Effect of metabolic acidosis on fetal breathing movements in utero

Experiments were carried out in 7 unanesthetized fetal lambs in utero in which catheters had previously been placed in the trachea (for intrathoracic pressure measurements), axillary artery (for pH and blood gas determinations), brachial vein (for infusions) and amniotic fluid. Fetal breathing movements (FBM) were noted as negative deflections in intrathoracic pressure recorded simultaneously with intraamniotic pressure and expressed as the incidence per 60-min period, in percent. Intravenous infusion of 0.2 N HCl lowered pH from 7.368±0.012 to 7.128±0.022 (with no significant change in PaCO₂) for an 8-hour period. The incidence of FBM did not increase above control levels for the first 4 hours after the initiation of the acidosis. However, in the 5th to 8th hours FBM's occurred 52%, 53%, 59% and 64% of the time, respectively. These are all significantly above the control levels of 23%, 30%, 27% and 21% observed during these same hours on other days in the same animals. We conclude that fetal respiratory activity in utero is stimulated by hydrogen ion (H⁺) possibly mediated by central chemoreceptive structures.