Thoracic aortic pressure-flow relationships and vascular impedance in fetal sheep.

Upper descending thoracic aortic pressures recorded from near-term fetal sheep resembled those recorded in mature animals, but diastolic thoracic aortic flows were much greater than those reported for adults. Furthermore, the pressure wave was not amplified during transmission along the aorta, as it is in adults. Descending thoracic impedance fell with increasing frequency to approximately 25% of resistance to steady flow and then showed oscillations typical of systems in which pulse wave reflections are important. Angiotensin II and norepinephrine infusions enhanced impedance oscillations and caused changes in apparent phase velocity in the aorta that were consistent with increased wave reflections. Angiotensin also increased aortic characteristic impedance from 0.0152 +/- 0.0013 to 0.0230 +/- 0.0024 mmHg.min.ml-1. The effects of nitroprusside infusions indicated reduced wave reflection effects. Embolization of the placenta and hindlimbs with 50-microns microspheres was used to elevate vascular resistance in these beds. Embolization caused aortic flow to fall to zero by late diastole, and changes in impedance and apparent phase velocity suggested increased wave reflections from the periphery. Characteristic impedance was increased to 0.0209 +/- 0.0031 mmHg.min.ml-1.