[Reversible dysfunction of the left ventricle in coronary disease (part two): hibernation and methods for detection of viability].

HIBERNATION

Hibernating myocardium is defined as a state of persistently impaired myocardial function, as a consequence of reduced coronary flow, which can be partially or completely reversed if the myocardial oxygen consumption/demand ratio is favorably altered. Since it indicates concordance between flow and function (flow-function relation), it can be concluded that hibernating myocardium, caused by reduced myocardial perfusion, improves its function after surgical revascularization, giving its detection a great clinical importance. Hibernating myocardium can be found in majority of patients with coronary artery disease and chronic left ventricular dysfunction. These patients, even without typical symptoms of angina, will benefit from myocardial revascularization. This beneficial effect is expressed by ejection fraction enhancement, which is directly proportional to the number of dysfunctional, but viable segments. Also, symptom improvement depends on the mass of revascularized myocardium, which is previously shown to be viable. Having that in mind, the mass of viable myocardium must be large enough, so that the degree of expected improvement of myocardial function after revascularization justifies the operation itself. Opposing this classical concept of hibernating myocardium, recent studies have shown that in patients with coronary artery disease, coronary flow at rest is normal or just slightly reduced, which cannot explain the degree of myocardial dysfunction. According to that, it is proposed that myocardial dysfunction is, like in myocardial stunning, the result of flow-function mismatch, meaning that pathophysiology of hibernating myocardium includes a component of stunning as well. Therefore, hibernating myocardium can be defined as a form of reversible left ventricular dysfunction, caused by chronic coronary artery disease, which is partially due to episodes of repetitive stunning and shows improvement after inotropic stimulation. From practical point of view, it is important to detect hibernating myocardium in all patients with coronary artery disease and left ventricular dysfunction, since their treatment and prognosis directly depend on whether the dysfunction is reversible or not. METHODS FOR IDENTIFICATION OF MYOCARDIAL VIABILITY: Detection of myocardial viability has great clinical importance, since both regional and global left ventricular function can significantly improve, either spontaneously or by myocardial revascularization. Noninvasive imaging procedures used for that purpose include positron emission tomography, thallium-201 imaging, technetium-99 imaging, dobutamine echocardiography and tissue characterization. Using these methods, it is possible to assess the presence of viable tissue through evaluation of metabolic activity, integrity of myocyte membrane and the inotropic reserve of myocardium.