A longitudinal study of the relationship between maternal cardiac output measured by impedance cardiography and uterine artery blood flow in the second half of pregnancy.

OBJECTIVE: To study serial changes in maternal systemic and uterine artery haemodynamics and establish reference ranges for the second half of pregnancy.
DESIGN: Prospective longitudinal observational study.
SETTING: University hospital in Norway. POPULATION: Low-risk pregnant women.
METHODS: Fifty-three low-risk pregnancies were evaluated at approximately 4-weekly intervals. Maternal systemic haemodynamics was assessed with impedance cardiography. Uterine artery blood velocity and diameter were measured using Doppler ultrasonography and uterine artery volume blood flow (Q(uta)) was calculated as the product of mean velocity and cross-sectional area of the uterine artery. The fraction of cardiac output (CO) distributed to the uterine circulation was calculated as: Q(uta)/CO x 100. MAIN OUTCOME MEASURES: CO, Q(uta), uterine vascular resistance (R(uta)) and the fraction of CO distributed to the uterine circulation.
RESULTS: The CO increased (P = 0.0063) until 34 weeks and remained stable until term. Total Q(uta) increased from 299 to 673 ml/minute and R(uta) halved from 0.26 to 0.13 mmHg/ml/minute (P < 0.0001). The fraction of CO distributed to the uterine circulation increased from 5.6% to 11.7% (P < 0.0001).
CONCLUSION: During the second half of pregnancy, Q(uta) and the fraction of maternal CO distributed to the uterine circulation increase approximately two-fold, mainly as a result of decrease in R(uta).