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’ Pz and a pure resistance R. By applying Ohm's law to the definition of pulsatility index PIflow of flow in the feeder artery, it is shown that PIflow is dependent only on the mean blood pressure (input), the pulse pressure and Pz, 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 Pz may be a more useful characterizing parameter than the value of PIflow as at present used by some workers. Copyright © 1991 International Society of Ultrasound in Obstetrics and Gynecology