When a heart attack or other condition damages heart muscle, a variety of proteins are released into the bloodstream including cardiac enzymes. Several cardiac enzymes can be measured by blood tests to help diagnose a heart attack. These include creatine kinase (CK) and a subtype of this enzyme called CK-MB. CK is found in the heart, skeletal muscle and brain, but CK-MB is found almost exclusively in the heart. For many years, CK and CK-MB testing was used to diagnose heart attacks. More recently, these tests have been largely replaced by measuring another protein called troponin. Troponin is most often indicative of heart muscle damage, whereas elevations in CK and CK-MB are associated with a host of other conditions.
Cardiac enzymes are measured when a person complains of chest pain, shortness of breath and other heart attack symptoms. During a heart attack, heart muscle cells die due to lack of oxygen. Cardiac enzymes from the dying cells leak into the bloodstream. In addition to a heart attack, other cardiac diseases that can cause elevated cardiac enzymes include:
-- heart failure, or weakness of the heart muscle
-- inflammation of the sac surrounding the heart, or pericarditis
-- inflammation of the heart muscle, or myocarditis
In some cases, people undergoing heart surgery or a procedure to open up a blocked heart artery experience elevations in CK and CK-MB.
Impaired blood flow to the brain can lead to a stroke and cause an elevation in cardiac enzymes. Many people who experience a stroke have risk factors -- such as high blood pressure, high cholesterol and diabetes -- that also increase their risk for heart disease. As a result, underlying heart disease may first become apparent during a stressful event like a stroke, also contributing to elevated cardiac enzymes. In addition, CK levels in particular may rise due to stroke, traumatic brain injury and brain cancer, as CK is found in brain tissue.
Certain lung disorders have also been linked to elevated cardiac enzymes. The most common of these is a pulmonary embolism, or a blood clot in the lungs. When a blood clot travels to or forms in the lungs, the heart typically has to pump harder to overcome increased pressure in the lungs. Researchers speculate that as a result of this added stress on the heart, CK, CK-MB and troponin are released. In severe cases, a pulmonary embolism can eventually lead to death of lung tissue, which is associated with an increase in CK as well.
People with chronic kidney disease are also prone to elevations in cardiac enzymes. High blood pressure and diabetes are leading causes of kidney disease. Likewise, people with kidney disease are at increased risk for heart disease and heart attacks. Nevertheless, CK and CK-MB are often elevated in people with kidney disease who are not experiencing a heart attack. Decreased kidney function likely contributes to chronically elevated cardiac enzymes, as the kidneys are responsible for clearing cardiac enzymes from the body. People with chronic kidney disease who have untreated hypothyroidism are more likely to have elevated CK and CK-MB as well.
A severe bloodstream infection called sepsis is associated with elevated cardiac enzymes. The body responds to sepsis with an inflammatory response that can affect multiple organs, including the heart. People with sepsis are critically ill and often experience a fast heart rate that can lead to elevated CK, CK-MB and troponin. Sepsis is also associated with other risk factors for cardiac enzyme elevations, such as impaired kidney function.
Since creatine kinase is present in skeletal muscle, muscle disorders are often associated with elevated CK levels. The inflammatory muscle disorders polymyositis and dermatomyositis -- a related condition that causes skin and muscle inflammation -- can lead to elevated CK. Muscle injury due to trauma, muscular dystrophy and connective tissue disorders, such as lupus, is also associated with elevations in CK. In addition, a number of medications including statin drugs used to treat high cholesterol, certain antidepressants and some antifungal medicines can cause high CK levels.