What is myocardial viability assessment? Cardiology Basics

What is myocardial viability assessment? Cardiology Basics

Myocardial viability means regions of myocardium which can recover its function if the blood supply is normalized. Viable myocardium will improve after restoring blood supply while non-viable myocardium will not.

So myocardial viability assessment is important in deciding whether the person will benefit from revascularization procedures like coronary angioplasty and coronary bypass surgery.

There are two types of viable myocardium, stunned myocardium and hibernating myocardium. Stunned myocardium needs only time to recover while hibernating myocardium needs restoration of blood supply. Stunned myocardium is transient delay in recovery of contractile function after restoration of blood supply as occurs after primary angioplasty for acute ST segment elevation myocardial infarction.

In hibernating myocardium, the heart muscle has minimized its function due to chronic ischemia. The contractions of hibernating myocardium will improve either partially or completely once the blood supply is restored. This is in contrast to a scarred myocardium which will not recover function even if the blood supply is restored.

In patients with heart failure, viability assessment will tell whether it can improve by revascularization. If the heart muscle is not viable, they will require advanced treatment modalities like left ventricular assist devices or heart transplantation. Currently LVADs are run on external batteries carried outside the body, which needs regular recharging.

There are several imaging tests which can assess myocardial viability. Easily available one is echocardiography. Echocardiography will show the thickness of the myocardium, its contractions and scarring if any. A thinned and scarred region of myocardium is unlikely to improve after revascularization. Such regions are considered non-viable.

Dobutamine stress echocardiography, is also useful in assessing myocardial viability. Contractions of viable segments will improve at low dose dobutamine infusion. Regional myocardial blood flow can be assessed by myocardial contrast echocardiography in which a special contrast opacifies the myocardium according to its blood flow.

Information about the contractions of myocardium, thinning and scarring can also be obtained from cardiac magnetic resonance imaging (CMR). In CMR, scarred regions will take up gadolinium based contrasts given intravenously, which is known as late gadolinium enhancement (LGE). Both echocardiography and MRI have the advantage that ionizing radiation is not used for the imaging.

Cost of echocardiography is much less than the other modalities. Availability of portable echocardiographic equipment for bedside assessment in a sick person is another advantage. Unlike cardiac magnetic resonance imaging, presence of implanted devices like pacemakers and defibrillators do not interfere with echocardiography.

Single photon emission computed tomography (SPECT) is imaging of the heart after injecting a radioactive tracer like technetium 99m sestamibi. Tests can be taken at basal level and after giving nitrates. For those capable of exercise, imaging after exercise is also done. By comparing these images, it is possible to assess the viability of myocardial segments.

The gold standard for myocardial viability assessment is positron emission tomography (PET) scan. But the low availability and high cost are the limiting factors. PET can assess both myocardial blood flow and metabolic activity by giving two different radioactive tracers. If there is a region with mismatch between the two, that region is considered as viable. In normal regions, both will be normal, while in scarred regions, metabolic activity will be absent.

PET can be done in patients with cardiac implantable electronic devices (CIED) like pacemakers, while MRI is not suitable in that situation. Compared to SPECT, PET gives better quality pictures and has less radiation risk. Still, it has some radiation risk when compared to echocardiography and CMR.