Response to Letter Regarding Article, “Prognostic Value of the Index of Microcirculatory Resistance Measured After Primary Percutaneous Coronary Intervention”
Authors: William F. Fearon, Adrian F. Low, Andy C. Yong, Ross McGeoch, Colin Berry, Maulik G. Shah, Michael Ho, Hyun-Sook Kim, Joshua P. Loh, Keith G. Oldroyd
Journal: Circulation (2014)

We thank Dr Poullis for his interest in our article.The index of microcirculatory resistance (IMR) is not a ratio like coronary flow reserve but is defined as pressure divided by flow during maximal hyperemia and therefore is a measure of the minimal achievable microvascular resistance.2][3] These studies have demonstrated that patients with higher IMR values immediately after primary percutaneous intervention for ST-segment-elevation myocardial infarction have higher levels of cardiac biomarkers, greater degrees of microvascular dysfunction on noninvasive imaging, and less recovery of left ventricular function over time.Measuring IMR in the nonculprit vessels was beyond the scope of this study.Others have shown that IMR in the nonculprit vessels at the time of ST-segment-elevation myocardial infarction is generally normal. 4Unfortunately, we do not have data on the prevalence of multivessel coronary disease in this study.Fewer than 5% of patients underwent subsequent coronary artery bypass graft surgery.The 4.3% mortality rate and the 13.8%rate of mortality or rehospitalization for heart failure at the 2.8-year follow-up are similar to or lower than the rates reported in other contemporary studies in this population.IMR at the time of ST-segmentelevation myocardial infarction does not distinguish between acute and chronic changes in the microvasculature.However, presumably in most cases, an elevated IMR is attributable to an acute change, and in either case, it is predictive of outcomes.IMR was compared with Thrombolysis in Myocardial Infarction (TIMI) myocardial perfusion grade in a previous article we published, which showed that TIMI myocardial perfusion grade did not correlate with cardiac biomarker elevation or improvement in left ventricular function, whereas IMR did. 1 The relationship between fractional flow reserve (FFR) and outcomes in this article is another very interesting finding and shows the independent contribution of both epicardial and microvascular dysfunction to adverse outcomes.The lack of inclusion of central venous pressure in the formula for calculating FFR in the setting of ST-segment-elevation myocardial infarction can lead to a higher FFR value in patients with very high central venous pressure.However, despite its exclusion, FFR was still predictive of adverse events.Moreover, other studies evaluating FFR in patients with acute coronary syndromes have not included central venous pressure measurements in the calculation. 4e did not include predictors of adverse outcomes after coronary artery bypass graft surgery in our analyses because the patients in this study were not undergoing coronary artery bypass graft surgery.The same applies to performance status and ejection fraction before the presentation.We disagree that further comparison between IMR, TIMI myocardial perfusion grade, and FFR is necessary before adoption.We do believe that IMR needs to be tested in the setting of a therapeutic intervention, as outlined by De Bruyne and Barbato 5 in the editorial accompanying the article.