A model based on the waterfall mechanism has been suggested to explain the relationship between pressure and flow in the non-autoregulating coronary arterial bed.1 We used the hydraulic waterfall model to study both the mean and the oscillatory pressure-flow relations. The hydraulic model consisted of a narrow thinwalled collapsible tube (diameter 1.4 mm) contained in a pressurised box (external pressure) to mimic intramyocardial pressure. The mean perfusion pressure-mean flow relation for a constant external pressure was straight with a slope equal to the (Poiseuille) resistance of the tube and an intercept determined by the external pressure (zero flow pressure). For oscillatory external pressure the mean pressure-mean flow relation was convex to the pressure axis at low flows and straight for high flows. The slope of the straight section was again equal to the tube's Poiseuille resistance. When perfusion pressure was sufficiently low and external pressure oscillated "systolic coronary flow" reversed in systole; when flow was stopped pressure remained oscillatory and above zero. The relationship between pulsatile perfusion pressure and pulsatile flow amplitudes was also described by the (Poiseuille) resistance of the collapsible tube. It is thus concluded that the hydraulic waterfall is a valuable model of the coronary bed.
The electrical analogue of the waterfall introduced by Downey and Kirk1 does not describe the experimental observations on oscillatory phenomena as pointed out by Spaan et al. 2 These investigators therefore suggested an electrical analogue that predicts these observations correctly. However, this model does not exhibit a zero flow pressure. One of the model's components arises from neglecting this intercept, and should be replaced by a resistor that varies such that an intercept pressure results. Both electrical models thus seem to be of limited use and a new model is suggested.