Pathogenesis and Differential Diagnosis
Enzyme diagnostics are based on resorption-necrotic syndrome. When a vessel supplying the heart muscle is occluded, ischemia and subsequent destruction of cardiomyocytes occur. As a result, cell membranes lose their integrity, and enzymes cascade into the bloodstream, causing hyperenzymemia.
This laboratory tool is critically important when a patient presents with shortness of breath and a clinical picture suspicious for acute coronary syndrome. Enzyme analysis helps differentiate true myocardial infarction from other life-threatening conditions, such as acute heart failure, unstable angina, or pulmonary embolism (PE).
Reference Ranges and Calculation Algorithm
To confirm the diagnosis, the patient's values must be compared with normal (reference) ranges. Traditionally, enzyme activity is expressed in international units per liter (IU/L).
Normal Values (Upper Limit):
- CK (Creatine Kinase): < 190 IU/L.
- AST (Aspartate Aminotransferase): < 47 IU/L.
- LDH (Lactate Dehydrogenase): < 250 IU/L.
Evaluation Algorithm:
- Record the patient's laboratory enzyme activity levels.
- Calculate the fold increase using the formula: `Patient Value / Normal Limit`.
- If the resulting figures for CK and AST significantly exceed normal limits, the diagnosis of myocardial infarction is confirmed.
Hourly Dynamics of Enzyme Activity
Each marker has its own kinetics, allowing clinicians to track the stages of pathological development. Dynamics are typically divided into three key periods following occlusion:
1. Early Period (12–24 hours):
- By 12 hours, levels already exceed normal limits: CK reaches 380 IU/L, AST reaches 75 IU/L, and LDH reaches 500 IU/L.
- By 24 hours, a peak maximum is observed for CK (570 IU/L). AST rises to 200 IU/L, and LDH rises to 1000 IU/L.
2. Intermediate Period (48–72 hours):
- At the 48-hour mark, CK begins to decline (380 IU/L). At the same time, AST and LDH reach their absolute maximums: 330 IU/L and 2000 IU/L, respectively.
- By 72 hours, the activity of all enzymes begins to smoothly decrease (CK — 285, AST — 190, LDH — 1750 IU/L).
3. Late Period (96–120 hours):
- By 96 hours, CK approaches the upper limit of normal (200 IU/L), AST drops to 140 IU/L, and LDH decreases to 1500 IU/L.
- By 120 hours, CK completely returns to normal (190 IU/L), and AST is 70 IU/L. However, LDH remains profoundly high (1250 IU/L), significantly exceeding the 250 IU/L threshold.
Interpretation of the Enzymatic Profile
The classical enzymatic profile of myocardial infarction is built upon marker kinetics:
- Creatine Kinase (CK): Characterized as an early marker of damage. Activity rises very rapidly (within 3–6 hours), peaks fastest at 24 hours, and then drops and normalizes relatively quickly.
- Aspartate Aminotransferase (AST): A medium-response marker. Begins to rise in 4–6 hours. Increases slower than CK, with peak activity occurring at 48 hours, followed by a gradual decline.
- Lactate Dehydrogenase (LDH): A late-detection and long-term monitoring marker. Rises latest (24–48 hours) and increases slower than the others. Its peak sometimes coincides with AST (around 48 hours), but its defining feature is that it remains elevated longer than any other marker.
Determining the Age of Infarction
A skillful combination of tests for CK, AST, and LDH gives the clinician the ability to accurately determine the timing of the ischemic event.
A high concentration of CK uniquely points to a "fresh" myocardial infarction that occurred within the past 1–2 days. Conversely, if the analysis reveals an isolated elevation of LDH against normal CK values, it strongly indicates a prior myocardial infarction that occurred several days ago, as the early fraction of enzymes has already been cleared from the circulation.