RESUMO
OBJECTIVE: Left ventricular ejection causes forward flow in the fetal aortic isthmus while the right ventricle has a retrograde influence. The aim of this study was to create reference values for an isthmic systolic index (ISI) reflecting the changing influence of right and left ventricular performance on Doppler flow velocity waveforms of the aortic isthmus throughout normal pregnancy. METHODS: Doppler recordings of 260 normal fetuses with a gestational age of 18-37 weeks were reviewed. Peak systolic velocity (PSV) and end-systolic velocity (or systolic nadir; Ns) were measured on all aortic isthmus waveforms. An ISI was derived from the ratio Ns/PSV. Left and right ventricular outputs were also calculated. RESULTS: Up to 22-23 weeks' gestation, the mean ISI is stable at around 0.2. At about 28 weeks, a brief end-systolic deceleration wave is observed on the aortic isthmus waveforms, progressing steadily with gestation and causing a fall of ISI towards a mean value of zero between 30 and 31 weeks. This trend continues thereafter and a mean value of -0.4 was observed at the end of pregnancy. An inverse correlation was found between right ventricular output and Ns (r = -0.334, P = 0.001). Simultaneous recordings of the isthmus and the ductus arteriosus Doppler waveforms demonstrated that the primary cause of the end-systolic deceleration and ultimate reversal of flow at the isthmus is the increasingly dominant flow from the right ventricle. CONCLUSION: The transitional changes of the respective right and left ventricular outputs throughout pregnancy are well profiled by the ISI. This index highlights the physiological increase in fetal right ventricle flow preponderance as pregnancy progresses. Alteration of the ISI profile could be expected in clinical conditions associated with unbalanced alteration of the fetal ventricular outputs.