The 3 Cardiac Enzymes of Heart Attacks

A heart attack, also known as a myocardial infarction (MI), happens when oxygen supply to a part of the heart (cardiac) muscle is cut off, which causes cardiac cells in the area to die. Cardiac enzymes (or also known as cardiac biomarkers) are chemicals released into the bloodstream when the heart muscle is injured. During a heart attack, these cardiac enzymes are discharged into the bloodstream when the cardiac cells become damaged from the lack of oxygen. They can be measured simply by using blood tests. 

Based on the concentrations of these substances detected in the blood, they may suggest a heart attack. Their levels may also be able to reflect the amount of heart damage or serve as an indicator of response to treatment. These tests could be repeated to monitor for any changes as well. 

What are the common cardiac enzymes and biomarkers physicians routinely use to diagnose heart attacks? Here are the 3 different types of cardiac enzymes / biomarkers!

1. Troponins

• Proteins found in the heart and skeletal muscles that help to control muscle contractions.
• There are two kinds of cardiac troponins: Troponin I and Troponin T. Both are equally sensitive, and usually only either one is measured. 

Role of Troponin in Heart Attacks

• Cardiac biomarker test of choice for diagnosing or evaluating patients suspected of having a MI. 
• Troponin is the most specific for heart muscle damage.
• High sensitivity troponin tests are available – able to detect very low levels of troponin, enabling early identification of any heart injury.
• Released into the bloodstream when there is damage to the heart.
• In general, an MI is defined by raised and increasing levels of troponin, along with other clinical evidence for a heart attack (e.g. electrocardiogram). Troponin is not used as a standalone test in the diagnosis of a heart attack.

Interpreting Troponin Test Results when a Heart Attack is Suspected

• Levels of troponin in the body typically increase within 2 to 3 hours, peaking in 24 to 48 hours, and can remain high for around 10 to 14 days following an attack.
• Troponin tests are often repeated several hours after the initial test, especially if the first test was negative, to monitor for any changes over time.
• The concentration of troponin released and detected in the blood is directly proportional to the extent of cardiac muscle damage. 
• Even a small rise could imply the presence of some form of damage to the heart, while a large increase in levels is likely to be indicative of a heart attack.
• High levels of troponin often suggest a current or recent heart attack. 

Other Things to Note

• Troponin levels are not only elevated in heart attacks. Alternative causes for increasing troponin levels include: 
  • Myocarditis (heart muscle inflammation)
  • Acute heart failure
  • Arrhythmia (abnormal heart rhythm)
  • Chest injury
  • Stroke
  • Pulmonary embolism (blood clot in the lung) 
• Possible causes for raised but stable values are: 
  • Chronic heart failure
  • High blood pressure
  • Severe infections
  • Kidney disease
  • Certain inflammatory muscle conditions
• Less useful in the detection of reinfarction as it stays in the body for a significant duration. 

 2. Creatinine Kinase-MB (CK-MB):

Creatinine kinase (CK) is an enzyme that aids in energy production. There are three major forms of CK: 

• CK-MB (mainly in the heart)
• CK-BB (mainly in the brain)
• CK-MM (mainly in skeletal muscles) 

Role of CK-MB in Heart Attacks

• Relatively high sensitivity and specificity for cardiac tissues. 
• Less sensitive and specific in comparison to high sensitivity troponin assays – may also be released into the blood when there is skeletal muscle damage.
• May be more useful than troponin for detecting reinfarction as it does not remain in the body for as long. 
• Used with other clinical investigations to confirm the diagnosis of MI or reinfarction. 

Interpreting CK-MB Test Results when a Heart Attack is Suspected

• Levels increases in response to heart muscle cell damage. 
• Can be detected about 4 to 6 hours after a heart attack, peaking at about 12 to 24 hours, and subsequently normalising quickly within 48 to 72 hours. 

Other Things to Note

• CK-MB levels may be elevated in other conditions such as: 
  • Substantial skeletal muscle damage (e.g. rhabdomyolysis)
  • Hypothyroidism
  • Chronic kidney failure
  • Trauma

3. Myoglobin

Myoglobin is a red protein found in the heart and skeletal muscles. It traps oxygen, which in turns allow muscles to work properly. Myoglobin levels can be obtained via a urine sample as well. 

Role of Myoglobin in Heart Attacks

• High concentrations suggest recent muscle damage.
• Largely replaced by cardiac troponins which is more specific and can remain in the bloodstream for longer.

Interpreting Myoglobin Test Results when a Heart Attack is Suspected

• Released into the blood and urine during muscle injury.
• Can be detected within an hour post-heart injury, peaking in about 4 to 12 hours, and normalizing almost immediately after. 

Closing words

Cardiac enzymes or biomarkers are valuable tests that can provide useful information about whether a patient has an MI. Out of these few main cardiac enzymes, troponin has become one of the most widely used at present to determine if a patient has a heart attack. 

Regardless, it takes time for body levels of all these biomarkers to rise after an infarct, and in critical cases, other therapeutic interventions and investigations may take precedence. Your physician will consider your condition holistically and decide what is most suitable in your situation.