Reduction of compartment compliance increases venous flow pulsatility and lowers apparent vascular compliance: implications for cerebral blood flow hemodynamics.

The global compliance of a fixed-volume, incompressible compartment may play a significant role in determining the inherent vascular compliance. For the intracranial compartment, we propose that the free-displacement of the cerebral spinal fluid (CSF) directly relates to cerebral vascular compliance. To test this hypothesis, an in vivo surrogate intracranial compartment was made by enclosing a rabbit's kidney within a rigid, fluid-filled container. Opening/closing a port atop the box modulated the free flow of box fluid (open-box state). We observed that the pulsatility of the renal venous outflow increased in response to hampering the free flow of fluid in-and-out of the container (closed-box state). To associate the observed pulsatility changes with the compliance changes, a parametric method was proposed for the computation of the apparent compliance (C(app)) of the whole renal vascular system. The calculated C(app) for each experiment's closed-box state was favorably compared to a time-domain compliance assessment method at the mean heart rate. In addition, it was revealed that C(app) in the open-box state was greater than that in the closed-box state only when the calculations were performed at frequencies lower than the heart rate and closer to the ventilation rate. These experimental results support the concept that the vessel compliance of vascular systems enclosed within a rigid compartment is a function of the global compartment compliance.