Optimal pulsatile timing and cardiac energy consumption in the human fetus

Introduction LV contraction and ejection of blood to the arterial system is an energy consuming process. During the transition of the pulse wave (PW) to the periphery reflection back to the LV occurs. Based on the recently published wave dynamic equation it can be shown that energy consumption of pulsatile cardiac action is minimized when the time of return Tr to the LV takes about 20 % of the cardiac cycle T, i. e. optimal pulsatile timing Tr/T ≈ 0.2. To our knowledge, optimal pulsatile timing has so far not been studied in fetal circulation.

Methods According to the wave dynamic equation: α = HR.L⁄c [equ 1], energy consumption of pulsatile cardiac action is minimized for α ≈ 0.1 (HR: heart rate, L: length of central arterial system, c: pulse wave velocity in the aorta, Pahlevan 2014). Multiplying [equ 1] by 2 yield:

2·α = HR·[2L/c] = (1/T)·Tr, (with: HR[Hz] = 1/T[sec], and: [2L⁄c] = Tr, Fig. 1),

thus: 2·α = Tr/T ≈ 0.2, indicates optimal pulsatile timing

While fetal cardiac cycle length T is simply the reciprocal of FHR (scaled in Hertz: FHR(Hz) = FHR(bpm)/60), hemodynamic modelling is requiered to obtain time of return, Tr, in the fetus: PWs arrive twice at cerebral circulation: 1^st by direct transmission, and 2^nd after reflection, return and subsequent transmission ([Fig. 1]). Flow re-acceleration may be observed, showing a systolic shoulder (S) in MCA Doppler (MCA-S) after a delay Δt. Thus the delay Δt, corresponds to the time needed for reflection and return, Tr (Mills 1970)

▶Fig. 1

Results Recently Tr data, of fetuses in pregnancies with IUGR and/or increased uteroplacental resistance, were obtained by this method (Gonser 2018, abstr.): Tr = 96 ± 15 ms (GA 31 ± 3w). Mean FHR reference value for 31w (= 140bpm) yield cardiac cycle length T = 423 ms. With these 2 parameters, we calculated pulsatile timing in these fetuses: Tr/T = 96ms/423 ms = 0.23 (23 %) and obtained good agreement with optimal pulsatile timing of 20 %, as given by the wave dynamic equation.

Discussion In spite or perhaps because of circulatory stress, the fetus maintains optimal reflection timing, obiously due to the priority of minimal energy consumption. Timing of PW reflection is governed by pulsatile energy optimization, and ampliude by the reflective conditions of the fetus, i. e. peripheral vasoconstriction. Thus the appearance of MCA-S is not an artifact, but a result of fetal physiology.

Publication History

Article published online:
04 December 2020

• References

• 1 Pahlevan NM. J Biomech 2014
• 2 Mills CJ. Cardiovasc Res 1970
• 3 Gonser M. Ultraschall in Med 2018