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

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.