Bustos et al. recently reported a new index for Doppler waveform quantification in the study entitled ‘Umbilical artery half peak systolic velocity deceleration time throughout pregnancy and its role in fetuses with bradycardia’1. I congratulate the authors on their work which may be of clinical value in assessing fetal hemodynamics in addition to other currently used parameters that quantify umbilical artery Doppler signals. However, whilst measuring flow deceleration time in the umbilical artery is certainly a novel way of assessing placental resistance, the described index, ‘velocity half-time’, is not a new parameter for Doppler waveform quantification, as stated by the authors. In 1990, my colleagues and I2 measured flow deceleration time from Doppler velocity signals obtained in the descending aorta of children with aortic coarctation and quantified the velocity waveform decay time as done by Bustos and colleagues1. A similar concept, ‘pressure half-time’, had been used previously to assess severity of mitral stenosis3. Derived from the mitral valve Doppler velocity profile, this allowed estimates of mitral valve area to be made non-invasively. Velocity and pressure half-time are not the same, the latter corresponding to 29% of the total deceleration time4 and the former relating to the time it takes for the peak systolic velocity to fall to half its value, as used by Bustos et al.1 and in our study of coarctation of the aorta2. We studied velocity half-time due to the resemblance between exponential decay and the pattern of velocity (but not pressure) deceleration in the descending aorta. This was considered to be a simple quantitative way of assessing prolongation of flow in the aorta—a pattern thought to be of diagnostic value in this condition, but which had not been quantified previously5. Additionally, we measured diastolic velocity half-time, or the time to half peak diastolic velocity, using the T-wave on the electrocardiogram as a marker. This was the single measurement with the highest accuracy to identify moderate to severe degrees of aortic narrowing compared with angiographic data. Similarly to Bustos et al.1, we did not correct our measurements for heart rate (based on the velocity profile being similar to exponential decay), but others have subsequently used deceleration half-time values indexed for heart rate6, 7. However, while prolongation of deceleration time in aortic coarctation seems to be related to severity of aortic narrowing, the characteristics of the arterial wall, i.e. aortic compliance, also influence this pattern. Using an in-vitro pulsatile flow model8 as well as a computer model of aortic coarctation9, researchers have shown that proximal aortic compliance is related to time of velocity decay and that downstream diastolic flow increases with proximal aortic compliance. While these models do not simulate the umbilicoplacental circulation, downstream (i.e. placental) resistance and vessel wall compliance are likely to play an important role in the diastolic flow runoff in the umbilical artery. This concept is certainly in agreement with the prolongation of umbilical artery half peak systolic velocity deceleration time as placental resistance decreases with advancing gestational age, as shown by Bustos et al.1. J. S. Carvalho*, * Brompton Fetal Cardiology, Royal Brompton Hospital, Fetal Medicine Unit, St. George's Hospital, University of London, London, UK