Lessons from experimental models of hibernating myocardium

Myocardial Viability Assessment Before Surgical Revascularization in Ischemic Cardiomyopathy: JACC Review Topic of the Week

Ischemic cardiomyopathy results from the combination of scar with fibrosis replacement and areas of dysfunctional but viable myocardium that may improve contractile function with revascularization. Observational studies reported that only patients with substantial amounts of myocardial viability had better outcomes following surgical revascularization. Accordingly, dedicated noninvasive techniques have evolved to quantify viable myocardium with the objective of selecting patients for this form of therapeutic intervention. However, prospective trials have not confirmed the interaction between myocardial viability and the treatment effect of revascularization. Furthermore, recent observations indicate that recovery of left ventricular function is not the principal mechanism by which surgical revascularization improves prognosis. In this paper, the authors describe a more contemporary application of viability testing that is founded on the alternative concept that the main goal of surgical revascularization is to prevent further damage by protecting the residual viable myocardium from subsequent acute coronary events.

Long-term preservation of myocardial energetic in chronic hibernating myocardium

We previously reported that the myocardial energetic state, as defined by the ratio of phosphocreatine to ATP (PCr/ATP), was preserved at baseline (BL) in a swine model of chronic myocardial ischemia with mild reduction of myocardial blood flow (MBF) 10 wk after the placement of an external constrictor on the left anterior descending coronary artery. It remains to be seen whether this stable energetic state is maintained at a longer-term follow-up. Hibernating myocardium (HB) was created in minipigs (n = 7) by the placement of an external constrictor (1.25 mm internal diameter) on the left anterior descending coronary artery. Function was assessed with MRI at regular intervals until 6 mo. At 6 mo, myocardial energetic in the HB was assessed by (31)P-magnetic resonance spectrometry and myocardial oxygenation was examined from the deoxymyoglobin signal using (1)H-magnetic resonance spectrometry during BL, coronary vasodilation with adenosine, and high cardiac workload with dopamine and dobutamine (DpDb). MBF was measured with radiolabeled microspheres. At BL, systolic thickening fraction was significantly lower in the HB compared with remote region (34.4 ± 9.4 vs. 50.1 ± 10.7, P = 0.006). This was associated with a decreased MBF in the HB compared with the remote region (0.73 ± 0.08 vs. 0.97 ± 0.07 ml · min(-1) · g, P = 0.03). The HB PCr/ATP at BL was normal. DpDb resulted in a significant increase in rate pressure product, which caused a twofold increase in MBF in the HB and a threefold increase in the remote region. The systolic thickening fraction increased with DpDb, which was significantly higher in the remote region than HB (P < 0.05). The high cardiac workload was associated with a significant reduction in the HB PCr/ATP (P < 0.02), but this response was similar to normal myocardium. Thus HB has stable BL myocardial energetic despite the reduction MBF and regional left ventricular function. More importantly, HB has a reduced contractile reserve but has a similar energetic response to high cardiac workload like normal myocardium.

Positron emission tomography for assessment of viability

PURPOSE OF REVIEW

The recent success of magnetic resonance imaging for viability assessment has raised questions about the future role of positron emission tomography and older imaging modalities in the assessment of viability. Recent information, however, indicates that positron emission tomography will remain a valuable tool.

RECENT FINDINGS

The primary positron emission tomography tracer used for assessment of viability is 18F-fluorodeoxyglucose, a glucose analogue that exhibits enhanced uptake in ischemic tissue. The finding of enhanced 18F-fluorodeoxyglucose uptake and a relative reduction in perfusion is considered the positron emission tomography correlate of myocardial hibernation. The mismatch pattern has been shown to identify patients with improvement in systolic function, heart failure symptoms, and prognosis with revascularization. Mismatch identifies a subset of patients with vulnerable myocardium who have a higher likelihood of a cardiac event compared with those without significant mismatch. Delay in revascularization may pose extra risk for those with mismatch. Positron emission tomography and magnetic resonance imaging demonstrate a close correlation in the detection of viable myocardium. The development of combined positron emission tomography/computed tomography scanners can reduce imaging time and improve functional-anatomic correlations.

SUMMARY

Positron emission tomography imaging utilizing 18F-fluorodeoxyglucose and perfusion tracers provides valuable diagnostic and prognostic information in patients with ischemic left ventricular dysfunction and has comparable accuracy to competing technologies for detection of viability.

Mechanisms leading to reversible mechanical dysfunction in severe CAD: alternatives to myocardial stunning

Patients with severe chronic coronary artery disease (CAD) exhibit a highly altered myocardial pattern of perfusion, metabolism, and mechanical performance. In this context, the diagnosis of stunning remains elusive not only because of methodological and logistic considerations, but also because of the pathophysiological characteristics of the myocardium of these patients. In addition, a number of alternative pathophysiological mechanisms may act by mimicking the functional manifestations usually attributed to stunning. The present review describes three mechanisms that could theoretically lead to reversible mechanical dysfunction in these patients: myocardial wall stress, the tethering effect, and myocardial expression and release of auto- and paracrine agents. Attention is focused on the role of these mechanisms in scintigraphically “normal” regions (i.e., regions usually showing normal perfusion, glucose metabolism, and cellular integrity as assessed by nuclear imaging techniques), in which stunning is usually considered, but these mechanisms could also operate throughout the viable myocardium. We hypothesize that reversion of these three mechanisms could partially explain the unexpected functional benefit after reperfusion recently highlighted by high-spatial-resolution imaging techniques.

Prediction of arrhythmic events with positron emission tomography: PAREPET study design and methods

BACKGROUND

In medically-treated patients with ischemic cardiomyopathy, myocardial viability is associated with a worse prognosis than scar. The risk is especially great with hibernating myocardium (chronic regional dysfunction with reduced resting flow), and the excess mortality appears to be due to sudden cardiac death (SCD). Hibernating myocardium also results in sympathetic nerve dysfunction, which has been independently associated with risk of SCD.

OBJECTIVES

PAREPET is a prospective, observational cohort study funded by NHLBI. It is designed to determine whether hibernating myocardium and/or inhomogeneity of sympathetic innervation by positron emission tomography imaging identifies patients with ischemic cardiomyopathy who are at high risk for SCD and cardiovascular mortality.

METHODS

Patients with documented ischemic cardiomyopathy, an ejection fraction of <or=35%, and with no plans for coronary revascularization will be recruited. Major exclusion criteria include: history of resuscitated SCD, sustained VT, ICD discharge, or unexplained syncope; recent myocardial infarction (30 days), percutaneous coronary intervention (3 months), coronary bypass surgery (1 year); or comorbidities that would be expected to reduce life expectancy to <2 years. All patients will undergo transthoracic echocardiography, and dynamic cardiac positron emission tomography to quantify resting perfusion (13N-ammonia), norepinephrine uptake as an index of sympathetic innervation (11C-meta-hydroxyephedrine), and metabolic viability (18F-2-deoxyglucose during glucose-insulin clamp). The development of SCD or cardiovascular mortality will be determined by telephone follow-up every three months. In patients with an implantable cardiac defibrillator, appropriate device discharge will be considered a surrogate for SCD.

CONCLUSION

The PAREPET study will prospectively determine whether the amount of viable dysfunction myocardium and/or cardiac sympathetic dysinnervation is associated with the risk of SCD. It is anticipated that the results of this trial will more specifically identify myocardial substrates of SCD. This will help target therapies intended to reduce arrhythmic death to those patients with the greatest likelihood of benefit.

Hibernation, Stunning, and Preconditioning: Historical Perspective, Current Concepts, Clinical Applications, and Future Implications

Despite considerable advances, coronary artery disease is the leading cause of morbidity and mortality in the Western world. The development of effective therapeutic strategies for protecting the myocardium from ischemia would have major impact on patients with coronary artery disease. It is now accepted that patients with coronary artery disease can experience prolonged regional ischemic dysfunction that does not necessarily arise from irreversible tissue damage, and to some extent, can be reversed by restoration of blood flow. The initial stages of dysfunction are probably caused by chronic stunning that can be reversed after revascularization, resulting in rapid and complete functional recovery. On the other hand, the more advanced stages of dysfunction likely correspond to chronic hibernation. After revascularization, functional recovery will probably be quite delayed and mostly incomplete. Over the past decade, the possibility that an innate mechanism of myocardial protection might be inducible in the human heart has generated considerable excitement. In the last two decades, there was phenomenal growth in the understanding of the mechanism known as ischemic preconditioning that is responsible for the innate myocardial protection. Continued research and progress in this area may soon lead to the availability of preconditioning-mimetic treatments. The current concepts, mechanisms, and potential clinical applications of myocardial hibernation, stunning, and ischemic preconditioning are reviewed.