Myocardial hibernation and stunning: from physiological principles to clinical practice.

In patients with coronary artery disease, one of the most powerful determinants of prognosis is left ventricular function. 2 Thus, if we assume that the prognostic benefit of revascularisation is mediated through an eVect on left ventricular function, two mechanisms might explain this benefit. First, coronary artery bypass surgery or angioplasty could reduce the impact of coronary artery disease on the attrition of left ventricular function—in other words, left ventricular function deteriorates more slowly following revascularisation. Second, revascularisation could improve left ventricular function per se; this mechanism assumes that there must be areas of the heart that are alive but not contracting properly, and that revascularisation will improve this contractile dysfunction. Thus impairment of left ventricular function does not necessarily mean that the myocardium is dead, but rather that it may be alive though with reduced function. This has stimulated much research into the mechanisms of reversible left ventricular dysfunction and how best to identify it. Traditionally, presurgical investigation of patients with ischaemic heart disease has involved an exercise test and coronary angiography, and so concentrates on the first mechanism while largely ignoring the concept of reversible left ventricular dysfunction, despite its documented potential importance. In unselected patients with left ventricular dysfunction (ejection fraction less than 30%) undergoing surgical revascularisation, ejection fraction at six months can improve by up to 50%. In patients with an ejection fraction of less than 40%, the presence of a mismatch between perfusion and viability predicted improvement in heart failure after revascularisation; furthermore, in patients treated medically, the presence of mismatch was associated with worse survival than in those without mismatch but with a similar initial ejection fraction. Thus the identification of these patients may be of clinical importance and, if so, requires some understanding of the physiological processes underlying reversible left ventricular dysfunction. The aims of this paper are to outline some of the physiological principles and controversies underlying reversible contractile dysfunction and to highlight current knowledge on how best to identify patients who might derive the most benefit from revascularisation. Causes of reversible left ventricular dysfunction From experimental studies, two main processes have been described, distinguished on the basis of resting myocardial blood flow. The concept of myocardial “hibernation” was introduced nearly two decades ago to describe impaired left ventricular function that improves following revascularisation.The original definition did not specify whether there was an abnormality in resting myocardial blood flow; however, it became widely accepted that hibernation was the result of reduced resting myocardial blood flow. 6 Another form of