A comparison of cross-sectional and longitudinal methods of assessing the influence of pregnancy on cardiac function during exercise

To evaluate the influence of pregnancy on resting and exercise cardiac function, we studied 16 nonpregnant and 26 pregnant females in a two-phase study. During phase I, all pregnant subjects (P1) (mean gestational age=29.5±7 weeks) were compared with 16 control (C) subjects. In phase II, 8 of the pregnant subjects (P2) (mean gestational age=32.6±6 weeks) were evaluated again at 12.5±7 weeks postpartum (PP). Using Collier's CO 2 rebreathing technique, exercise cardiac output (Q) was measured on a cycle ergometer during rest and at three submaximal work levels (25, 50, and 75 W), during the last minute of 5 to 7 minutes of steady-level power output. Metabolic parameters [ventilation (VE), beart rate (HR) and oxygen consumption (VO 2 )] were measured during the submaximal tests and throughout the progressive increase in work after the last measure of Q, until peak VO 2 was achieved. The results from the phase I comparison indicated (as expected) higher resting heart rates [(90.0±3 P1; 81.8±3 bpm C), ventilation (9.5±0.3 P1; 7.5±0.5 L min −1 C), and resting oxygen consumption (0.263±0.07 P; 0.221±0.01 L·min −1 C) associated with pregnancy, and these findings were similar when the pregnant subjects were compared with their postpartum values. Analysis of the various indices of cardiac function [Q, stroke volume (SV), and arteriovenous oxygen difference (A-VO 2 diff)] resulted in statistically higher resting Q in pregnancy when compared with C and PP conditions. No resting differences were observed for SV or A-VO 2 diff in either phase of the study. The results of the submaximal exercise tests indicated no significant differences in HR nor VO 2 between P1 and C; however, P2 had significantly higher submaximal heart rates and oxygen consumption when compared with PP. Further, no significant differences were observed between P1 and C for Q, SV or A-VO 2 diff, whereas P2 versus PP resulted in significantly lower SV and higher A-VO 2 diff in the pregnant subjects. These findings suggest that pregnancy/control versus pregnancy/ postpartum studies yield statistically different results and that an effort needs to made by the research community to develop standardized methodologies to evaluate physiological processes in pregnancy.