The “slow coronary flow” phenomenon: Evidence of preserved coronary flow reserve despite increased resting microvascular resistances

Introduction The expression “slow coronary flow phenomenon” (SCFP) indicates a slow progression of the contrast seen at the coronary angiography in the absence of epicardial stenosis and/or of other conditions associated with decreased coronary flow velocity. While microvascular abnormalities are suspected to underlie the mechanism of SCFP, they have never been directly demonstrated. Methods and results Fifteen anginal patients with a positive stress test and no evidence of epicardial lesions (obstructive coronary artery disease, coronary ectasia, or coronary spasm) were enrolled. In eight patients, the diagnosis of SCFP was made (TIMI frame count > average + 2SD). All subjects underwent measurement of the coronary flow reserve (CFR) and the index of microvascular resistance (IMR) using an intracoronary thermodilution method (RADI medical systems). There was no difference between groups in age, cardiovascular risk factors, blood pressure and heart rate, coronary artery diameter and fractional flow reserve (an index of the presence of epicardial stenosis). At rest, microvascular resistances (mean transit time × distal pressure) were significantly higher in the SCFP group (SCFP: 104 ± 31 versus 53 ± 27, P < 0.01). Showing normal responsiveness to vasodilators, this difference was abolished after induction of hyperemia (SCFP group: 34 ± 22; control: 22 ± 15, P = ns); coronary flow reserve was normal in the subjects with the SCFP (3.6 ± 1.6). Conclusions We provide the first human in vivo evidence that resting microvascular resistances are increased in patients with the SCFP. At the same time, showing an intact capacity to vasodilate, microvascular resistances were normal during hyperemia, and coronary flow reserve was not impaired in SCFP patients.