Magnetic Resonance Imaging, Myocardial Infarction and Development of Heart Failure.

Introduction. Cardiovascular disease is responsible for 30 % of worldwide mortality, accounting for approximately 15 million deaths per year. Improvement in medical treatment strategies during the last decades has produced a tremendous decrease in mortality in connection with acute myocardial infarction (AMI). Since the 1960's, short-term mortality (30 days) has decreased from approximately 30 % to the current mortality rate of 6,5%. The success of modern treatment of AMI has, however, led to an increasing number of patients surviving AMI, thus creating a growing group of high-risk patients that need further treatment and care. The development of heart failure and the risk of recurrent ischemia and reinfarction are the two main threats to this population. To further improve the treatment and outcomes in this high risk population, early risk stratification based on a thorough understanding of the operating mechanisms behind the transition from acute infarction to heart failure is necessary.

AMI, reperfusion and microvascular obstruction. Reperfusion therapy has been one of the major successes in the treatment of AMI, and there are numerous studies to support the idea of opening occluded coronary arteries, especially in the context of an AMI. However, even in the presence of a patent infarct related artery, there may still be inadequate reperfusion at the tissue level. This phenomenon, known as no-reflow, may preclude optimal reperfusion because of microvascular obstruction. It is estimated that microvascular obstruction occurs in 30-40% of all revascularised patients in spite of a patent artery; the exact underlying pathophysiological mechanisms are partially unknown. Microvascular obstruction may possibly be due to sequestration of neutrophils in the microvasculature that subsequently lead to microvascular occlusion by erythrocytes, leucocytes and cellular debris. It remains unclear whether the stimulus for the development of microvascular obstruction originates during coronary occlusion exclusively or if reperfusion plays an active role in progression of the phenomenon.

Microvascular obstruction and heart failure. The presence of microvascular obstruction following AMI, predicts unfavourable postinfarction prognosis and development of left ventricular dysfunction and left ventricular remodelling. Left ventricular remodelling is associated with development of heart failure and is directly related to the magnitude of microvascular obstruction early after experimental and clinical AMI1, as well as 6 months after the acute event. The mechanisms by which microvascular obstruction induces ventricular remodelling remain unknown. Possibilities include the potentiation of wall thinning and infarct expansion early after infarction, as well as potential impairment of infarct healing, given the association between presence of microvascular obstruction and greater transmural scar formation 6 months after AMI.

Assessment of microvascular obstruction. Until recently, microvascular obstruction could only be assessed by methods such as radioactive micro-spheres and other histopathological techniques, that only could be performed at the terminal phase of an experimental study, and not clinically. However, recent advances in the field of non-invasive cardiac imaging have enabled the serial assessment of this phenomenon by cardiac magnetic resonance imaging (CMR), thereby facilitating a much greater understanding of its pathophysiological and prognostic significance.

Assessment of left ventricular remodelling by CMR. The pathologic changes associated with the development of heart failure include changes in geometry and function, myocytes and extracellular matrix. Assessment of left ventricular remodelling includes estimation of left ventricular size and shape, left ventricular mass and a functional assessment including an estimation of ejection fraction.