Problems of coronary flow reserve.

Coronary flow reserve is used to aid understanding why myocardial oxygen consumption may fail to meet demand. Its general aspects are well known, but the problems of using it are not. This manuscript describes three important factors that need to be considered when assessing coronary flow reserve. (1) Maximal flow is usually achieved by giving either increasing doses or else what is thought to be a maximal dose of a vasodilator, or by examining peak reactive hyperemia. Evidence that both these approaches are flawed is provided. (2) Existing methods in humans allow only total reserve to be determined, but this might be inadequate because changes in total reserve might not reflect changes in subendocardial flow reserve. (3) Because there is marked heterogeneity of flow reserve in the left ventricle, measuring total flow reserve does not indicate when small regions are becoming ischemic. More basic research is needed to overcome these difficulties.