Biochemical markers of myocardial necrosis

The Damage of myocytes results in release of several proteins into the circulation. The estimation of the serum level of these proteins can be used as a marker of myocardial necrosis. These proteins include:

• Myoglobin
• Creatine kinase (CK) and its isozyme myocardial band creatine kinase (CK-MB)
• Cardiac troponins I and T (cTnI and cTnT)
• Lactate dehydrogenase
• Aspartate aminotransferase (AST) or (SGOT)
• Ischemia modified albumin
• Heart Fatty acid binding protein
• Myosin light chain

The criteria for ideal biomarker for myocardial necrosis are:

1. High sensitivity: abundant in myocardial tissue.
2. High specificity: not present in extramyocardial tissues.
3. Rapid release from damaged cells
4. Cost effective detection
5. Can be detected precisely

Elevation of biochemical markers is essential for diagnosis of myocardial infarction. From the definition of myocardial infarction "it is the typical rise and fall of biomarkers of myocardial ischemia with at least a single value above the 99^th percentile of the upper reference limit, combined with the presence of any of ischemia symptoms, ischemic ECG changes (Q-waves, ST elevation or ST depression) or coronary intervention". However this definition considers only 2 of the cardiac biomarker, namely troponins and CK-MB. Due to the lag between the onset of chest pain and appearance of biochemical markers in blood, a second sample should be obtained and tested after 6 hours of the presentation before exclusion of myocardial infarction.

Myoglobin:

• It is a small heme protein found in myocardium and skeletal muscles.
• The earliest marker to rise (within 1-2 hours) because of its small molecular weight that facilitates its diffusion from the damaged tissue to circulation.
• Peaks after 6-8 hours
• Returns to normal after 24 hours
• It is highly sensitive.
• It is not specific to myocardium because it is found also in greater amounts in the skeletal muscles. So, not used in clinical practice to detect myocardial necrosis.
• Normal level is 30-90 ng/ml in males and 10-55 ng/ml in females.

Creatine kinase (CK) and its isoenzyme (CK-MB):

• It is found within the striated muscle (skeletal and cardiac) cells and is essential for ATP production (creatine phosphate shuttle).

• It is a dimer formed of two subunits. In the skeletal muscle the two subunits are of M type. In the cardiac muscle there is one M subunit and the other is of B type.
• The CK-MB isozyme is abundant in the cardiac myocytes (40%). The CK-MM isozyme is dominant in skeletal muscle (97%).
• Detection of total CK is less sensitive and less specific than detection of CK-MB.
• Both CK and CK-MB are elevated in other causes than MI, e.g. rhabdomyplysis. In such cases, CK-MB to total CK fraction of >10% is diagnostic of MI.
• CK-MB is detectable after 4-6 hours of onset of chest pain in MI, peaks after 24 hours and remains elevated for 3-4 days.
• CK-MB is more useful in detection of re-infarction because of its faster return to normal value as compared with troponins.
• Normal CK: 15-170 U/L
• Normal CK-MB: 0-15 U/L

Troponins:

• Troponin complex is a regulatory protein responsible for regulation of myocyte contraction. It is formed of 3 subunits. Troponin C binds to the calcium, troponin T binds to tropomyosin and troponin I inhibits actin and myosin interaction.

• The bulk of troponin is found within the contractile apparatus but a small fraction is found free in the cytoplasm (cytosolic) and it is the first part that detected in the plasma.
• Normal cTnI is <0.1>
• - It is elevated 4-6 hours after the onset of MI.
• It peaks after 24 hours.
• It remains elevated for 1-2 weeks after onset (TnT longer than TnI).
• It is highly sensitive and specific for myocardial damage.
• It adds prognostic value to the diagnosis as patients with negative troponins are considered of low risk. Also, the level of elevation is correlated to the risk. Patients with elevated troponins <6>
• Troponins are not useful for diagnosis of re-infarction because it takes long duration to return to the normal value. So, concomitant estimation of CK-MB is needed.
• Elevation of troponins with normal CK-MB levels identifies the patients who will gain greatest benefits of GP IIb/IIIa inhibitors.

• other conditions that cause elevation in cardiac troponins are:

- other causes of cardiac injury:

cardiac contusion

Pulmonary embolism

Acute decompensated heart failure

Coronary spasm

Hypertensive crisis

Myocarditis

DC cardioversion/defibrillation/ablation procedures

- Renal failure: elevated troponins level is found in high percentage of asymptomatic patients with end stage renal disease. cTnI is much more specific than cTnT in this group of patients.

- Other infrequent causes:

Subarachnoid hemorrhage and cerebrovascular accidents.

Endocrinal diseases.

Hematological malignancies.

Skeletal muscle diseases: dermatomyositis and polymyositis.

Sepsis

Lactate dehydrogenase (LDH):

• This enzyme is widely distributed in many tissues and organs.
• It is not specific for myocardial injury as it is found also in RBCs WBCs, lungs, kidneys, liver, skeletal muscles, pancreas, placenta and other tissues.
• It is elevated in MI after 24 hours, peaks after 3-4 days and returns to normal after 14 days.
• The isoenzyme LDH₁ is the form found in cardiac myocytes and RBCs. Normally LDH₂ is the abundant form. So, flipped LDH pattern (LDH₁>LDH₂) is found in MI and hemolytic and megaloblastic anemias.
• Normal LDH range: 100-225 U/L

Aspartate aminotransferase (AST also SGOT):

• This enzyme is found in myocardial cell, skeletal muscle cells and liver cells. It is elevated in injury to any of these tissues, so, it is not specific to myocardial injury.
• In MI, the AST level is increased after 6-8 hours of onset, peaks after 24 hours and returns to normal within 5-7 days. The AST level reaches 4-10 times the upper limit of normal in MI.
• The normal value is 5-30 U/L.

Heart Fatty acid binding protein (H-FABP):

• It is a small protein (14.5 KDa) responsible for transport of hydrophobic long chain fatty acids from cell membrane to mitochondria. The H-FABP is immunologically different from the corresponding protein found in liver and intestines.
• It is released rapidly (1-3 hours) and appears early in urine. Its urinary level correlates to the extent of infarction.
• It peaks at 6-8 hours from onset.
• It returns rapidly to normal level after 24-36 hours (excellent for detection of re-infarction and perioperative infarction).

Myosin light chain 1 (MLC1):

• Myosin is a part of the sarcomere (the basic contractile unit of the skeletal and cardiac muscles).
• Myosin is heteropolymer formed of 2 heavy chains and 2 pairs of light chains. There are 2 types of myosin light chains: MLC1 and MLC2.
• MLC2 is very labile and can not be measured clinically. Thus, it is not clinically significant.
• MLC1 appears after 3-6 hours and peaks after 4 days. It remains elevated for 10-14 days.
• Its peak level correlates to the infarction size and prognosis.
• It can not be used as a marker of reperfusion or re-infarction.

Ischemia modified albumin (ILA):

• Albumin loses its ability to bind some metals such as cobalt, after exposure to ischemic myocardium due to some conformational changes to its N-terminus.
• This test poorly discriminates between myocardial ischemia with and without infarction.

N.B.: The normal levels mentioned above may vary with different methods of estimation and between different populations.