Doppler Characterization of Murine Embryonic Umbilical Blood Flow: Insights into Developing Embryonic-Placental Circulation and Cardiac Function

Abstract 159 Poster Session II, Sunday, 5/2 (poster 260)

Recent innovations in noninvasive imaging have begun to provide insights into the development of the embryonic circulation. In addition to high-resolution 40 MHz ultrasonic backscatter microscopy (UBM) providing 2-D imaging, we have developed 45 MHz pulsed-wave (PW) Doppler capabilities. We sought to characterize umbilical blood flow and thus, cardiovascular hemodynamics, in the early mouse embryo. We hypothesized that flow patterns would show changes typical for a growing heart and blood volume, with decreasing placental impedance (Z_p). Methods: 45 Swiss-Webster mouse embryos were studied between embryonic day (E) 9.5-14.5, in anesthetized dams. Rectal temperature was closely maintained at 37±1° C. Umbilical cord angle assessed by UBM and Doppler interrogation from multiple views were used to maximize the velocity signal. PW Doppler allowed insonation of umbilical vessels, separate from intracardiac/aortic signals. Doppler signals signs were converted by short-time Fourier transform into a spectral display on a LabView data management program. Results: Heart rate (HR) increased from 215±10 [SD] bpm at E9.5, to 251±52 bpm at E14.5 (r=0.96, p<0.003, by linear regression analysis). Peak arterial velocity (PAV) increased from 20.9±7.6 mm/sec at E9.5 to 86.8±33.5 mm/sec at E14.5, while peak venous velocity (PVV) increased from 9.3±1.3 mm/sec at E9.5 to 38.8±11.7 mm/sec at E14.5 (r>0.97, p<0.002). Concomitant increases were seen in arterial (AVTI) and venous velocity time integrals (VVTI) (r=0.99, p<0.0002). Ejection time as a proportion of cycle length (ET/CL) also increased (r=0.94, p<0.005). Trough arterial velocity (TAV) was essentially zero so that the placental resistive index ([PAV-TAV]/PAV) was constant at one from E9.5-E14.5. We therefore assessed Z_p by a more sensitive index, the time delay between arterial and venous velocity peaks (AVD), since Z_p influences transmission of the arterial wave into the umbilical vein. Both AVD and AVD/CL increased with gestation (r>0.87, p<0.03). Also, venous pulsatility ([PVV-trough venous velocity]/PVV) decreased with gestation (r= -0.92, p<0.009). Acceleration time, an angle- and load-independent peripheral arterial Doppler index of cardiac contractility (Pediatr Res 1991; 30:375), did not change with gestation (p=0.28). Conclusions. We have developed a PW Doppler system capable of measuring low-velocity blood flow in the early mouse embryo. HR was faster than previously described, likely due to the noninvasive nature of measurement and stringent thermoregulation. PAV, AVTI, ET/CL, PVV, and VVTI indicate increasing stroke volume through umbilical vessels, from E9.5-E14.5. AVD, AVD/CL, and venous pulsatility indicate decreasing Z_p, which very likely contributes to increasing stroke volume. Cardiac contractility does not seem to change between E9.5-E14.5. These quantifiable changes form the basis for the study of transgenic mice with aberrant cardiovascular and placement development.