A common reason for testing cardiac enzyme levels is to determine if someone is having a heart attack. However, that's not the only reason levels rise. Pulmonary embolism, sepsis, drug toxicities, overexertion and other triggers can also raise levels. Elevated enzymes may signal something is wrong.
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What are Cardiac Enzymes?
Cardiac enzymes are proteins or chemicals that are released into the bloodstream in response to some catalyst or process in the body. These substances serve as biomarkers, which help clinicians detect certain heart problems.
Cardiac enzymes are measured with blood tests. The National Center for Biotechnology Information (NCBI) reports that cardiac enzyme testing has been used since the mid-20th century to determine if someone is having or has had a heart attack.
Types of cardiac enzymes include creatine kinase (CK) and a subtype 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, tests measuring CK and CK-MB were used to diagnose heart attacks.
More recently, says NCBI, these tests have been largely replaced by measuring another protein called troponin. There are three subtypes of troponin: troponin C, troponin I and troponin T. Elevated troponin I and troponin T are most often indicative of heart muscle damage, whereas elevations in troponin C, CK and CK-MB can be associated with a host of other conditions.
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Measure of a Heart Attack
Ben Shepple, MD, a cardiologist at the University of Tennessee Medical Center, explains that when cardiac enzyme levels are elevated, additional blood tests may be done to keep an eye on the numbers.
"If cardiac enzymes are elevated, then they are typically followed over time to see if they increase, decrease or stay the same," Dr. Shepple says. "Rapidly increasing cardiac enzymes are typically due to a heart attack.
"Cardiac enzyme tests are a lagging indicator and can take hours to show up in the blood after a heart attack has started," he adds. "If a patient has cardiac enzymes tested soon after a heart attack starts, levels are often normal. For this reason, cardiac enzyme tests are typically repeated four to six hours later."
Depending on the symptoms, a doctor may recommend additional tests, says the American Heart Association. These may include an echocardiogram, stress test or coronary angiography.
Other Reasons for Elevated Enzymes
Elevated cardiac enzymes can be an indication of health conditions or reasons other than heart attack. According to a research review published in the Journal of Cardiology & Current Research (JCCR) in January 2018, this includes open heart surgery, strenuous exercise, chemotherapy, atrial fibrillation and blunt trauma to the chest. Other possibilities, per the JCCR review, include:
- Brain disorders. People with elevated troponin have increased risk factors for stroke. Additionally, after a stroke, troponin levels help determine prognosis for short-term and long-term outlook.
- Respiratory conditions: Certain lung disorders have been linked to elevated cardiac enzymes. The most common is a pulmonary embolism, which is a blood clot in the lungs. About 30 to 50 percent of people with pulmonary embolism have elevated troponins, according to the JCCR review.
- Kidney disease: People with kidney disease are at increased risk for heart disease and heart attacks. Nevertheless, cardiac enzymes 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.
- Severe infection: 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.
- Skeletal muscle injury: Trauma to the muscle, alcohol and drug use, serious illness, certain prescribed medications and overexertion during exercise can lead to a skeletal muscle injury called rhabdomyolysis. As cells are damaged, cardiac enzymes are released, boosting levels of CK and troponins. People with rhabdomyolysis who have elevated levels of troponins are sicker for longer than individuals who have this condition but do not have elevated levels of this biomarker.
- National Center for Biotechnology Information: “Cardiac Enzymes”
- Ben Shepple, MD, cardiologist, University of Tennessee Medical Center
- Journal of Cardiology & Current Research: “A Review of Cardiac and Non-Cardiac Causes of Troponin Elevation and Clinical Relevance Part II: Non Cardiac Causes”
- American Heart Association: "Non-Invasive Tests and Procedures"