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

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 (Rd). 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 (Ri). 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--Ri/Rd and was found to decrease with increasing flow. The (partial) reflection of travelling waves depends on both the characteristic impedance (Zc) and the instantaneous resistance rather than on differential or peripheral resistance. Furthermore it is the product of the instantaneous resistance (Ri) and vascular compliance (C) that determines the time constant of a vascular bed.