INCREASE OF DOPPLER INDEXES IN THE LOWER ABDOMINAL-AORTA DURING INFUSION OF NITROPRUSSIDE INTO CHRONICALLY INSTRUMENTED FETAL SHEEP

Objective: The change of blood flow profiles in the lower abdominal ao rta of fetal sheep during arterial hypotension was investigated. Metho ds: Five fetal sheep (gestational age 119-127 days) were chronically i nstrumented with catheters in the abdominal aorta, the inferior vena c ava, and the amniotic cavity. Experiments were performed at the fifth postoperative day recording fetal heart rate (FHR), arterial blood (FA P), and amniotic fluid (AFP) pressures, and aortic blood flow velocity (pulsed duplex Doppler) before, during, and after infusion of sodium nitroprusside (22.5-42 mu g min(-1)). Pulsatility index (PI) and resis tance index (RI), corrected FAP as FAP - AFP, Doppler aortic blood flo w rate (mi min(-1)), and peripheral resistance (cFAP/flow rate) were c alculated. Results: During infusion of nitroprusside, cFAP reached a p lateau significantly lower than control values (from 42.0 +/- 5.4 to 3 2.4 +/- 2.1; mean +/- SEM, five animals). Peripheral resistance decrea sed significantly from 0.073 +/- 0.013 to 0.057 +/- 0.01 mm Hg min ml( -1), and FHR increased from 141 +/- 10 beats per min (bpm) to 162 +/- 14 bpm (not significant), PI and RI increased significantly (from 1.24 +/- 0.1 to 1.49 +/- 0.2 and from 0.57 +/- 0.04 to 0.70 +/- 0.03, resp ectively), due to increased systolic maximal and decreased diastolic m inimal velocities. Flow rates and mean velocities did not change. Cont rol blood gas values stayed within normal ranges. When nitroprusside i nfusion was terminated, all values returned to control levels. Conclus ions. In the ovine fetal abdominal aorta, resistance indicators RI and PI may increase when the peripheral vascular resistance actually decr eases. It is suggested that nitroprusside might influence the vascular impedance, but that also the ratio of downstream (mainly umbilical-pl acental circulation) to upstream resistance might be increased.