What are hibernating and stunned myocardium?

What echocardiographic techniques are useful for detecting them? How do these methods compare with others available?

The physiologic abnormalities that are associated with resting myocardial dysfunction and viable myocardium range from reduced resting myocardial flow and preserved metabolic uptake of 18F-2-Deoxyglucose (FDG) (hibernating myocardium) to patients in whom resting myocardial flow is preserved (stunned myocardium). Animal studies1 suggest that stunning may progress to hibernation as part of an adaptive response. As coronary flow reserve decreases, fasting FDG uptake increases
while resting flow remains normal (chronic stunning). Later on, during continuing ischaemia, flow is reduced while FDG uptake continues, characteristic of hibernation.

Assessing myocardial viability is important in coronary artery disease patients with ventricular dysfunction because its presence improves left ventricular function and survival
following revascularisation.2,3 Diagnostic methods include positron emission tomography (PET), based on the detection of metabolic activity, 201Tl single-photon emission computed tomography (Tl-SPECT), to assess cell membrane integrity by rest/redistribution and the assessment of contractile reserve by dobutamine stress chocardiography. Echocardiography can assess the presence of myocardial viability by looking at contractile reserve following inotropic stimulation with dobutamine (dobutamine stress echocardiography). This differentiates viable myocardium (presence of contractile reserve) from non-viable scarred myocardium (absence of contractile reserve) in patients with ventricular dysfunction at rest. More recently, myocardial contrast echocardiography (MCE) has been proposed as a method to assess myocardial perfusion and viability. Myocardial opacification produced by the presence of microbubbles in the coronary microcirculation has been considered synonymous with preserved microvascular integrity.

Using detailed histology from explanted hearts in patients undergoing heart transplantation, Baumgartner et al. compared PET, SPECT and echo to detect viable myocardium4. While segments with >50% of viable myocytes were equally well predicted by all three non-invasive tests, in segments with <50% of viable myocytes the response to dobutamine was poor in relation to SPECT and PET, which showed equal sensitivities. However, taking survival as an end point, patients with at least 42% of viable segments during dobutamine stress echocardiography had a better long term survival following revascularisation.