PHYSICOCHEMICAL ANALYSIS OF CHANGES IN ACID-BASE REGULATION IN EARLY HUMAN PREGNANCY

Information is needed on physiological changes at rest and during exercise throughout pregnancy to provide guidelines for safe exercise participation. Pregnancy is accompanied by a partly-compensated respiratory alkalosis, but the mechansims and time course of changes in the first trimester have not been established. Such information is important to verify maternal capacity to protect the fetus from metabolic acidosis during exercise in early pregnancy. PURPOSE Stewart's physicochemical approach (Can J. Physiol. Pharmacol. 61: 144, 1983) was used to assess serial changes in [H*] and its physiological determinants at rest and during exercise in the first trimester of pregnancy. METHODS Healthy, active women (n = 4) were studied pre-conceptually and at 6–7, 10–11 and 14–15 wks gestation. Respiratory measurements were recorded and arterialized blood samples were taken at rest and during upright cycling at 85% of each subject's ventilatory threshold (measured pre-conceptually). Blood samples were analyzed for plasma [H*], PCO2, total protein to reflect total weak anion [Atot] and electrolytes to calculate the strong ion difference ([SID]). RESULTS Ventilation increased significantly (P < 0.05) over pre-conceptual values by 6–7 wks gestation in the resting state, and by 10–11 wks during exercise. [H*] decreased significantly by wks 10–11 in the resting state. The early decrease in [H*] was due primarily to a significant reduction in PCO2 (caused by increased ventilation), since [Atot] and [SID] (the other determinants of plasma [H*]) did not change significantly. Concomitant with these effects was a significant reduction in plasma bicarbonate during exercise by wks 6–7. CONCLUSION Evidence for a partly-compensated respiratory alkalosis is present by wks 10–11 of healthy pregnancy. Practical implications for safe exercise participation in early pregnancy will be discussed. Supported by Ontario Thoracic Society, OTS Block Term Grant, NSERC (Canada)