Flow and velocity waveform indices in the ovine fetal abdominal aorta with changes in behavioural state

The purpose of this study was to determine the extent to which flow and velocity waveform indices are comparable as measured from the ovine fetal abdominal aorta and reflect downstream vascular resistance with changes in electrocortical activity. Nine chronically catheterized fetal sheep were studied near term with continuous measurement of electrocortical activity, perfusion pressure, and fetal heart rate. A cuffed Doppler crystal and a Transonic flow probe on the descending abdominal aorta were used for the simultaneous measurement of velocity waveforms and flow waveforms, with the Resistance Index (RI), calculated for each. Mean blood flow was also obtained from the Transonic flow probe. Whereas the change in cardiovascular parameters between the low- and high-voltage electrocortical activity (ECOG) states was small, with little relationship to the change in RI measurements, across all animals there was an inverse correlation between the RI measurements and fetal heart rate, perfusion pressure and aortic blood flow during both low- and high-voltage ECOG states. The RI as measured from the velocity waveforms showed only a modest correlation to those measured from the flow waveforms. It is concluded that over the range of circulatory change generated with spontaneous change in behavioural state, the RI as measured in the abdominal aorta of the ovine fetus is primarily determined by pulsatile cardiovascular variables rather than lower body vascular resistance.