Evaluating acid–base status and perfusion: arterial and venous blood gases during pregnancy

Abstract

Objective: Developing a conceptual framework that enables the interpretation of arterial and venous blood gases and perfusion parameters in pregnant women, integrating the physiological adaptations of pregnancy to support clinical decision-making.

Materials and methods: Based on a hypothetical case that practically illustrates the topic, an explanatory document was developed and used to analyze the main physiological changes of pregnancy that affect the interpretation of arterial and venous blood gases, such as progesterone-induced hyperventilation, which causes compensated respiratory alkalosis. Furthermore, the study addressed maternal hemodynamic and perfusion adaptations, fetal oxygenation, and their clinical implications.

Results: During pregnancy, a series of physiological changes occur that impact maternal and fetal homeostasis, leading to subsequent modifications in arterial and venous blood gases and perfusion parameters. Respiratory changes, such as increased oxygen consumption, minute ventilation and tidal volume, are responsible for the decrease in PaCO₂ levels, resulting in a compensated respiratory alkalosis and, secondarily, an increase in arterial pH. At the hemodynamic level, a progressive increase in cardiac output and a decrease in peripheral vascular resistance are observed, resulting in a highvolume, low-resistance circulation, characteristic of a hyperdynamic state. These aspects are key when providing care to critically ill obstetric patients.

Conclusions: There are limitations in current knowledge regarding the interpretation of arterial blood gases as indicators of respiratory function and tissue perfusion in pregnant women, as well as regarding the interaction between maternal and fetal oxygenation and placental perfusion, particularly under conditions of critical stress or at high altitudes. At present, interpretation parameters derived from nonpregnant women should guide the management of pregnant women with serious complications. Further studies are required to better characterize the changes observed in critically ill pregnant women, in light of the physiological changes that accompany pregnancy, in order to allow a more accurate interpretation of arterial and venous blood gases as indicators of ventilation and tissue perfusion in the mother, the fetus, and the placenta.