Invasive Assessment of the Coronary Microvasculature : The Index of Microcirculatory Resistance

Diabetes mellitus increases the risk of adverse cardiac outcomes and is considered a coronary artery disease (CAD) equivalent. We examined whether coronary vascular dysfunction, an early manifestation of CAD, accounts for increased risk among diabetics compared with nondiabetics. A total of 2783 consecutive patients (1172 diabetics and 1611 nondiabetics) underwent quantification of coronary flow reserve (CFR; CFR=stress divided by rest myocardial blood flow) by positron emission tomography and were followed up for a median of 1.4 years (quartile 1-3, 0.7-3.2 years). The primary end point was cardiac death. Impaired CFR (below the median) was associated with an adjusted 3.2- and 4.9-fold increase in the rate of cardiac death for diabetics and nondiabetics, respectively (P=0.0004). Addition of CFR to clinical and imaging risk models improved risk discrimination for both diabetics and nondiabetics (c index, 0.77-0.79, P=0.04; 0.82-0.85, P=0.03, respectively). Diabetic patients without known CAD with impaired CFR experienced a rate of cardiac death comparable to that for nondiabetic patients with known CAD (2.8%/y versus 2.0%/y; P=0.33). Conversely, diabetics without known CAD and preserved CFR had very low annualized cardiac mortality, which was similar to patients without known CAD or diabetes mellitus and normal stress perfusion and systolic function (0.3%/y versus 0.5%/y; P=0.65). Coronary vasodilator dysfunction is a powerful, independent correlate of cardiac mortality among both diabetics and nondiabetics and provides meaningful incremental risk stratification. Among diabetic patients without CAD, those with impaired CFR have event rates comparable to those of patients with prior CAD, whereas those with preserved CFR have event rates comparable to those of nondiabetics.

More than 20% of patients presenting to the cardiac catheterization laboratory with angina have no angiographic evidence of coronary artery disease. Despite a "normal" angiogram, these patients often have persistent symptoms, recurrent hospitalizations, a poor functional status, and adverse cardiovascular outcomes, without a clear diagnosis.

The prognostic impact of microvascular status in patients with high fractional flow reserve (FFR) is not clear.