Experimental validation of uterine artery volume blood flow measurement by Doppler ultrasonography in pregnant sheep

Objective: To test the hypothesis that Doppler-derived (calculated) uterine artery volume blood flow (cQ_UtA) reflects accurately volume blood flow measured directly (mQ_UtA) in an experimental setting. Methods: Five pregnant sheep were instrumented at 122-130 days of gestation under general anesthesia. After a 4-day recovery period, maternal hemodynamics were varied by administering to the sheep under general anesthesia noradrenaline, beta-blocker, low oxygen gas mixture, epidural bupivacaine and ephedrine, consecutively. The central venous pressure was obtained with the help of a thermodilution catheter. The mean arterial pressure and acid-base status were monitored using a 16-gauge polyurethane catheter inserted into the descending aorta via a femoral artery. A 6-mm transit-time ultrasonic perivascular flow probe was used to measure the mQ_UtA. Doppler ultrasonography of the uterine artery was performed and volume blood flow was obtained simultaneously by the transit-time ultrasonic perivascular flow probe during each phase of the experiment. Results: A total of 31 observations were made. The mQ_UtA varied between 90 and 800 (mean ± SD, 419 ± 206) mL/min during the experiments. The corresponding values for the cQ_UtA were 110 and 900 (mean ± SD, 459 ± 211) mL/min. There was a significant correlation (R = 0.76; P < 0.0001) between mQ_UtA and cQ _UtA. The mQ_UtA correlated positively with Doppler-derived uterine artery absolute velocities, i.e. peak systolic (R = 0.50; P = 0.004), end-diastolic (R = 0.53; P = 0.002) and time-averaged maximum (R = 0.69; P < 0.0001) and time-averaged intensity weighted mean (R = 0.75;P < 0.0001) velocities. Conclusion: cQ_UtA correlates well with volume blood flow measured directly. Doppler-derived uterine artery absolute blood flow velocities reflect uteroplacental volume blood flow in pregnant sheep.