ENZYME PATTERN IN MYOCARDIAL INFARCTION

Enzyme diagnosis is required for MI as :

a) 25 to 30% of MI cases or not diagnosed antemortem.

b) Clinical diagnosis and angiographic studies do not correlate in 25 to 30% of patients

c) ECG findings may not be helpful if

• Prior left bundle branch block is present
• Presence of Old changes that may mask current ECG interpretation
• Intravascular infarction may not change ECG pattern
• Diaphragmatic infarction often missed on ECG

Enzyme assays that are carried out in MI are :

A)Commonly done

• Creatine phosphokinase (CPK)
• Aspartate Transaminase (AST)
• Lactate dehydrogenase (LDH)

B) Other enzymes which have been studied but not commonly done

• Gamma-glutamyl transpeptidase (GGT)
• Histaminase
• Pseudocholinesterase

1. Creatine Phosphokinase(CPK)

• The enzyme catalyzes the following reaction

Creatine phosphate + ADP ------> Creatine + ATP

• Found in high concentration in skeletal muscle myocardium and brain
• Normal value 4- 60 IU/L
• After MI, 
• it starts increasing at 6 hours 
•  reaches Peak level in 24 to 30 hours and 
• return to normal level in 2 to 4 days (approximately 72 hours)
• More sensitive indicator in early-stage of MI
• Potentially more useful in subendocardial infarction
• In heart failure and coronary insufficiency, there is no increase noted.
• CK-MB Fraction is more specific

2. Serum Aspartate Transaminase(AST)

• The concentration of the enzyme is very high in myocardial injury
• Normal value 10-35 IU/L
• In acute MI 
• rises sharply within the first 12 hours 
• peak level at 24 hours and 
• returns to normal within 3 to 5 days
• Serum AST levels correlate well with prognosis
• > 350- due to massive infarction
• > 150- high mortality
• > 50--- low mortality
• Elevation has been noted in the absence of ECG change
• Highest incidence of abnormal levels occurs on the second day of infarction
• Rise depends on the size of infarction
• Re-Infarction results in a secondary rise in AST
• AST increases due to muscle or hepatic diseases can be distinguished by clinical examination of elevated ALT levels

3.Lactate dehydrogenase

• Normal value- 60-250 U/L
• In acute MI, 
• serum activity rises 12-24 hours, 
•  attain peak at 48 hrs (2-4 days) reaching about 1000 1U/ /L 
•  return gradually to normal from 8th to 14th day
• The peak rise in LDH is roughly proportional to the extent of injury to the myocardial tissue
• LDH elevation may persist for more than a week after CPK and AST levels have returned to a normal level
• LDH > 1500IU/L in AMI suggests the grave prognosis
• LDH is non-specific for myocardial tissue as it may also rise in carcinoma acute leukemia, muscle disease, etc.

4. ɣ- Glutamyl transpeptidase

• Catalyzes the transfer of ɣ- Glutamyl group from one peptide to another peptide or to an amino acid
• Normal value :
• men: 10-47 IU/L
• women: 7-30 IU/
• The highest activity is found in kidneys, liver, lungs, pancreas and prostate.

• The normal heart contains very little GGT
• In MI increase is found to be late. 
• peak activity is seen between 7-11th day and lasts as long as a month.