How Do Cord Compressions Affect the Umbilical Venous Flow Resistance? An In Vitro Investigation of the Biomechanical Mechanisms

Umbilical vessels, that provide blood oxygenation and fetal nourishment in utero, are encased and protected against external forces by the umbilical cord. The biomechanics of this peculiar structure has not been deeply investigated so far. The purpose of this study is to investigate the hydraulic behaviour of human umbilical veins (UV) and its changes in presence of an external cord compression. Five umbilical cords were subjected to in vitro tests. UV was accurately cannulated and connected to a perfusion circuit, while the cord was subjected to an external compression. Pressure drops across UV were measured for various venous flow rates and various degrees of cord constriction. Compressive forces were measured, too. The UV hydraulic resistances measured in unloaded cords (0.029 ± 0.016 mmHg min cm-1 L-1) correspond to placenta-abdomen pressure drops well consistent with in utero measurements. As expected, at fixed flow rate, flow resistance augments when cord is compressed. Interestingly, resistance does not substantially change until a 30-50% cord thickness reduction, whereas slightly larger constriction cause a steep increase. Compressive forces becomes critical for values above 0.5-2 N, depending on the length of cord compression and on considered specimen. Moreover, at high cord constriction, hydraulic behaviour of UV is very peculiar. Namely, the slope of the pressure-flow relationship decreases at increasing flow rates and, in few cases, a surprising reduction of pressure drop was even observed. The biomechanical behaviour of the umbilical cord during compression is very complex, with high non-linearity of venous hydraulic behaviour