Plasma acid-base regulation above and below ventilatory threshold in late gestation

Stewart's physicochemical approach was used to study the effects of pregnan cy on acid-base regulation in arterialized blood. Responses of 15 healthy p regnant women (PG; gestational age, 37.1 +/- 0.2 wk) were compared with tho se of 15 nonpregnant controls (CG) at rest and during cycling at 70 and 110 % of the ventilatory threshold (T-vent). Hydrogen ion concentration ([H+]) was lower in the PG vs. CG at rest and during exercise (P < 0.05 at rest an d 70% T-vent). Exercise-induced changes in [H+] were similar between groups . Lower resting [H+] values in the PG vs. CG resulted from lower values for arterialized PCO2 (Pa-CO2) and total weak acid ([A](tot)), which were part ly offset by a lower strong-ion difference ([SID]). Reductions in [A](tot) and [SID] at rest were primarily the result of reductions in albumin [Alb] and sodium [Na+], respectively In the transition from rest to 70% T-vent, s mall increases in PaCO2 and [A](tot) contributed to moderate increases in [ H+] in both groups, however [SID] increased in the PG and decreased in the CG (P < 0.05 between groups). In the transition from rest to 110% T-vent, d ecreases in [SID] made a significantly greater contribution to changes in [ H+] in the CG vs. PG. Exercise-induced increases in [H+] are similar in the pregnant vs. nonpregnant state, but there is a reduced contribution of [SI D] both above and below T-vent during pregnancy.