Response to the Article by Spaan on “Coronary Diastolic Pressure‐Flow Relation and Zero Flow Pressure Explained on the Basis of Intramyocardial Compliance” which appeared in Circ. Res. 56: 293–309, 1985

BOTH studies in the "Controversies* show that models play an important role in attempts to understand the physics of the coronary circulation. A model is a representation of and is supposed to be analogous to a different, more complex system. If the physics of the system is well understood and the boundary conditions are well defined, a model can be used as an analogous computer. Thus can the sailing characteristics of a mammoth tanker be designed by studies on a model no longer than a few meters. Unfortunately, in coronary physiology, there is as yet a dramatic lack in knowledge of anatomy and mechanical behavior of the microcirculation. Hence, models are used to test whether hypothetized mechanisms may indeed be operative at locations where observations are not yet possible. The chance that mechanisms are as hypothetized is greater, as more measured phenomena are being correctly described. Hence, an appraisal of these phenomena is important. In this respect, the two reviews are complementary. To many researchers, but especially to clinicians, this discussion about models may seem very theoretical. However, the discussion has practical implications. This is dear from the different recommendations resulting from the two different papers in these 'Controversies." According to the model based on intramyocardial compliance alone, the ratio between mean pressure and mean flow in the beating heart is an adequate index of vasomotor tone. If a waterfall element is thought to be essential, both the slope of the diastolic pressure-flow line and Pf_0 should be measured. In addition, a number of experimental requirements must be met.