Relationship between mean arterial pressure and parameters derived from blood flow velocity waveforms in the systemic circulation of fetal sheep.

In 8 fetal sheep under anesthesia, we examined the relationship between the fetal aorta blood flow velocity waveform and fetal mean arterial pressure after administration of norepinephrine and angiotensin II. It was hypothesized that the pulsatility index (PI) of the waveform would change during fetal hypertension. Measurements were performed using a 20-MHz Doppler transducer on the aorta directly beneath the diaphragm and a catheter inserted via the femoral artery into the abdominal aorta. Further instrumentation included a Doppler transducer and an electromagnetic flowmeter on the common umbilical artery and a catheter in the inferior vena cava. Fetal hypertension was induced by bolus administration of either norepinephrine or angiotensin II or by infusion of norepinephrine. With the Doppler transducer on the common umbilical artery, it was possible to study external iliac artery blood velocity waveforms in 4 fetal lambs, whereas aorta blood velocity waveforms were recorded in 7 fetal lambs. The measurements of external iliac artery PI and mean arterial pressure were characterized by a linear regression with a correlation coefficient of 0.72, whereas the measurements of aorta PI and mean arterial blood pressure were characterized by a linear regression with a correlation coefficient of 0.40. However, notable changes were observed in the aorta blood velocity waveform, which were not expressed by a substantial increase in the aorta PI. These changes can be described as a narrowing of the systolic peak and an earlier occurrence of the first dicrotic notch.(ABSTRACT TRUNCATED AT 250 WORDS)