FAQ: What Is Hibernating Heart Muscle?

The Importance of Hibernating Myocardium

Cardiac MRI
Cardiac MRI. BSIP/UIC/Getty Images

Q. My husband has heart failure because of his coronary artery disease.  The first doctor we saw told him that bypass surgery wouldn’t do any good because the blocked artery is supplying heart muscle that is already dead.  But the second doctor says it’s possible the heart muscle isn’t permanently damaged, but just “hibernating.”  He says that bypass surgery might allow the muscle to “wake up,” and his heart failure to improve. What can you tell me about hibernating heart muscle, and do you think the second cardiologist might be right?

Answer:

The concept of “hibernating myocardium” (myocardium means heart muscle) still seems to be a foreign idea to some doctors, but most cardiologists are well aware of it. In some people with coronary artery disease (CAD), portions of heart muscle that appear to be severely damaged and non-functional are actually still viable, and can be “revived” if the blood supply is restored.

According to the way many doctors think about CAD, the heart muscle functions normally as long as there is sufficient blood flow. If the blood flow becomes insufficient to meet the needs of the heart muscle (for instance, when a person with CAD begins exercising), the muscle becomes transiently ischemic (starved for oxygen), and angina may occur. Ischemic heart muscle does not function normally. In fact, performing an echocardiogramduring exercise is one way to diagnose ischemia, since the echo test can visualize segments of heart muscle that fail to contract normally when they are deprived of sufficient oxygen.

The way doctors classically thought about CAD, either the ischemia would soon go away (because, for instance, the person with CAD would stop exercising when angina appeared), or the ischemia would persist until a myocardial infarction (death of heart muscle) occurred.

So classically, the myocardium supplied by a diseased coronary artery can exist in one of three states: normal, ischemic, or dead.

But it turns out heart muscle might also persist in a fourth state, a state referred to as hibernation.

What Is Hibernating Myocardium?

Hibernating myocardium is just what it sounds like. Like a bear hibernating through the winter, despite all appearances hibernating heart muscle is not dead, but rather has just assumed a “dormant” state.  It no longer functions normally — it does not contract with each heartbeat, and is not contributing to the work of the heart.

But neither is it dead.  It is merely in a state of self-protective inactivity.  It has shut down every one of its functions that is not immediately critical to its staying alive.

Heart muscle may enter a state of hibernation when the CAD is severe enough to produce ischemia that is chronic and relatively constant, rather than the more typical ischemia that comes and goes relatively infrequently (which is the case in most people with angina).  So, essentially, the heart muscle is never really getting enough blood flow to function normally, but it is — just barely — getting enough blood flow to stay alive.

Why Is Hibernating Myocardium Important?

Hibernating heart muscle is an important concept because the muscle is still potentially viable, and the hibernation can be reversed. If the hibernating muscle’s blood supply can be restored — through bypass surgery or stenting— there’s a reasonably good chance the hibernating myocardium can “wake up,” and begin once again contributing to cardiac work. In a person with heart failure, this increased cardiac work capacity might make all the difference.

In your husband’s case It sounds like the second doctor saw evidence that your husband may have hibernating myocardium, and that opening up one or more of his coronary arteries with bypass surgery might allow at least a portion of his heart muscle to begin functioning again.

There are special tests that cardiologists can do to help differentiate hibernating myocardium from heart muscle that is non-viable (that is, dead), including MRI studies, and special echocardiographic testing.  Since this kind of testing is non-invasive and essentially risk-free, pursuing the possibility of hibernating myocardium would seem to be entirely reasonable in your husband’s case.

If this assessment reveals that he is likely to have a substantial amount of hibernating myocardium, then “waking up” that portion of heart muscle could potentially improve his heart failure significantly. So it would make a lot of sense to strongly consider the bypass surgery the cardiologist is recommending if the testing supports the presence of hibernating myocardium. 

Sources:

Allman KC, Shaw LJ, Hachamovitch R, Udelson JE. Myocardial Viability Testing and Impact of Revascularization on Prognosis in Patients with Coronary Artery Disease and Left Ventricular Dysfunction: a Meta-analysis. J Am Coll Cardiol 2002; 39:1151.

Kim SJ, Peppas A, Hong SK, et al. Persistent Stunning Induces Myocardial Hibernation and Protection: Flow/function and Metabolic mechanisms. Circ Res 2003; 92:1233.

Parasher PS, Daher IN. Myocardial Recovery after Hypoxia: Stunning Recovery. Echocardiography 2008; 25:1011.

Rahimtoola SH, La Canna G, Ferrari R. Hibernating Myocardium: Another Piece of the Puzzle Falls into Place. J Am Coll Cardiol 2006; 47:978.

Continue Reading