Objective: Recent studies in lambs using wave intensity (WI) analysis suggest that a characteristically low fetal lung blood flow is related to an extremely large backward-running compression wave (BCW) present in the left (LPA) and right pulmonary arteries (RPA), which is partially transmitted via the main pulmonary artery (MPA) into the pulmonary trunk (PT) as a pressure-increasing, flow-reducing wave, and into the ductus arteriosus (DA) as a pressure- and flow-increasing wave. The aim of this study was to more fully elucidate the circulatory fates of the fetal pulmonary BCW by assessing transmission and reflection properties of the LPA/RPA/MPA and MPA/PT/DA junctions.
Methods: Pressure, flow and WI reflection and transmission coefficients at the LPA/RPA/MPA and MPA/PT/DA junctions were calculated using linear transmission line theory. Vessel diameter and wave speed values required for these calculations were obtained from fetal lamb studies.
Results: LPA and RPA BCW were partially transmitted into the MPA (pressure/flow/WI coefficients: 0.5/0.92/0.24 and 0.58/0.92/0.32 respectively). The resulting MPA BCW was partially transmitted into the PT (coefficients: 0.52/1.16/0.25) and, with lesser flow and WI consequences, into the DA (coefficients: 0.52/0.33/0.15). LPA and RPA BCW were also partially re-reflected at the LPA/RPA/MPA junction as pressure-reducing waves. However, the re-reflected wave in the LPA was largely cancelled by partial transmission of the RPA BCW into the LPA, and vice versa.
Conclusion: Fetal pulmonary BCW have multiple circulatory fates, but the most haemodynamically important are transmission into the PT and DA, causing flow reduction and augmentation respectively, with pressure augmentation in both vessels.