Ventilation during rest and exercise in pregnancy and postpartum.

The expiratory minute volume (Ve), respiratory frequency (f), tidal volume (VT), carbon dioxide production (Vco2), and end-tidal carbon dioxide concentration (FETCO2) and pressure (PETCO2) were measured at monthly intervals in 12 normal women during pregnancy and two, six and 12 weeks, and six months postpartum. At eacy study, measurements were made sitting at rest and during steady-state exercise at 306 kpm/min on a bicycle ergometer. During pregnancy, a significant increase in VE occurred, both at rest and during exercise, due to a significantly greater VT. Although VCO2 was significantly increased at rest throughout pregnancy and with exercise in late pregnancy, the respiratory exchange ratio (R) was not significantly altered during pregnancy. The FETCO2 was lower during pregnancy than postpartum, both at rest and during exercise. The ventilatory equivalent for oxygen (VE/VO2) was greater at rest near term and during exercise throughout pregnancy. However, resting physiologic dead space increased during pregnancy. Alveolar ventilation (VA) was calculated on the basis of three alternative assumptions: (1) that PETCO2 during exercise accurately reflects mean alveolar PCO2; (2) that the physiological dead space does not change during exercise; and (3) that mean alveolar PCO2 does not change from rest to exercise. Exercise VA, calculated on the basis of any of these three assumptions, is greater during pregnancy than postpartum.