Myocardial compression and recoil as determinants of coronary blood flow.

Coronary blood flow is conventionally analysed as a continuous flow of blood through a tube with the input of energy in the epicardial coronary arteries and the principal resistance to flow in the arterioles and the small arteries. This model has been studied in detail and is accepted by all expert groups. However, this essay argues that this model is valid but incomplete. Coronary blood flow is not continuous. Coronary blood flow is phasic and asynchronous. During systole, at rest, there is no forward flow to the left ventricle in the epicardial coronary arteries. But there is forward flow in systole in the coronary veins. The blood forced during systole into the coronary veins is expelled from the coronary microcirculation by myocardial contraction and represents the compressible volume of the coronary microcirculation. During diastole, inflow of blood from the epicardial coronary artery begins abruptly, accelerated by myocardial recoil, refilling the compressible volume of the coronary microcirculation, and then flowing through the coronary vein. Accordingly, the hydrostatic energy in the epicardial coronary artery is not the only energy propelling blood through the coronary circulation. Myocardial compression and recoil also contribute. Disorders of myocardial compression and myocardial recoil in diastole should be considered in the differential diagnosis of disorders of the microcirculation. Thus, reduced coronary flow reserve in heart failure with preserved ejection fraction might be a consequence, rather than a cause, of myocardial dysfunction.