Microvascular resistance reserve (MRR) measured directly after primary PCI was an independent predictor of all-cause mortality or hospitalization for heart failure (HF) in patients with a STEMI over long-term follow-up, according to the results of a pooled analysis by Rob Eerdekens, MD, MSc, et al., which the authors presented during EuroPCR 2024 and simultaneously published in JACC.

The analysis examined individual patient data from six cohorts totaling 446 patients (average age, 61.6; 82.1% men) with STEMI who had both their index of microcirculatory resistance (IMR) recorded directly after primary PCI and had their MRR available to be calculated. Median IMR was 32 (19-54; p<0.001) and MRR was 1.8 (1.4-2.6; p<0.001). Researchers found that the optimal cutoff value to predict the primary outcome was 1.25. MRR was ≤1.25 in 19.7% of patients (the low MRR group) and was >1.25 in 80.3% (the high MRR group).

Results showed that during a mean follow-up of 3.8 years, the primary endpoint, the composite of all-cause mortality or HF hospitalization, occurred in 27.3% of patients in the low MRR group and 5.9% of patients in the high MRR group (hazard ratio, HR, 4.16; 95% CI, 2.31-7.50; p<0.001). The composite of cardiac death or HF hospitalization occurred in 14.8% and 1.4% of patients in the low and high MRR groups, respectively (HR, 10.08; 95% CI, 3.59-28.29; p<0.001.) MRR, like IMR, was an independent predictor of the primary composite endpoint (MRR [per 1 increase]: HR, 0.48; 95% CI, 0.30-0.78; p=0.003).

"These observations highlight the importance of invasive coronary physiology assessment immediately after primary PCI," Michel Zeitouni, MD, PhD; Ghilas Rahoual, MD; and Gilles Montalescot, MD, PhD, of the ACTION study group, write in an accompanying editorial comment. "First, MRR offers a direct and reliable evaluation of coronary microvascular function after acute mechanical reperfusion, irrespective of epicardial flow. Second, the additional prognostic value of MRR enables physicians to estimate the potential recovery of the infarct territory. Third, MRR facilitates immediate risk stratification, to consider more intensive cardioprotective therapies."

One limitation of the study, the ACTION study group notes, is that the MRR was measured by bolus thermodilution, which, "has shown poor reproducibility and tends to overestimate MRR and coronary flow reserve" compared with continuous thermodilution.

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