Cardiac Markers and Enzymes

Cardiac Markers Overview - Heart's SOS Signals

• Substances released into blood when heart muscle (myocardium) is damaged.
• Used to diagnose, assess risk, and determine prognosis in Acute Coronary Syndromes (ACS).
• Types: Enzymes (e.g., CK-MB) and structural proteins (e.g., Troponins).

⭐ Ideal cardiac markers are highly sensitive and specific for myocardial injury, rise rapidly in blood, have a diagnostic window that allows for both early and late detection, and their levels should correlate with extent of injury.

Key Enzymes (CK-MB) - Enzyme Alarms

Creatine Kinase (CK) has isoenzymes: CK-MM (skeletal muscle), CK-BB (brain), and CK-MB (primarily myocardium).

• CK-MB (Myocardial Band):
  • Historically a key marker for Myocardial Infarction (MI).
  • Rises in serum: 3-6 hours post-MI.
  • Peaks: 12-24 hours.
  • Returns to normal: 48-72 hours.
  • Not entirely cardiospecific; small amounts in skeletal muscle.
  • Can be useful for detecting re-infarction if levels rise again after normalizing.
• CK-MB Relative Index (RI):
  • Calculated as: $(CK-MB / \text{Total CK}) * 100$.
  • Threshold suggestive of myocardial injury: > \textbf{4-6}%.

⭐ The CK-MB relative index is crucial to differentiate skeletal muscle injury from myocardial injury when total CK-MB is elevated, particularly by applying this percentage threshold.

Cardiac Troponins - Heart's True Detectives

• Gold standard for Myocardial Infarction (MI) diagnosis.
• Types:
  • Cardiac Troponin I (cTnI): Myocardium-specific.
  • Cardiac Troponin T (cTnT): Myocardium-specific.
  • Troponin C (cTnC): Cardiac & skeletal muscle.
• Kinetics (Post-MI):
  • Rise: 2-4 hrs (conventional), earlier with hs-cTn.
  • Peak: cTnI (12-24 hrs), cTnT (12-48 hrs).
  • Duration: cTnI (5-7 days), cTnT (7-14 days). 📌 Troponin T lasts longer (up to Two weeks), I is shorter.
• Other causes of ↑ Troponin: Myocarditis, pericarditis, PE, CKD, sepsis, strenuous exercise.

⭐ High-sensitivity cardiac troponins (hs-cTn) can detect myocardial injury earlier and with greater accuracy than conventional troponin assays, enabling rapid 0h/1h or 0h/2h rule-in/rule-out protocols for MI.

Other Biomarkers - The Wider Net

• Myoglobin
  • Early marker of muscle injury (cardiac/skeletal).
  • Rises in 1-3 hours, peaks at 6-9 hours, normalizes in 24 hours.
  • Low specificity for MI due to presence in skeletal muscle.
• BNP (Brain Natriuretic Peptide) & NT-proBNP
  • Released from ventricles in response to stretch (volume/pressure overload).
  • Used for diagnosis, severity assessment, and prognosis of heart failure.

  ⭐ BNP (Brain Natriuretic Peptide) and NT-proBNP are key markers for diagnosing and assessing the severity of heart failure, and also carry prognostic value in Acute Coronary Syndromes (ACS).

• High-sensitivity C-reactive Protein (hs-CRP)
  • Marker of inflammation.
  • Predicts risk of future cardiovascular events.
  • Levels <1 mg/L: Low risk; 1-3 mg/L: Average risk; >3 mg/L: High risk for CVD events (AHA/CDC guidelines).

Marker Kinetics & Use - Timing is Everything

• Troponins (cTnI, cTnT): Most sensitive/specific. Rise 2-4h, peak 12-48h, normal 5-14d.
• CK-MB: Reinfarction. Rise 3-6h, peak 12-24h, normal 2-3d.
• Myoglobin: Earliest. Rise 1-4h, normal 24h. Non-specific.
• Serial sampling is key for diagnosis & prognosis.

⭐ For suspected reinfarction when troponin levels are already elevated, a significant rise (e.g., >20% increase from nadir) in serial troponin values or a re-elevation of CK-MB (if it had normalized) is indicative.

High‑Yield Points - ⚡ Biggest Takeaways

• Troponins (TnI, TnT): Gold standard for MI; most sensitive & specific, elevated 7-14 days.
• CK-MB: Early MI marker, peaks ~24 hrs; useful for re-infarction detection.
• Myoglobin: Earliest marker (1-4 hrs post-MI), but low specificity.
• LDH1 > LDH2 (flipped pattern): Late indicator of MI (2-3 days post-event).
• BNP/NT-proBNP: Key markers for heart failure diagnosis, severity, and prognosis.
• hs-CRP: Marker of inflammation; indicates increased cardiovascular risk.