Hemodynamic changes in early pregnancy in chronically instrumented, conscious rats

To explore the onset of and the interrelationship between maternal hemodynamic changes in pregnancy, serial hemodynamic measurements were performed in chronically instrumented, conscious rats using electromagnetic flow probes around the ascending aorta and arterial catheters. The rats were studied daily from day 4 to day 12 and on days 14, 18, and 20 of pregnancy. Nonpregnant (NP) rats matched for age and days postsurgery served as controls. In the pregnant (P) group, the hematocrit started to decrease by postconceptional day 6 (day of implantation) to reach a value of 9 +/- 3% below the initial level by day 8. In these 2 days, cardiac output (CO) increased by 9 +/- 4%, as a result of a rise in stroke volume (SV). No changes in mean arterial pressure were observed. In both groups, heart rate (HR) had decreased by day 7. Aortic flow acceleration and peak aortic flow, indicators of myocardial contractility, increased from day 10 on only in the P group. In this context, it should be emphasized that the results of this study do not allow differentiation between the contractile properties of the cardiac muscle and the contractile changes as a result of the altered preload and afterload. By days 12 and 18, CO had increased by 20 +/- 5 and 29 +/- 9%, respectively, above the initial value and by 25 and 40%, respectively, above the value observed in the NP group. The rise in CO after day 14 was accomplished by a concomitant increase in HR and SV and was accompanied by a further increase in myocardial performance. We conclude that hemodynamic changes can already be identified by day 8 of rat pregnancy, 2 days after implantation. The rise in CO in early pregnancy results from a selective increase in SV and is accompanied by a rise in myocardial performance.