Venous and arterial behavior during normal pregnancy.

To assess the contribution of the arterial and venous systems in the hemodynamic changes of normal pregnancy, we studied blood flow, vascular resistance, venous tone, and the viscoelastic properties ("creep") of the upper and lower limbs (using plethysmography), aortic distensibility (using pulse wave velocity measurements), and cardiac dimensions (using echocardiography) in nine healthy women. Studies were longitudinally performed at the first (10-13 wk) and third (33-38 wk) trimesters of pregnancy in comparison with the period between the third and sixth month after delivery. From the first trimester, heart rate significantly increased while systemic blood pressure and limb vascular resistances did not change significantly and aortic distensibility increased (P < 0.05). Lower limb viscoelastic properties decreased at the third trimester (P < 0.05) and venous tone increased from the first trimester (P < 0.01), whereas little changes were observed at the site of upper limbs. The decrease in calf venous tone was significantly correlated with the increase in left ventricular diastolic diameter at the first (P < 0.001) and the third trimester (P < 0.05). The study provides evidence that during normal pregnancy, changes in the arterial and venous sides of the circulation occur independently of pressure alterations. The increase in venous tone, contributing to preload augmentation, and the decrease in aortic stiffness, reducing afterload, both optimize cardiac function until delivery.