Relationships among Doppler-derived umbilical artery absolute velocities, cardiac function, and placental volume blood flow and resistance in fetal sheep

We hypothesized that umbilical artery (UA) absolute blood flow velocities measured by Doppler ultrasonography reflect placental volume blood flow (Q _UA) and placental vascular resistance (R_UA) in a late gestation fetal sheep model. In addition, we examined the relationships between umbilical artery absolute blood flow velocities and parameters of fetal cardiac function. Twenty-six sheep fetuses were instrumented at 112-132 days of gestation. After a 5-day recovery period, experiments were performed under general anesthesia in 16 normal fetuses, in 5 fetuses after maternal administration of phenylephrine, and in 5 fetuses after placental embolization. The Q_UA and arterial blood pressures were measured using a transit-time ultrasonic flow probe and a catheter placed into the descending aorta, respectively. UA peak systolic velocity (PSV), end-diastolic velocity (EDV), time-averaged maximum velocity (TAMXV), pulsatility index (PI), mean velocity (V_mean), fetal cardiac output, ventricular ejection forces, and the proportion of isovolumetric relaxation time (IRT%) in the cardiac cycle were measured with the use of Doppler ultrasonography. Significant positive linear correlations were found between UA EDV, TAMXV, and V_mean versus Q_UA, whereas UA PI had a significant negative correlation with Q_UA. Significant negative correlations were shown between UA EDV, TAMXV, and V_mean versus R_UA. A significant positive correlation was present between UA PI and R_UA. Doppler-derived UA parameters did not correlate with fetal arterial blood pressures, cardiac output, ventricular ejection forces or IRT%. In fetal sheep, Doppler-derived UA PI and absolute velocities, except PSV, are closely related to directly measured Q_UA and R_UA, validating the use of noninvasive Doppler velocimetry in the assessment of placental circulation.