Abstract
Extract: Response of the fetal lamb to metabolic acidosis in the absence of hypoxia or maternal acidosis was studied and compared with that of the newborn lamb. Experiments were conducted on nonanesthetized fetuses intact in utero, catheters having been implanted previously at hysterotomy. Metabolic acidosis was produced by infusion of 28 mmoles of lactic or hydrochloric acid (0.25 m) into the femoral vein of fetus or newborn.
Similar changes in acid-base state were produced with hydrochloric and lactic acids. In fetuses, pH fell 0.28 pH units; this decrease was greater than that observed in newborns (0.20 pH units). Newborns hyperventilated during and following infusion, lowering by an average of 8 mm Hg and partially compensating for the metabolic acidosis. In fetuses, arterial rose during infusion by an average of 6 mm Hg, but values for in umbilical vein and maternal artery did not change. Thus fetal A-V difference for across the placenta increased, whereas the gradient between fetus and mother remained constant. These observations suggest that fetal ability to compensate for metabolic acidosis is limited by the maternal .
Upon completion of the acid infusion, pH rose and base deficit fell at approximately the same rate in both fetus and newborn, but the rate of recovery was slow. After 1 hr, pH was still significantly lower than the control value (by 0.1 pH unit) and base deficit was lower by 9 mEq/liter. By 12 hr, all acid-base values had returned to normal.
Speculation: These experiments indicate that the transfer of hydrogen ion across the placenta is relatively slow, even when the circulatory status of both mother and fetus is normal, and there is no fetal hypoxia or uterine contraction. In this respect the placental membrane has properties similar to the blood brain “barrier.” With strong uterine contractions, maternal perfusion of the intervillous space is reduced or ceases, which leads to successive episodes of fetal hypoxia and acidosis. Slow placental transfer of hydrogen ion may explain the development of metabolic acidosis in the fetus during the course of labor.
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Daniel, S., Baratz, R., Bowe, E. et al. Elimination of Hydrogen Ion by the Lamb Fetus and Newborn. Pediatr Res 6, 584–592 (1972). https://doi.org/10.1203/00006450-197206000-00007
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DOI: https://doi.org/10.1203/00006450-197206000-00007
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