Umbilical artery half peak systolic velocity deceleration time throughout pregnancy and its role in fetuses with bradycardia.

OBJECTIVES: To describe a new technique for Doppler waveform quantification of the fetal umbilical artery, the half peak systolic velocity (h-PSV) deceleration time, which measures the time that it takes a single flow velocity waveform to halve its maximum systolic velocity. As this measurement is independent of fetal heart rate, its role in the evaluation of placental vascular resistance in fetuses with bradycardia was also explored. METHODS: The umbilical artery impedance indices and the h-PSV deceleration time were measured in 532 normal singleton fetuses from 17 to 41 weeks of gestation. A nomogram was established and its usefulness in assessing fetuses with bradycardia was evaluated. RESULTS: The relationship between umbilical artery h-PSV deceleration time and gestational age was best described by a linear formula (y = 6.5523x + 12.41; R(2) = 0.6346), h-PSV deceleration time increasing by 6.6 ms per week. The correlation coefficients for the 95(th), 50(th) and 5(th) centiles were 0.97, 0.98 and 0.98, respectively. Measurement of h-PSV deceleration time was reproducible, with interobserver and intraobserver variabilities of 10.7% and 7.4%, respectively. Among fetuses with bradycardia (n = 7), the h-PSV deceleration time was above the 5(th) centile in all cases, suggesting normal placental function in spite of abnormally increased impedance indices obtained with traditional Doppler indices in six. CONCLUSIONS: The h-PSV deceleration time increases linearly during the second and third trimesters of pregnancy. As its measurement is reproducible and independent of heart rate, it may be useful in the assessment of well-being in fetuses with bradycardia.