Blood flow in the fetal descending aorta.

Doppler estimation of the blood flow in the descending aorta of the fetus and waveform analysis of the maximum aortic velocity provide valuable information on fetal circulation. When estimating fetal aortic flow, it is important to consider possible sources of error; even when recording the maximum aortic velocity for waveform analysis, to obtain reproducible results it is important to use a low high-pass filter, a well-defined angle of insonation, and a standardized site of measurement and to avoid periods of fetal breathing and activity. In normal pregnancy the time-averaged mean velocity in the descending aorta is stable throughout the third trimester. The weight-related flow is stable until 37 weeks, after which it falls slightly. The placental proportion of the flow in the thoracic descending aorta decreases toward term. In the aorta of normal fetuses there is positive flow throughout the whole heart cycle, which is due to the low vascular resistance in the placental circulation. The waveform of the aortic velocity can be characterized by the PI, which is stable during the last trimester of gestation. Caution is required when interpreting changes in the aortic PI, as it is not only affected by the peripheral resistance but also by the heart performance. Furthermore, PI is related to fetal heart rate. Near term different values of PI are found in different fetal behavioral states. In fetuses with retarded growth and in fetuses at distress, characteristic changes of the aortic velocity waveform have been reported by several researchers: the end-diastolic velocity diminishes and disappears, and in extreme cases a brief reversal of flow in diastole was observed. Consequently, the PI increases in such cases. The absence of the end-diastolic aortic velocity can easily be determined and is the best indicator of fetal status: in fetuses with absent end-diastolic velocity the incidence of perinatal mortality and morbidity is significantly higher than in fetuses with positive flow throughout the cycle. In pregnancies with hypertension or diabetes mellitus, normal aortic flow has been reported, as long as the fetuses were not growth retarded. In cases of severe Rh-isoimmunization, the mean aortic velocity correlates with fetal hematocrit. In hypoxic fetuses the mean velocity was reported to correlate with the degree of hypoxia, hypercarbia, and acidosis.(ABSTRACT TRUNCATED AT 400 WORDS)