Myocardial Hibernation

Myocardial Viability Testing in the Management of Ischemic Heart Failure

Although major advances have occurred lately in medical therapy, ischemic heart failure remains an important cause of death and disability. Viable myocardium represents a cause of reversible ischemic left ventricular dysfunction. Coronary revascularization may improve left ventricular function and prognosis in patients with viable myocardium. Although patients with impaired left ventricular function and multi-vessel coronary artery disease benefit the most from revascularization, they are at high risk of complications related to revascularization procedure. An important element in selecting the patients for myocardial revascularization is the presence of the viable myocardium. Multiple imaging modalities can assess myocardial viability and predict functional improvement after revascularization, with dobutamine stress echocardiography, nuclear imaging tests and magnetic resonance imaging being the most frequently used. However, the role of myocardial viability testing in the management of patients with ischemic heart failure is still controversial due to the failure of randomized controlled trials of revascularization to reveal clear benefits of viability testing. This review summarizes the current knowledge regarding the concept of viable myocardium, depicts the role and tools for viability testing, discusses the research involving this topic and the controversies related to the utility of myocardial viability testing and provides a patient-centered approach for clinical practice.

Estrogen deficiency compromised the β2AR-Gs/Gi coupling: implications for arrhythmia and cardiac injury

Estrogen and β₂-adrenergic receptors (β₂AR) play important roles in the processes that protect the heart. Here, we investigated how ovariectomy influenced the β₂AR downstream pathways in the context of catecholaminergic stress. In vivo and in vitro stress models were developed in female Sprague-Dawley (SD) rats by epinephrine (Epi) treatments. The cardiac function was evaluated at in vivo and in vitro levels in terms of contraction, rhythm, and injury. We found that myocardial contractility was not significantly different between Sham and ovariectomized (OVX) group rats in the normal state. However, Epi pretreatment decreased the contractility and increased abnormal rhythms especially in OVX group, which were attributed to lack of estrogen. Inhibition of the β₂AR-Gi-PI3K/p38MAPK pathway with ICI118,551, PTX or LY294002 increased contractility and aggravated Epi-induced injury on cardiomyocytes, decreased p38MAPK phosphorylation, and only increased arrhythmia in Sham group. These results indicated that OVX exacerbated cardiac injury and abnormal rhythms through β₂AR-Gi-PI3K and β₂AR-Gi-p38MAPK pathways, respectively. In normal state, the levels of activated Gi were similar in both groups, but those of cAMP and activated Gs were higher in OVX group. Epi treatment increased activated Gi (especially in Sham group) and activated Gs and cAMP in Sham group but decreased it in OVX group. These results suggested that estrogen increased the Gi activity in normal and stress states and Gs activity in stress state. These results indicated that lack of estrogen impaired the β₂AR-Gs/Gi coupling during stress which compromised cardiac contractility and increased abnormal rhythms.

Recovery of hibernating myocardium: what is the role of surgical revascularization?

Myocardial responses to chronic ischemia represent a continuum of adaptations resulting, over time, in a stress-resistant phenotype. One such adaptation, hibernating myocardium (HM), has increased antioxidant capacity that protects against ischemia-induced oxidative stress. Studies have suggested that revascularization alone may not fully restore cardiac function, highlighting the need for targeted therapies to serve as adjuncts to the innate healing process following revascularization. In our review, we discuss current understanding of HM and the recovery process following surgical revascularization, focusing on animal models of HM to understand implications for human patients.

Mechanisms of Cell Survival in Myocardial Hibernation

Myocardial hibernation represents a condition of regional ventricular dysfunction in patients with chronic coronary artery disease, which reverses gradually after revascularization. The precise mechanism mediating the regional dysfunction is still debated. One hypothesis suggests that chronic hypoperfusion results in a self-protecting downregulation in myocardial function and metabolism to match the decreased oxygen supply. An alternative hypothesis suggests that the myocardium is subject to repetitive episodes of ischemic dysfunction resulting from an imbalance between myocardial metabolic demand and supply that eventually creates a sustained depression of contractility. It is generally agreed that hibernating myocardium is submitted repeatedly to ischemic stress, and therefore one question persists: how do myocytes survive in the setting of chronic ischemia? The hallmark of hibernating myocardium is a maintained viability of the dysfunctional myocardium which relies on an increased uptake of glucose. We propose that, in addition to this metabolic adjustment, there must be molecular switches that confer resistance to ischemia in hibernating myocardium. Such mechanisms include the activation of a genomic program of cell survival as well as autophagy. These protective mechanisms are induced by ischemia and remain activated chronically as long as either sustained or intermittent ischemia persists.

Myocardial hibernation: a delicate balance

The pathophysiology of myocardial hibernation is characterized as a situation of reduced regional contractile function distal to a coronary artery stenosis that recovers after removal of the coronary stenosis. A subacute "downregulation" of contractile function in response to reduced regional myocardial blood flow exists, which normalizes regional energy and substrate metabolism but does not persist for more than 12-24 h. Chronic hibernation develops in response to one or more episodes of myocardial ischemia-reperfusion, possibly progressing from repetitive stunning with normal blood flow to hibernation with reduced blood flow. An upregulation of a protective gene program is seen in hibernating myocardium, putting it into the context of preconditioning. The morphology of hibernating myocardium is characterized by both adaptive and degenerative features.