A Korean study has found that the degree of resistance in the microvessels of the coronary arteries, which supply blood to the heart, can serve as an early warning of the risk of life-threatening cardiovascular events such as heart attacks. Researchers expect the finding to open a way to identify high-risk patients by assessing the function of microvessels themselves, which has been difficult to detect even with coronary angiography.
Samsung Medical Center said on the 11th that a joint research team of Professor Lee Joo-myung of its division of cardiology and Professors Hong Young-joon, Lee Seung-hun and Ahn Joon-ho of the division of cardiology at Chonnam National University Hospital had identified a new indicator for screening patients at risk of coronary microvascular dysfunction.
Coronary microvascular dysfunction is a condition in which the small vessels that supply blood to the heart muscle fail to work properly and cannot deliver sufficient blood flow. Even when imaging shows no clear narrowing of the large coronary arteries, the heart muscle may not receive enough blood and oxygen, or chest pain may occur. Until now, clinicians have assessed microvascular dysfunction by combining several indicators, including coronary flow reserve (CFR) and the index of microcirculatory resistance (IMR). More recently, microvascular resistance reserve (MRR), which examines the function of the small vessels themselves while excluding the influence of the large coronary arteries on the epicardium, the outermost surface of the heart, has emerged as an alternative. MRR indicates how much the coronary microvessels can lower their resistance to increase blood flow when the heart needs more blood. A lower value means microvascular function has deteriorated. But evidence had been lacking on how useful the measure is in predicting cardiovascular risk in actual patients.
The team analyzed 1,003 patients who underwent coronary angiography and blood flow function tests at seven medical institutions in Korea between April 2022 and November 2024 for suspected ischemic heart disease. Using a threshold of an MRR of 2.5 or below, microvascular dysfunction was confirmed in 33.3%, or 334, of the participants. The researchers defined major cardiovascular events as death from any cause, heart attack, repeat procedures for treatment and hospitalization for heart failure, and followed the patients for a median of 1.9 years.
Patients with microvascular dysfunction were 1.94 times more likely to experience a major cardiovascular event than those without it. The estimated rate of major cardiovascular events within two years also differed significantly, at 18.2% for patients with microvascular dysfunction and 8.6% for those with normal function. The results suggest MRR can be used not only to check the function of coronary microvessels but also to screen patients at high risk of future cardiovascular disease.
"We confirmed the possibility of assessing microvascular dysfunction far more simply with a single indicator, MRR, where several indicators had to be considered together," Professor Lee Seung-hun said. "It is highly significant that we confirmed this new assessment indicator is also linked to actual patient outcomes."
"Coronary microvascular dysfunction is hard to detect with standard coronary angiography alone, so there are cases where no clear cause is found even when a patient complains of chest pain," Professor Lee Joo-myung said. "We hope this study helps establish standards for diagnosing microvascular dysfunction more accurately and assessing patient risk."
The findings were presented in the Late-Breaking Clinical Trial session at the ESC Congress 2026, the annual meeting of the European Society of Cardiology held recently in Munich, Germany, and were published in a recent issue of the European Heart Journal.
