Hemodynamic validation of Doppler assessment of fetoplacental circulation in a sheep model system

Abstract

Several clinical studies suggest that the Doppler indices (DI) derived from Doppler waveforms analysis may be powerful predictors of perinatal compromise. In contrast, significant paucity exists in experimental verification of the central premise that the DI reliably reflect hemodynamic state downstream from the measurement point. This paper presents the results from two sets of acute animal experiments in which experimentally induced changes in the ovine fetoplacental circulation as reflected by the independently measured hemodynamic parameters were correlated with the corresponding variations in the DI. Doppler velocimetric measurements were performed under general anesthesia in ovine fetuses with gestational ages ranging from 100 to 110 days. A continuous wave Doppler instrument with a 4-MHz transduce was used. In three fetuses, variable vascular constriction was produced by a ligature placed approximately 10 cm from the probe location. In four fetuses, vasopressin was rapidly infused. The hemodynamic parameters measured included the peripheral resistance, volumetric flow, and pressure. The DI included the systolic-diastolic ratio, pulsatility index, and resistance index. The results indicate that, in hemodynamically stable and well-defined obstruction to downstream flow as achieved in the constriction experiments, the DI highly correlated with the hemodynamic state (p < .005); however, during the more complex and unstable circulatory changes associated with vasopressin administration, these correlations were inconsistent and poor (p > .05). This study experimentally establishes in an in vivo animal system the hemodynamic validity of the DI within certain limits. However, the validity does not extend to hemodynamic changes associated with vasopressin administration. Further studies are required to address this issue.