Characterizing post-myocardial infarction microvascular obstruction by ECG: we could learn more from cardiac magnetic resonance imaging.

Despite successful restoration of blood flow of an infarct related coronary artery, a wealth of experimental and clinical data has confirmed the common existence of poor tissue perfusion. 1,2 This no-reflow phenomenon due to microvascular obstruction (MVO) is known to have serious clinical consequences including significant ventricular arrhythmias, heart failure, adverse remodeling, and death. Impairment of microcirculatory flow after reperfusion remains complex spatially and temporally. Swelling of injured myocytes and endothelial cells, infiltration and activation of neutrophils and platelets, and deposition of fibrin cause capillary obstruction and eventually rupture, with resultant debris deposition and even hemorrhage. The mechanisms that lead to formation of MVO from prolonged ischemia followed by late reperfusion are rapid. It has been suggested that each 30-minute delay in reperfusion increases the risk of MVO by 20%. 6 Challenging further understanding of the mechanistic basis of MVO, experimental setting of ischemia-reperfusion does not account for some of the clinical factors that contribute to MVO, such as distal microembolization of atherosclerotic or thrombotic debris commonly seen from percutaneous coronary intervention. All of these factors contribute to the currently incomplete understanding of the underlying pathophysiology of MVO. In addition, MVO is unfortunately common. The incidence of MVO varies widely in the literature but has been reported to exist in as high as 70% of patients presenting with an acute infarction. 7 Therefore, a lack of optimal therapy against MVO remains an obstacle in achieving the next breakthrough in improving the mortality of patients post-myocardial infarction. The need to diagnose and quantify MVO has been well known for more than 3 decades. By coronary angiography, the corrected-TIMI frame count assesses tissue perfusion by counting the number of cine frames required for contrast to reach distal coronary landmarks in the culprit artery. A high corrected-TIMI frame count at 90 minutes after administration of a thrombolytic agent is associated with adverse cardiac outcomes despite achieving angiographic TIMI flow grade 3. 8 Other angiographic parameters such as the semi-quantitative myocardial blush grade estimates tissue reperfusion by grading myocardial contrast density on the final angiogram. These semi-quantitative angiographic methods have demonstrated prognostic implication but are limited by their invasiveness and tissue perfusion can only be assessed immediately after achievement of epicardial patency, when microvascular flow is known to be hyperemic. It has been observed that there is a threefold increase in perfusion in the first few hours after emergent primary coronary intervention of the …