Aortic Doppler velocity measurement and cardiac function in the fetal lamb

Aortic haemodynamic parameters, and Doppler waveforms in particular, were investigated in acute experiments with fetal lambs. Cardiovascular changes were produced by central infusion of the drugs esmolol and dopamine. Pulsed Doppler waveforms were obtained from the descending thoracic aorta, simultaneous with recordings of pulsatile aortic volume flow rate, diameter and blood pressure. The relation between Doppler-derived velocities and the corresponding full vessel lumen velocities was shown to be fairly linear and consistent across different animals. The aortic volume flow per beat decreased with esmolol (p < 0.003, repeated measures ANOVA); the Doppler and vessel lumen mean velocities also decreased, whether measured only at peak systole or over the full cardiac cycle (at most p < 0.003). With dopamine the aortic flow per beat increased (p < 0.001), as did the Doppler and vessel lumen mean velocities (at most p < 0.02). An inverse relation between the aortic flow per beat and the peripheral resistance was observed. To identify inotropic changes in the presence of vascular effects, a theoretical model based on cardiac power output changes was implemented. The data were divided into three groups, according to whether the model did or did not identify a definite inotropic effect (positive or negative). The Doppler velocity changes for these three groups were different (p < 0.0001). The mean Doppler velocity increased by 7 cm s^-1 in the positive inotropic effect group, and decreased by 4 cm s^-1 in the negative group. The aortic flow parameters of the human fetus are very similar to those of the fetal lamb. Decreased aortic velocities have been reported in human fetal compromise, and the results of this study support the hypothesis that this can be evidence of impaired fetal cardiac function.