Effect of pregnancy on heart rate/oxygen consumption calibration curves

The purpose of this investigation was to determine heart rate (HR)/oxygen consumption (VO2) calibration curves for exercising and sedentary women during pregnancy and the postpartum periods.

Fifty-two women were studied at three time points: 20 wk gestation, 32 wk gestation, and 12 wk postpartum. Subjects were grouped either as regular exercisers (N = 27) or sedentary controls (N = 25). At each time point, each woman had HR and VO2 measured at rest (lying, seated, and standing) as well as during steady-state treadmill exercise performed at three increasing intensities. Flex HRs were defined and calculated by averaging the value seen during the lowest exercise intensity and highest value during rest. Individual HR/VO2 calibration curves were generated for exercise at each time point. Statistical analyses of all dependent variables included comparisons of subject groups (exercise and sedentary) and pregnancy status (20 wk, 32 wk, and 12 wk postpartum).

Resting VO2 (mL.kg-1.min-1) was approximately 6.5% greater during pregnancy compared with postpartum conditions (P < 0.005). Also, both resting and flex HRs were greater during pregnancy compared with postpartum (P < 0.01). Resting HR was lower in exercising women compared with sedentary controls at all time points (P < 0.01). Slopes of HR/VO2 regression curves were flatter during pregnancy (P < 0.005), but there was no difference between groups. Y-intercepts were less at 20 wk compared with 36 wk postpartum.

A woman's HR, VO2, and the relationship between these two parameters are altered during pregnancy. Change in slope of HR/VO2 regression curves indicates less energy expenditure at a given HR as pregnancy progresses, compared with postpartum conditions. A woman's true energy expenditure would be overestimated at rest, and underestimated during physical activity, if these physiological changes are not taken into account.