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Myocardial Viability

Myocardial viability is the ability of heart muscle to contract and have adequate muscle tone around the left ventricle to pump blood throughout the body.

If blood flow to the heart muscle has been decreased or absent due to coronary artery disease, the heart tissue may appear to have irreversible myocardial damage. In some cases, if the blood flow is restored to the heart through angioplasty or coronary bypass surgery, the heart tissue will recover.

Diagnosis

A key issue for patients who have been diagnosed with coronary artery disease is to determine whether there has been permanent damage to the heart muscle (myocardium) due to reduced supply of nutrients. This means looking at areas of the heart that are not functioning properly and determining whether the tissue is still alive (viable). PET/CT imaging using the radiopharmaceutical 2-Deoxy-2-[18F]fluoro-D-Glucose (FDG), is utilized by physicians to determine myocardial viability and whether the heart can recover if blood supply is restored by revascularization.

Myocardial viability imaging utilizes FDG, a form of glucose, which helps the physician determine if areas of the heart tissue demonstrate the ability to metabolize glucose. If the heart muscle has been damaged permanently, it will not show any glucose metabolism, and the patient will not benefit from revascularization or by having blood supply re-established. Such a patient would need medical therapy or a heart transplant. About 35% of coronary artery disease patients who receive bypass surgery or angioplasty to revascularize the heart do not show improvement in cardiac function because the affected tissue is permanently damaged and not reversible.1

PET/CT imaging is a sensitive technique for the identification of viable myocardial tissue in patients with coronary disease. Metabolic assessment with 18F FDG appears to be the most accurate test available for determining myocardial viability and therefore is considered by many to be the gold standard for predicting recovery of left ventricular function prior to surgical revascularization.2

Treatment

If blood flow to the heart muscle has been decreased or absent due to coronary artery disease, the heart tissue may appear to have irreversible myocardial damage. In patients where the left ventricle is not functioning adequately, treatment options are limited and many of these patients are advised that a heart transplant may be their only option. However, some of these patients may benefit significantly in terms of symptom improvement, enhanced quality of life, and improved survival if they undergo successful revascularization or bypass surgery.

PET/CT imaging is used by physicians to determine the viability of heart muscle prior to revascularization and can help the physician determine if there is permanent damage or whether bypass surgery would reverse the effects of the blockage and improve the function of the heart. The term "hibernating myocardium" is used by physicians to describe the heart tissue that adapts to decreased blood flow by shutting down and downgrading function, but can be restored to full function by restoring adequate blood flow.

The use of PET/CT imaging to select patients for surgery can reduce mortality and complication rates and is a cost-effective way to avoid unnecessary bypass surgeries, and even unnecessary heart transplants.

Follow-up

Myocardial viability imaging can be used to assess whether the treatment was successful. The physician can measure changes in myocardial viability with PET/CT imaging, before and after treatment. The highly sensitive PET/CT scan can evaluate improvement of heart function by showing the presence of FDG uptake in heart tissue, after bypass surgery or angioplasty.

Myocardial Viability Utilization

PET/CT imaging provides a way to assess the severity of heart disease and measure its impact on heart function. Clinical studies show an important role for PET/CT in screening for coronary heart disease, assessing flow rates and flow reserves, and distinguishing viable from nonviable heart tissue.

Myocardial viability imaging indications

  1. Identify ischemic disease, reduced blood supply to the heart
  2. Distinguish viable from nonviable myocardium for bypass and transplant candidates
  3. Demonstrating whether a surgery to reverse the effects of a blockage will improve the function of the heart
  4. Distinguish viable myocardium from infarcted tissue in patients with suspected hibernating or stunned myocardium
  5. Evaluate extent of disease in patients being considered for interventional revascularization or transplantation procedures

Source: Gambhir et al. A Tabulated Summary of FDG PET Literature. J Nucl Med 2001; 42:1S-93S

Source: Clinical Application and Economic Implications of PET in the Assessment of Myocardial Viability: A Retrospective Study. ICP Cardiology Task Force.