Cardiac marker – Part 1 – Diagnosis of Acute Myocardial Infarction, AMI

Sample

1. The best sample is Venous blood to get a clear serum.
2. Heparinized plasma can be used.
3. Rapid Troponin-T can be done on whole blood (some methods are available).
   1.  Current recommendation on admission with the possibility of acute MI, blood sample should be taken at intervals of:
      1. Blood sample at admission.
      2. Second sample at 2 to 4 hours.
      3. Third sample at 6 to 8 hours.
      4. Fourth sample at 12 hours.
4. The serum can be stored at room temperature for 4 to 8 hours.
   1. At 4 °C serum is stable for  1 to 2 days.
   2. For troponin-T serum is stable for 24 hours at 2 to 8 °C.

Precautions

1. I/M injection may raise the level of CPK.
2. Strenuous exercise and recent surgery may raise the CPK level.
3. Early pregnancy may decrease the CPK level.

Definition of Acute Myocardial Infarction

1. This is characterized by the ischemia of the myocardial muscles. Also, this myocardial ischemia results in irreversible myocardial cell damage or death (necrosis).
2. Or when there is obstruction of the blood supply to any part of the heart and necrosis of the muscles or massive cell death occurs.
3. Silent myocardial infarction where 40 to 50% of the patient does not notice the typical signs and symptoms of acute myocardial infarction.
   1. These are diagnosed on the ECG.
   2. This is usually seen in diabetic patients and in hypertension.
4. The AMI classified on the basis of anatomic, morphologic, and diagnostic clinical data as:
   1. Nontransmural MI when there is the involvement of only the endocardium or both endo and myocardium.
   2. Transmural MI when all the three layers endo, myo, and epicardium layers are involved.

Pathophysiology

1. There is an increased risk of AMI with the increasing age.
   1. 4 to 5 times more risk in men between the ages of 45 to 54 years as compared to women.
   2. The risk is the same in both sexes (men and women) after the age of 80 years.
2. Before the acute myocardial infarction takes place there is transient ischemia where the oxygen supply is deprived of muscle and this is called an Anginal attack.
   1. In angina, there is no necrosis.
3. The major cause of AMI is atherosclerotic plaques and thrombus formation.
4. Ischemia starts from the endocardium and then spreads outwards.
5. Irreversible damage to the muscles occurs when the ischemia is for at least 15 to 20 minutes.
   1. Vasospasm and platelet aggregation may contribute to coronary occlusion.

6. Myocardial necrosis starts:
   1. The necrosis process starts in 20 to 30 minutes.
   2. Infarcts start in the subendocardial area.
   3. After several hours there is a mid and subepicardial area of the myocardium is involved.
   4. After 3 to 6 hours there is full infarct formation.
      1. Streptokinase injection can limit the infarct.
7. The ischemic necrosis involvement of the heart muscle depends upon the involvement of the anatomical blood vessel occlusion.
   1. The anterior left descending coronary artery involves the Anterior and apical part of the left ventricle and adjacent interventricular septum.
   2. The left circumflex coronary artery involves the Lateral wall of the left ventricle.
   3. The right coronary artery involves a Posterior and basal portion of the left ventricle.

8. Risk factors for AMI are:
9. Hypertension.
10. Tobacco use.
11. Diabetes mellitus.Hyperlipidemia.
12. Sex when more common in males.
13. More common in the case of family history.

Signs and symptoms:

1. Chest pain in the mid-thorax, and there is crushing substernal pain.
2. Pain may radiate to teeth or jaw, or shoulder, or arm, or back.
3. There may be dyspnea or shortness of breath.
4. There may be sweating.
5. There may be epigastric discomfort with or without nausea and vomiting.
6. The patient may go into syncope.
   1. In 50% of the patient, the AMI is preceded by angina pectoris.
7. The pain of AMI is not relieved by nitroglycerin.

Acute Myocardial infarction is characterized by:

1. The typical rise of CK-MB.
2. Raised level of Troponin.
3. ECG changes.
4. A coronary angiogram shows coronary artery abnormality.

To diagnose the Acute Myocardial infarction:

1. H/O chest pain.

2. Changes in the ECG.
   1. The initial ECG is diagnostic in more than 50% of the cases.
   2. 15% no initial changes seen in ECG.
   3. Follow-up for 24 hours shows positive ECG in 75% of the cases.
      1. These changes are reflected from the ST-segment, T-wave, and enlarged Q-wave.

1. Abnormal cardiac enzymes:
   1. CK.
   2. CK-MB.
   3. LDH.
   4. Myoglobin.
   5. Troponin T and I.
   6. SGOT.

CK and CK-MB

1. CK Can reflect timing, quantity, and resolution of MI.
   1. CK has isoenzymes:
      1. CK-1 =BB
      2. CK-2 =MB
      3. CK-3 =MM
2. CK-MB is more specific for cardiac muscle injury.
3. This also helps in quantifying and give a degree of myocardial injury.
   1. If there is no further myocardial damage then:
      1. CK-MB detected in the first 3 to 6 hours.
      2. The peak level is at 20 to 24 hours.
      3. This returns to normal in 12 to 48 hours (in another reference becomes normal in 72 hours).
      4. Serial measurement is more important and diagnosis reaches 100%.
4. Total CK may be normal but CK-MB will be raised.
5. CK-MB is not as specific as Troponin and there are false-positive reports of non-ischemic cardiac injuries like pericarditis and myocarditis.
   1. CK-MB is also positive in muscular dystrophy, exercise, and rhabdomyolysis.
6. CK-MB usually does not arise in the case of angina, pulmonary embolism, or congestive heart failure.
   1. CK-MB may be seen rising in case of shock, malignant hyperthermia, myopathies, or myocarditis.
7. There is a small amount of CK-MB in the skeletal muscles.
   1. In case of severe skeletal muscle injury may give rise to a significantly high level of CK-MB.
8. CK-MB is more helpful and more specific than Myoglobin when the patient comes with chest pain after 10 to 12 hours.
9. CK-MB relative index:
   1. This is done to avoid skeletal muscle injury with myocardial muscle damage.
   2. Calculation:
      1. CPK-MB/total CPK
         1. If CPK-MB = 3.0 ng/mL
            1. Relative index = ≥2.5
            2. This is highly suggestive of myocardial injury.
         2. If CPK-MB = >3.0 ng/mL
            1. Relative index = <2.5
            2. Not diagnostic for myocardial injury.
               

               CK- MB profile in acute myocardial infarction

LDH (lactate dehydrogenase enzyme):

1. This is present in the cytoplasm of the cells and the highest concentration seen in the skeletal muscles, heart, liver, RBCs, kidneys, brain, and lungs.
   1. LD-1 (17 to 27%) has the highest concentration is found in the RBCs, heart, and kidneys. It comes mainly from the heart.
   2. LDH-2 ( 27 to 37%) comes from the reticuloendothelial system.
   3. LDH-3 (18 to 25%) comes from the lungs and other tissues.
   4. LDH-4 (3 to 8%)is present in the kidneys, placenta, and the pancreas
   5. LD-5 (0 to 5%) has the highest concentration in the skeletal muscles and liver.
2. LDH is not tissue-specific because of its distribution throughout the body.
   1. It is raised in a variety of diseases, including myocardial injury.
   2.  It is also not specific for cardiac diseases.
3. LDH in the myocardial infarction:
   1. Rises within 24 to 48 hours of Acute MI.
   2.  The peak level is 2 to 3 days.
   3. Returns to normal in 5 to 10 days.
4. LDH has significance in other diseases like:
   1. The elevated level in the urine indicates malignancy or injury to the urinary system.
   2. LDH also found in the effusions like pleural, cardiac, or peritoneal.
      1. When effusion LDH / serum LDH ratio is >0.6 indicate exudate in the effusion fluid.

Myoglobin:

1. Myoglobin is an oxygen-binding protein of the skeletal and cardiac muscles.
2. Myoglobin most sensitive to the cardiac marker and is the earliest marker of acute myocardial infarction.
   1. But this is the least cardio-specific of cardiac markers.
3. This is cardiac and skeletal muscle protein. This can increase after trauma to skeletal muscle or cardiac muscle injury.
   1. This is not specific for the cardiac muscle injury because it will rise even with there is a minor injury to skeletal muscle.
   2. It is raised in the trauma, inflammation, or ischemic changes to skeletal muscles.
4. But Myoglobin is very sensitive than CK and CK-MB during the first hour of chest pain.
   1. It starts rising in the first 1 to 4 hours and detectable in 6 to 9 hours.
   2. If Myoglobin remains within reference range 8 hours after the start of chest pain, then essentially Acute MI can be ruled out.
5. Myoglobin is not recommended in patients with renal failure because they will have a raised level due to decreased clearance by the kidneys.

SGOT

• This was the first enzyme marker for MI but it lakes cardiac specificity, so lost its value.

Troponin (Tn) T

1. The troponin protein complex is situated on the thin filament of skeletal and cardiac muscles.
   1. Troponin is highly concentrated in cardiomyocytes.
2. The role of Troponin is calcium-mediated contraction through its action with actin and myosin.
3. Troponin complex can be separated by the monoclonal antibodies consists of:
   1. TnT (Troponin – T) is a tropomyosin binding subunit.
   2. TnI (Troponin – I) is the myosin ATPase inhibiting subunit, block the myosin movement in the absence of calcium.
   3. TnC (Troponin – C) is the calcium-binding subunit.
   4. Cardiac troponins are separated by the immunoassay with the help of monoclonal antibodies into:
      1.  Troponin – T.
      2. Troponin – I.
   5. Troponin is localized in:
      1. Myofibrils are 94 to 97%.
      2. A cytoplasmic fraction is 3 to 6%.
4. Troponin -T allows the early and late diagnosis of MI.
5. The serum level remains raised beyond 7 days.
   1. It can be detected as early as 3 hours after the myocardial injury.
   2. Troponin -T remains elevated for 10 to 14 days.
   3. Troponin – I remain elevated for 7 to 10 days.
      

      Troponin-T in acute myocardial infarction

6. The sensitivity of Troponin – T is 100% from 12 hours to 5 days after the chest pain.
   1. The raised level is also significant, because it may go even up to 200 times.
7. Serial measurement of Troponin – T, and Troponin – I is advisable and specific for the diagnosis of AMI (myocardial damage).
8. Raised levels of Troponin – T in unstable angina predict poor outcomes in some patients.
9. Troponin measurement is better than LDH, particularly in patients who seek medical attention more than 24 to 48 hours after the onset of symptoms.
10. Troponins are measured by:
    1. Monoclonal antibody immunoassay.
    2. Enzyme-linked immunoassay.
    3. Monoclonal sandwich antibody qualitative technique.

Advantage of Troponin over CK-MB:

	Troponins	CK-MB
Specificity	More specifically for cardiac muscle injury. Normal in noncardiac muscle injury	Increased in skeletal muscle injury, brain, lung, and renal failure
Increased level	Increased early and remains elevated longer than CK-MB
Sensitivity	More sensitive to cardiac muscle injury than CK-MB	Less sensitive to cardiac muscle injury
Importance	More important for the evaluation of chest pain	Less important for the evaluation of chest pain

Normal values of cardiac markers:

Source 1

• CK-MB = 0 to 3 ng/mL
  • Or 0 to 3 µg/L

Source 2

• Troponin T = <0.2 ng/mL
• Troponin I = <0.3 ng/mL

Source 4

• TRoponin – T = <0.2 ng/mL
  • Or <o.2 µg/L
• Troponin – I = <0.35 ng/mL
  • Or <0.35 µg/L
• LDH = 140 to 280 U/L
• Myoglobin = <55 ng/mL

Table of cardiac markers:

Marker	Time rise in hours	When detectable in hours	Peak hours	days to become normal
Creatine Kinase	5 to 8	6 to 8	24 to 36 hours	3 to 4 days
CK -MB	5 to 15	4 to 6	12 to 24 hours	2 to 3 days
LDH	2 to 4	8 to 12	72 to 144 hours	8 to 14 days
Myoglobin		1 to 3	6 to 9 hours	1 day
Troponin I		3 to 8	14  to 18 hours	3 to 10 days
Troponin T		3 to 8	72 to 100 hours	5 to 10 days
AST (SGOT)	3 to 5	6 to 8	24 to 48 hours	4 to 6 days

• Thrombolytic therapy is useful if applied within the first 12 hours of AMI.
• Note: You may find some differences in these tests to become normal in different references.

Critical values 

• Troponin I = >1.5 ng/mL

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