Assessment of pulse wave velocity in the human umbilical artery: a first approach by Doppler blood flow analysis

Objectives:

In physiology the concept of pulse wave (PW) propagation along the arterial systems is well established (Nichols 2011). In the human fetus with UA notch, we found evidence that timing and shape of the notch may be explained on the bases of PW propagation and reflection. Timing is based on PW velocity (PWV) in the fetal Aorta (fAo) and in the UA (Gonser 2017). While PWV in the human fAo is about 250 cm/s, to our knowledge no such information is available for the human UA.

Methods:

To assess UA-PWV by Doppler ultrasound, two Doppler signals should be registered simultaneously from two separate sampling sites with known distance ΔL. With the time shift Δt between both signals UA-PWV can be calculated (foot-to-foot velocity method, Laurent 2006): UA-PWV =ΔL/Δt.

However in the human fetus such an arrangement seems unfeasible, perhaps with the exception of a nuchal cord encirclement using a long Doppler sample volume to cover the loop twice. Then two immediately successive, overlapping Doppler waveforms can be obtained, showing a very short time shift Δt, which corresponds to the PW transit time around this loop, and the outer circumference is a surrogate for path length ΔL.

Results:

In several singletons with cord encirclement we could sample two time shifted Doppler waveforms as expected. But only in 5 fetuses the loop was fairly enough visible for measurement of the outer circumference. With these 5 pairs of transit times and path lenghts, the UA-PWV vields 630 ± 38 cm/s and GA 33 ± 3wks (mean ± SD).

Conclusion:

In the human fetus we found rough evidence for considerable higher PWV along the UA (630 cm/s) than along the fAo (250 cm/s). In the fetal sheep, with a comparable UC length (60 cm), also a much faster UA-PWV was reported (623 ± 12 cm/s, Adamson 1992). Obviously our result obtained by insonating a cord loop around the neck allows only a rough estimate of PWV.

The considerable faster UA-PWV is due to higher vessel stiffness, which might prevent UC bending. The whole travel time for a fetal PW along the fAo and the UA and back will take roughly 300 ms (Gonser 2017). The fetal cardiac cycle is 400 ms (FHR 150bpm) and 300 ms in fetal tachycardia (FHR 200 bpm). Thus the fast UA-PWV might avoid that a placental site reflected PW can impact the next outgoing ventricular PW, except in fetal tachcardia.

References:

Nichols WW. McDonald's Blood Flow in Arteries: Theoretical, Experimental, and Clinical Principles. 6th ed. London: Hodder Arnold; 2011.

Adamson SL. Pulsatile pressure-flow relations and pulse-wave propagation in the umbilical circulation of fetal sheep. Circ Res 1992.

Gonser M. Umbilical artery notch: appearance and timing may be explained by pulse wave reflection and propagation. UOG 2007, abstract accepted.

Laurent S. Expert consensus document on arterial stiffness: methodological issues and clinical applications. Eur Heart J 2006.