U01.07.042 Diagnosis of myocardial infarction

Early Diagnosis (First 6 Hours)

Electrocardiogram (ECG) is the gold standard within the first 6 hours of symptom onset.

Cardiac Biomarkers

Cardiac biomarkers help confirm MI and assess infarct severity.

Key Biomarkers in Myocardial Infarction

Biomarker	Rise Time	Peak Level	Duration	Clinical Significance
Cardiac Troponin I (cTnI)	4 hours	24 hours	7–10 days	Most specific marker for MI. Elevated for a prolonged period.
CK-MB (Creatine Kinase-Myocardial Band)	6–12 hours	16–24 hours	Returns to normal in 48 hours	Useful for detecting reinfarction due to rapid clearance. Found mainly in myocardium but also in skeletal muscle.

Large MIs cause greater elevations in troponin I and CK-MB.
Biomarker levels and their kinetics depend on the testing procedure used.

Electrocardiographic (ECG) Changes in MI

Different MI types present with characteristic ECG changes:

ST-Segment and Wave Abnormalities

ECG Finding	Associated Condition
ST Elevation (STEMI)	Transmural infarct
ST Depression (NSTEMI)	Subendocardial infarct
Hyperacute (Peaked) T Waves	Early sign of ischemia
T-Wave Inversion	Ischemia or infarction
New Left Bundle Branch Block (LBBB)	Possible myocardial infarction
Pathologic Q Waves / Poor R Wave Progression	Old or evolving transmural infarct

Key Points to Remember

ECG is essential for rapid MI detection, especially in the first 6 hours.
Troponin I is the most specific biomarker and remains elevated for 7–10 days.
CK-MB is useful for diagnosing reinfarction due to its shorter duration (returns to normal within 48 hours).
STEMI vs. NSTEMI differentiation is crucial for treatment decisions.

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