Anaerobic threshold and respiratory compensation in pregnant women.

In an effort to explore why CO2 output (VCO2) at peak exercise is lower during pregnancy than postpartum despite little change in the peak O2 uptake (VO2), we determined the VCO2/VO2 relationship during rapidly incremental exercise and estimated the anaerobic threshold (AT) and the respiratory compensation (RC) point. We measured heart rate, VO2, VCO2, and minute ventilation (VE) at rest and during cycle exercise tests with rapidly increasing exercise intensities until maximal effort in 33 volunteers at 16-, 25-, and 35-wk gestation and postpartum. Through modification of the V-slope method, we estimated the AT and RC point for each test by nonlinear regression analysis in a three-dimensional space (defined by VE, VO2, and VCO2) for a line assumed to have two breakpoints; we found a good fit for all tests. The AT and RC points were found at exercise intensities of approximately 50 and 80% peak VO2, respectively, with no significant differences between test periods. VE was significantly higher during pregnancy than during postpartum at rest and throughout incremental exercise. A lower peak VCO2 relative to peak VO2 during pregnancy compared with postpartum was reflected by a more shallow slope of VCO2 vs. VO2 above the AT point. This suggests that during pregnancy the buffering of lactic acid is reduced.