Site-dependent effects of increases in placental vascular resistance on the umbilical arterial velocity waveform in fetal sheep

Placental vascular resistance was increased by embolization or by arterial vasoconstriction with angiotensin II and the effect on the flow and velocity waveforms in the umbilical artery was examined. Fetal sheep ( n = 6) were studied under halothane anesthesia following the implantation of vascular catheters and an umbilical arterial electromagnetic flow probe. Blood flow and velocity (measured using Doppler ultrasound) gradually decreased during diastole and ultimately reversed in direction as cotyledon resistance was increased up to 14 fold. Cotyledon resistance was significantly correlated with the ratio of diastolic to systolic values (D/S ratio) of blood flow ( r = −0.94) and velocity ( r = −0.79). In contrast, increasing placental resistance up to 16 fold by constricting the umbilical arteries caused no significant change in the shape of the flow or velocity waveform