What is myocardial viability assessment?

What is myocardial viability assessment?

Myocardium is the technical term for heart muscle. Myocardial viability means regions of heart muscle 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 procedures like coronary angioplasty and coronary bypass surgery. In coronary angioplasty, blocks in blood vessels of the heart are removed using small tubes known as balloon catheters introduced through the blood vessels of the groin or wrist. In coronary bypass surgery, the blocked blood vessels are bypassed in a surgery which involves opening up the chest fully or partially (minimally invasive).

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 the recovery of contractile function after restoration of blood supply as occurs after an angioplasty for a heart attack. In hibernating myocardium, the heart muscle has minimized its function (sleeping!) as its blood supply has come down due to blocks in the blood vessel. 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 removing the blocks in the blood vessels. If the heart muscle is not viable, they will require advanced treatment modalities like left ventricular assist devices or heart transplantation. Left ventricular assist devices are pumps used to support the function of the left lower chamber of the heart. Currently they 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, ultrasound imaging of the heart. Echocardiography will show the thickness of the heart muscle, its contractions and scarring if any. A thinned and scarred region of heart muscle is unlikely to improve even if the blood supply is restored. Such regions are considered non-viable. Using medications which can increase the contractions during echocardiography in a test known as dobutamine stress echocardiography, is also useful in assessing myocardial viability. Blood flow to various regions of the heart can be assessed by injecting another medication into the blood vessels, in a test known as myocardial contrast echocardiography.

Information about the contractions of heart muscle, thinning and scarring can also be obtained from a magnetic resonance imaging (MRI) of the heart. In MRI, scarred regions will take up a dye known as gadolinium injected into the blood vessels, which is known as late gadolinium enhancement. 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. Tests can be taken at basal level and after giving medications to enlarge the blood vessels of the heart. For those capable of exercise, imaging after exercise is also done. By comparing these images, it is possible to assess which regions of the heart muscle are viable.

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 give the blood flow to each region of the heart muscle and assess the metabolic activity of the heart muscle, 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 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 MRI.