Factors determining aortic and umbilical blood flow pulsatility in fetal sheep.

The link between the flow pulsatility index (P.I.) and vascular resistance was examined by infusing vasoactive agents or by progressive embolization of the peripheral vascular bed. Experiments were performed on fetal sheep near term, 3 days after implantation of a flow probe, catheter-tip pressure transducers, and fluid-filled catheters to monitor hemodynamics in the umbilical artery and the descending thoracic aorta. Increasing placental and hind limb resistance by embolization with microspheres caused a progressive increase in flow P.I. whereas altering resistance with vasoactive agents had variable results. Results showed that flow P.I. was sensitive to impedance to pulsatile flow and pressure pulsatility in addition to vascular resistance. We conclude it is hazardous to use the flow P.I. to detect changes in vascular resistance particularly when these changes are induced by physiological adjustments or vasoactive agents.