could possibly be involved in the mechanisms of n eonatal apnea or sudden infant death syndrome.