title: Feto-placental circulatory competence.
abstract: Doppler assessment of the pulsatility index (PI) of umbilical artery blood velocities offers promise as a simple, non-invasive method for early diagnosis of placental failure. The present report identifies the hemodynamic factors determining this PI, based on current knowledge of feto-placental physiology. It is postulated that the umbilical artery PI is mainly determined by the ratio of capillary to arterial resistances in the fetal placental circulation. However, the biological PI variations on a minute by minute basis are caused by short-term variations in blood pressure pulsatility. The ratio of resistances declines markedly with advancing pregnancy, indicating that the site of predominant resistance determining the volume of placental blood flow gradually migrates from the peripheral capillary to the arterial compartment. After 28 wk of gestation, umbilical artery PI values decrease below 1, reflecting the normal reserve capacity of the functional capillary bed of the placenta. PI values above 1 in the last trimester reflect a critical enhancement of placental capillary resistance, which must impede blood flow to functional chorionic villi. It is concluded that this Doppler technique may provide a simple method for clinical assessment of placental circulatory competence.
mesh_terms: D001011:Aorta / Q000502:physiology; D001783:Blood Flow Velocity; D005314:Embryonic and Fetal Development; D005260:Female; D006801:Humans; D008955:Models, Cardiovascular; D010920:Placenta / Q000503:physiopathology*; D010927:Placental Insufficiency / Q000175:diagnosis / Q000503:physiopathology; D011247:Pregnancy; D012212:Rheology; D014469:Umbilical Arteries / Q000033:anatomy & histology / Q000502:physiology*; D014655:Vascular Resistance