Dynamic responsiveness of the vascular bed as a regulatory mechanism in vasomotor control

The dynamics of blood supply to a vascular bed depend on lumped mechanical properties of that bed, namely the compliance (C), resistance (R), viscoelasticity (K), and inertance (L). While the study of regulatory mechanisms has so far placed the emphasis largely on R, it is not known how the remaining properties contribute collectively to the play of dynamics in vasomotor control. To examine this question and to establish some benchmark values of these properties, simultaneous measurements of pressure and flow waveforms in the vascular bed of the forearm were obtained from three groups: young healthy individuals, older hypertensives with controlled blood pressure, and older hypertensives with uncontrolled blood pressure. The values of R and C were found to vary within a wide range in each of the three groups to the extent that neither R nor C could be used independently as an indicator of health or age of the subjects tested. However, higher level dynamic properties of the bed, such as the time constants and damping index, which depend on combinations of C,K, and L, and which may reflect measures of the dynamic responsiveness or “sluggishness” of the system, were found to be maintained over a wide range of pulse pressures. These findings support a hypothesis that the pulsatile dynamics of blood supply to a vascular bed are adapted to the individual baseline values of R and C in different subjects with the effect of optimizing the level of dynamic responsiveness to changes in pressure or flow, and that this dynamic property of the vascular bed may be a protected and/or regulated property.