EFFECT OF MILD HYPOCAPNIA ON FETAL BREATHING AND BEHAVIOR IN UNANESTHETIZED NORMOXIC FETAL LAMBS

We hypothesized that the level of arterial PCO2 (Pa-CO2) affects the incidence of fetal breathing movements and electrocorticographic (ECoG) states in chronically instrumented fetal sheep. Six fetuses of 128-132 days gestational age were instrumented for recording fetal behavior and for later connection to an extracorporeal membrane oxygenation (ECMO) system to change fetal blood gases. Before ECMO fetal arterial pH and blood gases were pH 7.40 +/- 0.01, Pa-CO2 42.9 +/- 1.5 Torr, and arterial Pot (Pa-O2) 19.2 +/- 1.7 Torr; during ECMO in normocapnia they were pH 7.37 +/- 0.01, Pa-CO2 46.1 +/- 0.7 Torr, and Pa-O2, 27.6 +/- 3.0 Torr; and during ECMO in mild hypocapnia they were pH 7.47 +/- 0.01, Pa-CO2 35.3 +/- 1.7 Torr, and Pa-O2 26.6 +/- 1.7 Torr. The overall incidence of breathing movements, the incidence of breathing movements during low-voltage (LV) ECoG activity, and the mean duration of periods of breathing decreased significantly during hypocapnia. Fetal ECoG activity showed normal cycling during the periods of mild hypocapnia, and the mean duration of LV ECoG periods did not-change. During mild hypocapnia, eye movements remained associated with LV ECoG activity and nuchal electromyographic activity remained associated with high-voltage ECoG activity. These results suggest that the presence of breathing movements in fetal life is not only dependent on the behavioral state but also on the level of fetal Pa-CO2.