Characteristics of flow velocities in the umbilical arteries, assessed by Doppler ultrasound

Measuring blood velocities by Doppler ultrasound has introduced a new noninvasive technique into obstetrical diagnostics. This study evaluates the information about fetoplacental hemodynamics that can be extracted from Doppler signals obtained from umbilical arteries. The most significant blood flow characteristic is blood flow pulsatility expressed as pulsatility index (PI). The PI is the difference between systolic and diastolic velocity, divided by the mean velocity. The PI strongly correlates with downstream impedance to flow. It is demonstrated that sophisticated spectral analysis of the Doppler signals has no practical advantages over the simple zero-crossing technique for establishing PI values, since blood flow in umbilical arteries has a blunt-to-flat profile. This study shows that the PI is a gestational-age-related, intra- and inter-observer-reproducible variable. The significant decrease of PI values with advancing gestational age reflects a progressive reduction of placental vascular resistance to umbilical blood flow. The PI holds promise as a simple method for early diagnosis of impaired umbilical circulation