Epicardial inflow versus myocardial distribution: average regional transmural coronary flow is not enough

The collaborative paper from the multinational ILIAS Registry on hyperaemic stenosis resistance (HSR) by Boerhout et al in this issue of EuroIntervention1 provides the occasion to discuss a key distinction between flow in the epicardial coronary arteries versus its subsequent distribution across the layers of the myocardium. In short, measures of regional average transmural perfusion do not capture clinically essential information for managing many patients.

An important mechanistic paper from over 50 years ago offers insights regarding transmural flow distribution across the left ventricular (LV) wall2. In an animal model, the investigators studied simultaneous epicardial blood flow and its subendocardial versus subepicardial distribution. Across a range of perturbations, coronary flow increased or decreased relative to baseline. For each condition, the relative ratio of subendocardial to subepicardial blood flow was computed in tissue samples from the inner third to outer third circumferentially across the LV wall. Figure 1 plots data from Table 1 of their manuscript, demonstrating two major points.

First, when coronary blood flow decreases below baseline levels, subendocardial flow always decreases faster than...