Novel Imaging Strategies for Predicting Remodeling and Evolution of Heart Failure: Targeting the Renin-angiotensin System

Myocardial Viability: Survival Mechanisms and Molecular Imaging Targets in Acute and Chronic Ischemia

Myocardial responses to acute ischemia/reperfusion and to chronic ischemic conditions have been studied extensively at all levels of organization. These include subcellular (eg, mitochondria in vitro); intact, large animal models (eg, swine with chronic coronary stenosis); as well as human subjects. Investigations in humans have used positron emission tomographic metabolic and myocardial blood flow measurements, assessment of gene expression and anatomic description of myocardium obtained at the time of coronary artery revascularization, ventricular assist device placement, or heart transplantation. A multitude of genetic, molecular, and metabolic pathways have been identified, which may promote either myocyte survival or death or, most interestingly, both. Many of these potential mediators in both acute ischemia/reperfusion and adaptations to chronic ischemic conditions involve the mitochondria, which play a central role in cellular energy production and homeostasis. The present review is focused on operative survival mechanisms and potential myocardial viability molecular imaging targets in acute and chronic ischemia, especially those which impact mitochondrial function.

Imaging left ventricular remodeling: targeting the neurohumoral axis

Left ventricular remodeling is a key determinant of the clinical course and outcome of systolic heart failure. The myocardial renin-angiotensin system (RAS) has been closely linked to the major maladaptive cellular and molecular changes that accompany left ventricular remodeling. Direct inhibition of various components of the RAS, such as the angiotensin-converting enzyme, angiotensin II type 1 receptor, and aldosterone, has resulted in favorable clinical responses in heart failure. Many questions, however, remain unanswered regarding the timing of initiation, optimum doses, need for simultaneous use of RAS inhibitors, and proper monitoring of RAS blockade. Additionally, significant variation has been noted in individual responses to RAS blockade as a result of genetic differences. Answering these questions requires direct access to the myocardial component of RAS, which is largely independent of its systemic component. Molecular imaging using radiotracers with high affinities for myocardial angiotensin-converting enzyme and angiotensin II type 1 receptors can provide direct access to tissue RAS and thus provide a better understanding of the pathophysiology of left ventricular remodeling in individual patients. This Article briefly reviews the potential for evaluating the tissue expression of angiotensin in heart failure by targeted RAS imaging.