Steady state and instantaneous pressure-flow relationships: characterisation of the canine abdominal periphery

Abstract

This study was performed to characterise a vascular bed in terms of pressure-flow relationships. Steady state and instantaneous relationships were obtained in the flow perfused isolated femoral beds of six mongrel dogs. The steady state pressure-flow relations were obtained by applying a series of stepwise changes of flow in random order. The relations were found to be straight and to have a zero-flow pressure intercept (P₀). The slope of this relation is the differential resistance (R_d). On each steady state flow level a ramp-flow was superimposed. The pressure response was measured between 1.5 and 5 s after the start of the ramp-flow, to exclude compliance effects and (auto) regulatory effects, respectively. In this way instantaneous pressure-flow relations were obtained, the slope of this relation is the instantaneous resistance (R_i). The instantaneous resistance expresses the true physical resistance value at a working point of the steady state pressure-flow relation before the bed has performed its (auto) regulatory adaptation after a change in flow. Instantaneous resistance therefore characterises the vascular state that exists at that particular working point. After this particular vascular state has been modified by (auto)-regulation the steady state pressure-flow relation is reached again. Instantaneous resistance increases with increasing flow thereby approximating the value of the differential resistance. At the same flow a vasodilator decreases and a vasoconstrictor increases instantaneous resistance. The gain (G) of the system, that characterises the (auto)regulatory capability, was calculated as G = 1 − R_i/R_d and was found to decrease with increasing flow. The (partial) reflection of travelling waves depends on both the characteristic impedance (Z_c) and the instantaneous resistance rather than on differential or peripheral resistance. Furthermore it is the product of the instantaneous resistance (R_i) and vascular compliance (C) that determines the time constant of a vascular bed.