Objective: Recent studies in lambs using wave intensity (WI) analysis indicate that a characteristic mid-systolic fall in fetal pulmonary artery (PA) blood flow is related to a large pressure-increasing, flow-decreasing backward-running compression wave (BCWms), which originates in the pulmonary microvasculature via cyclical vasoconstriction and reflection of the early-systolic forward-running compression wave (FCW) associated with ventricular ejection. This BCWms is partially transmitted into the pulmonary trunk (PT) as a smaller BCWms. This study sought to determine if the PA BCWms was also transmitted into the ductus arteriosus (DA), and the hemodynamic effects of any transmission.
Methods: Simultaneous PT, left PA and DA WI analysis was performed in 8 anaesthetized late-gestation fetal sheep instrumented with PT, left PA and DA micromanometer catheters to measure pressure (P) and transit-time flow probes to obtain blood velocity (U). The main PA was briefly occluded in a subgroup (n=5) to abolish wave transmission from the lungs. WI was calculated as the product of P and U rates of change.
Results: The left PA WI profile displayed a striking BCWms 4-fold larger than the PT BCWms (P<0.001), with both waves increasing P but decreasing U. By contrast, the DA WI profile demonstrated a prominent mid-systolic FCW, which increased both DA P and U. Main PA occlusion abolished not only the PT BCWms, but also the DA mid-systolic FCW.
Conclusion: These results suggest that transmission of the fetal PA BCWms into the DA as a FCW augments forward flow across this shunt, and thus descending aortic flow.