Noninvasive Index of Microvascular Resistance as a Predictor of Left Ventricular Performance Recovery in Patients with STEMI Undergoing Primary PCI

Background: Coronary microvascular function can predict the infarct size and Left Ventricular (LV) functional recovery in patients diagnosed with ST-Elevation Myocardial Infarction (STEMI). Multiple invasive and non-invasive methods are used to evaluate coronary microvascular function. The Non-invasive Index of Microvascular Resistance (NiMR) is a method to evaluate microvascular resistance.

Objectioves: This study aimed to assess the relationship between NiMR and Left Ventricular Ejection Fraction (LVEF) measured by Transthoracic Echocardiography (TTE) in patients with acute STEMI undergoing primary Percutaneous Coronary Intervention (PCI).

Methods: This prospective observational study was conducted on 39 patients with STEMI. NiMR was measured instantly after primary PCI. After that, the patients were divided into two groups based on their NiMR: Group 1 (n = 20) with slighter microvascular dysfunction (NiMR < 24) and Group 2 (n = 19) with more severe microvascular dysfunction (NiMR ≥ 24). In the first 24 hours (Echo1) and one month after primary PCI (Echo2), LVEF and the Global Longitudinal Strain (GLS) were measured by TTE.

Results: The mean age of the patients was 58.0 ± 11.3 years, and 34 ones (87.1%) were male. In Echo1, there were no significant differences between the two groups regarding LVEF (39.6 ± 7.8% vs. 38.8 ± 8.6%; P = 0.761) and GLS (−10.2 ± 2.5 vs. −10.9 ± 3.2; P = 0.487). However, LVEF improvement was higher in Group 1 than in Group 2 (Δ LVEF = 5.8 ± 7.3% in Group 1 vs. Δ LVEF = 1.3 ± 8% in Group 2; P = 0.073), but the difference was not statistically significant (OR: 2.8, 95% CI: 0.72 - 10.7; P = 0.13). GLS also exhibited an improvement in both study groups after a month (Δ GLS = 5.4 ± 3.1 in Group 1 vs. Δ GLS = 2.4±3.2 in Group 2; P = 0.005), but this improvement was statistically significant only in Group 1 (OR: 5.5, 95% CI: 1.32 - 22.8; P = 0.01).

Conclusions: In patients with lower NiMR values, LV systolic function recovery (defined by improvement in GLS) was significantly higher one month after STEMI. Thus, NiMR can be used as an early marker of LV performance recovery after acute STEMI.