Maternal blood-gases, PAo2--Pao2), hysiological shunt and VD/VT in normal pregnancy.

Serial measurements of maternal blood-gases, alveolar-to-arterial oxygen tension difference (PAO2--PaO), calculated pulmonary venous admixture (physiological shunt), deadspace/tidal volume ratio (VD/VT), and respiratory minute volume have been made in a carefully selected group of normal pregnant patients at 12, 24, 32 and 38 weeks of gestation and 5 weeks after delivery. All measurements were made in the semi-recumbent position with a left lateral pelvic tilt. Mean arterial PO2 was consistently greater than 100 mm Hg throughout pregnancy, although the value decreased from 106.4 mm Hg at 12 weeks of gestation to 101.8 mm Hg at the 38th week. Despite this decrease there was no change in (PAO2--PaO2) VD/VT, or percentage shunt with increasing gestation; nor did these values differ significantly from non-pregnant values in the same patients.