Dynamic changes in the haemodynamic severity of coronary artery stenosis in a canine model.

Abstract

Coronary artery stenosis diminishes and eventually abolishes the peak reactive hyperaemic response following momentary coronary occlusion. If stenosis is increased beyond this point, it is generally believed that resting coronary blood flow and reactive blood flow will continue to be identical. This assumption was investigated in 14 anaesthetised dogs. Aortic pressure, distal coronary pressure, and coronary flow were measured. Stenotic resistance was calculated as the pressure gradient across the stenosis divided by the coronary blood flow. Before coronary artery constriction, the usual large coronary blood flow increase and distal coronary resistance decrease were observed. With moderate stenosis, distal coronary resistance decreased and coronary blood flow increased, although attenuated. With severe stenosis, coronary blood flow remained unaltered, the distal coronary resistance decrease being nullified by the stenotic resistance increase. With further constriction, coronary blood flow actually decreased following momentary coronary occlusion. The coronary blood flow decrease was associated with a large stenotic resistance increase. We believe the most plausible explanation for the observed stenotic resistance increase was a passive collapse of the vessel due to the distal coronary pressure reduction. This hypothesis is strengthened by our demonstration of similar stenotic resistance increases in an in vitro, constant flow perfused carotid artery, induced by reducing the peripheral resistance. This study demonstrates that stenotic resistance is not fixed, but is dynamic. Conceivably, in the presence of severe coronary artery stenosis, vasodilatation could increase stenotic resistance and thereby reduce coronary blood flow.