The active and passive components of extravascular coronary resistance

Intramyocardial stresses compress the coronary arteries raising their resistance to flow. The present study measures the relative contribution of ventricular pressure and the contractile state of the myocardium to this extravascular component of the coronary resistance. The extravascular resistance, which was found to be as much as one third of the total coronary resistance at normal ventricular and perfusion pressures, consists of two components. The major component, which persists when myocardial tension development is prevented, is attributed to passive stresses which are in equilibrium with the ventricular pressure. It accounts for 75% of the extravascular resistance. The remaining 25% is due to a modification of these passive stresses by active contraction of the myocardium. Massive increases in contractility resulted in only an additional 19% increase in the extravascular resistance. It is concluded that changes in contractility per se have only a slight effect on the amount of compression experienced by the coronary arteries and that ventricular pressure is the major determinant of the extravascular resistance.