Interpretation of pulsatility index in feeder arteries to low‐impedance vascular beds

Abstract

A simple electrical circuit is proposed as a model for peripheral vascular beds of low impedance. The model consists of a flow and pressure source, i.e. the 'feeder' artery, with the vascular bed represented by a Zener diode Z giving rise to a 'back pressure' P(z) and a pure resistance R. By applying Ohm's law to the definition of pulsatility index PI(flow) of flow in the feeder artery, it is shown that PI(flow) is dependent only on the mean blood pressure (input), the pulse pressure and P(z), and is independent of R. For those vascular beds where the impedance is largely resistive, e.g. the brain, the kidney and the uterine-placental complex, it is suggested that P(z) may be a more useful characterizing parameter than the value of PI(flow) as at present used by some workers.

Renal Resistive Index

“…Thus, when downstream properties (representing vascular impedance) are incorporated into mathematical or physical models, blood flow can become dependent on resistance. 4,5 Although this has been corroborated during postischemic hyperemia in the brachial artery 6 and hypercapnea in neonatal cerebral arteries, 7 the dependence of RI on vascular resistance varies considerably among vascular beds. In rabbit kidneys perfused ex vivo with increasing concentrations of phenylephrine, 8 there was only a slight increase in RI despite a broad range of resistance not observed in vivo ( Figure 1) and the inability of vasoconstrictors or vasodilators to appropriately alter RI has also been observed in the uterine, 9 carotid, 10 and retinal 11 circulations.…”

“…This independence of RI on resistance has been demonstrated in simple artificial circuits. 4,5 However, this basic analysis does not account for nonuniformity of blood velocity and, more importantly, downstream vascular properties such as compliance or capacitance. The more compliant the distal arteries are, the more flow they can accommodate during systole compared with diastole.…”

Attenuation of Blood Flow Pulsatility along the Atlas Slope: A Physiologic Property of the Distal Vertebral Artery?

“…The pulsatility index and resistance index are mainly used to describe the arterial resistance of a downstream capillary bed 25 and upstream vessel stenoses. 26 However, it is known that vascular compliance can also affect these indices.…”

Is There a Direct Link Between Cerebrovascular Activity and Cerebrospinal Fluid Pressure-Volume Compensation?

“…10 As an index of the compensatory reserve of CSF space, we determined the correlation coefficient between pulse amplitude of ICP and mean ICP (RAP). 6,8 An RAP close to 0 indicates a good compensatory reserve, whereas an RAP close to ϩ1 indicates an exhausted compensatory reserve.…”