STEPTRACKERCARDIOVASCULAR | PATHOLOGY | HIGH-YIELD REVIEW

Myocardial Infarction Timeline

What the infarct looks like — grossly and under the microscope — from the first hour to the final scar, and the danger in each window.

FOCUS ON
Memorize the cells by day. Day 0: nothing visible. Days 1–3: neutrophils. Days 3–7: macrophages, the softest point and rupture window. Week 1–2: granulation tissue. Week 2 onward: collagen scar. Troponin stays up 7–10 days, while CK-MB clears in 2–3 days, which makes CK-MB the marker for reinfarction.
TIME
GROSS
MICROSCOPIC
MAIN RISK IN THIS WINDOW
0–4 hours
No visible change
None, or wavy fibers at the border
Arrhythmia (VF) → sudden death; cardiogenic shock, acute HF
4–24 hours
Dark mottling; pale on TTC stain
Early coagulative necrosis: eosinophilic fibers, edema, hemorrhage; contraction bands if reperfused
Arrhythmia
1–3 days
Hyperemia at margins
Coagulative necrosis with loss of nuclei and striations; neutrophils flood in
Fibrinous pericarditis (transmural infarcts)
3–7 days
Yellow-brown, soft center with red border
Macrophages clear dead myocytes; wall at its weakest
Papillary muscle, septal and free-wall rupture; pseudoaneurysm
1–2 weeks
Red-gray depressed border
Granulation tissue (new vessels, fibroblasts) grows in from the margin
Free-wall rupture still possible
2 weeks–2 months
Gray-white scar
Collagen scar, mostly complete by ~2 months
True ventricular aneurysm, mural thrombus, Dressler syndrome, HF
Reperfusion
any time it occurs
Hemorrhagic (red) infarct
Contraction band necrosis (Ca²⁺ influx), free-radical injury
Troponin may keep rising after successful PCI
Most vulnerable zone
Subendocardium
Highest wall stress, farthest from epicardial supply
Irreversible injury from ~20–30 min; wavefront reaches epicardium by 3–6 h

Mechanism — the wavefront of necrosis

Occlusion
→
SecondsATP ↓, contraction stops
→
20–30 minSubendocardium irreversible
→
3–6 hTransmural
→
InflammationNeutrophils → macrophages
→
Scar~2 months

Opening the artery early saves the myocardium that the wavefront hasn't reached — "time is muscle."

Markers and ECG over time

MARKER / ECG
APPEARS
PEAK · CLEARS
Troponin I / T
2–4 h
Peak ~24–48 h; elevated 7–10 days
CK-MB
4–6 h
Peak ~24 h; normal by 48–72 h
Hyperacute T waves
Minutes
Transient
ST elevation
Minutes–hours
Resolves over days
Pathologic Q waves
Hours–days
Usually permanent
T-wave inversion
Days
Weeks–months

How it's tested

COMMON CONFUSIONNeutrophils peak in the first few days; macrophages peak around days 3–7. Granulation tissue means the infarct is about 1–2 weeks old.
THE TRAPChest pain recurs on day 4 after an MI and troponin is still high from the first event. Use CK-MB, which should have normalized, to detect the reinfarction.
YOUR APPROACHRead the histology or the day → place it on the timeline → name the complication most likely in that window.

If you see… think…

Death within an hour of MI → ventricular fibrillation
Wavy myocytes, no inflammation → very early MI (hours)
Neutrophil-rich necrotic myocardium → 1–3 days
Macrophages eating myocytes → 3–7 days — rupture window
Granulation tissue at the infarct edge → 1–2 weeks
Dense collagen scar → ≥ 2 months
Contraction bands after a stent → reperfusion injury
New chest pain on day 4 with rising CK-MB → reinfarction
Friction rub on day 2 of a transmural MI → fibrinous pericarditis