VASCULAR LACTIC-ACID INFUSIONS DO NOT ALTER THE INCIDENCE OF FETAL BREATHING MOVEMENTS OR THEIR INHIBITION BY ACUTE HYPOXEMIA

Hypoxemia transiently inhibits the incidence of fetal breathing movements (FBM), but their incidence returns to normal after several hours despite maintained hypoxemia. We hypothesized that the lactic acidosis associated with prolonged systemic hypoxemia might mediate the adaptation of the hypoxemic inhibition of FBM. In sheep fetuses, the incidence of FBM was measured in a control hour and during 6 h of i.v. infusion of L-lactic acid, which raised the blood lactate concentrations to levels seen with moderate hypoxemia. FBM were observed at the same incidence as during control during each of the first 4 h (all approximately 40%). In the 5th h of lactic acid infusion, fetal hypoxemia was induced by lowering maternal inspired oxygen fraction and FBM occurred only 8 +/- 1% (SEM) of that hour. In a subsequent normoxemic recovery hour, the incidence of FBM remained below control levels. In the same animals on a different day, a similar hypoxemia induced without the acid infusion caused a comparable inhibition of FBM, but the incidence of FBM returned to the control level in a subsequent recovery hour. A moderate peripheral lactic acidosis does not blunt the inhibition of FBM evoked by acute hypoxemia and is not a likely explanation for the return of FBM during prolonged hypoxemia but actually might mediate some of the inhibition.