Mechanism of the effect of coronary artery stenosis on coronary flow in the dog.

The hemodynamic mechanism of the effect of coronary artery stenosis on coronary flow was studied in the circumflex artery of 10 open-chest dogs by simultaneously measuring coronary flow, aortic pressure, and coronary artery pressure distal to an adjustable constrictor; while the distal coronary bed was intermittently maximally vasodilated by intracoronary injections of angiographic contrast media (Hypaque-M, 75 per cent). For each stenosis, the pressure gradient across the stenosis varied directly with the flow through the stenosis (r equals 0.99), the slope of the regression indicating the severity of the stenosis. An important observation was that this regression line did not intercept the flow axis at zero flow, but at a positive flow, meaning that for a given regression line slope the pressure gradient was much less than expected. At rest, distal bed resistance decreased as progressive stenosis lowered the distal bed pressure, maintaining flow at control level until the distal bed pressure dropped below 60 mm. HG. However, at maximum hyperemia, distal bed resistance was at a fixed minimum value such that flow was directly proportional to distal bed pressure. Hence, progressive stenosis decreased the ratio of hyperemic to resting flow by: (1) causing the vasodilatory reserve to be used to maintain resting flow, decreasing that available for hyperemia, and (2) dropping the distal bed pressure relatively more for smaller increases in flow. This study provides a hemodynamic explanation for the known fact that progressive stenosis initially limits the maximum hyperemic flow, and only after this flow is decreased almost to resting level, does resting flow fall.