Myocardial Infarction Pathology
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Direct answer
Coagulative necrosis of myocytes in a coronary territory, triggered by an occluding thrombus over a ruptured atherosclerotic plaque, defines myocardial infarction. Irreversible injury begins after roughly twenty to thirty minutes of total ischaemia and spreads as a wavefront from subendocardium to epicardium over the next three to six hours, which is why door-to-balloon time decides salvage. Cardiac troponin I rises within three to four hours and stays raised for seven to ten days; CK-MB clears within two to three days and marks re-infarction. Half of all deaths occur within the first hour from ventricular arrhythmia, before the patient reaches hospital — the statistic behind community defibrillation.
What you must remember
- Tissue clock: wavy fibres and coagulative necrosis by 12-24 hours; neutrophils peak days one to three (grossly a yellow-tan soft myocardium); macrophages and granulation tissue days three to seven; collagen from week two; a grey-white scar by four to six weeks.
- Serological clock: troponin I up at 3-4 hours, peak at 18-24 hours, elevated 7-10 days (troponin T even longer); CK-MB rises 4-6 hours, peaks near 24 hours, normalises by 48-72 hours — the re-infarction marker.
- Territory map: left anterior descending artery in roughly 40-50% (anterior wall, apex, anterior septum), right coronary in 30-40% (inferior wall, posterior descending branch), left circumflex in 15-20% (lateral wall).
- Rupture window: days three to seven, when neutrophil-laden tissue is softest — papillary muscle, interventricular septum or free wall; free wall rupture causes haemopericardium and tamponade.
- Papillary muscle rule: the posteromedial papillary muscle, supplied by the posterior descending artery alone, ruptures far more often than the dual-supplied anterolateral one.
- Late complications: true ventricular aneurysm (apical, after LAD occlusion, dyskinetic bulge with thrombus), mural thrombus with embolism, and Dressler syndrome — immune pericarditis two to ten weeks after infarction.
- Type 1 versus type 2: plaque rupture with thrombosis versus supply-demand mismatch (anaemia, tachyarrhythmia, shock, sepsis); the pathology and the treatment differ.
A five-day deterioration, worked through
On day five after an anterior infarction, a monitored patient suddenly develops a loud new pansystolic murmur and shock. Reason anatomically. If the septum has ruptured, a step-up in oxygen saturation occurs in the right ventricle; if the posteromedial papillary muscle has given way, acute mitral regurgitation floods the lungs with a murmur radiating to the axilla; if the free wall has split, the jugular veins distend, the pressure equalises and electromechanical dissociation arrests the patient within minutes. All three belong to the same day-three-to-seven window, when the infarct is a soft neutrophil soup with no collagen yet laid down. Weeks later a different complication appears: a bulging apical aneurysm with a haemodynamically stable but persistently raised ST segment, or Dressler's fever, pleuritic chest pain and friction rub responding to non-steroidal drugs. The timeline is the diagnosis.
Troponin without infarction
A raised troponin means myocyte injury, not necessarily atherosclerotic occlusion — the commonest exam trap in the modern era. Renal failure slows clearance, myocarditis lyses myocytes diffusely, pulmonary embolism strains the right ventricle, sepsis and tachyarrhythmias create supply-demand mismatch, and all produce positive assays. The delta matters: a rising pattern with chest pain and electrocardiographic change argues for type 1 infarction, demanding antiplatelet therapy and angiography; a flat, low-level elevation in a dialysis patient does not. Indian experience adds another twist — patients frequently present a day or more after symptom onset, so troponin has peaked and the history, not the curve, dates the event and governs reperfusion decisions.
Frequently asked questions
When does ventricular free wall rupture typically occur after infarction?
Between days three and seven, when the neutrophil-rich infarct is softest; it produces haemopericardium, tamponade and often electromechanical dissociation.
Why does the posteromedial papillary muscle rupture more often?
It receives blood from the posterior descending artery alone, whereas the anterolateral papillary muscle has a dual supply.
Which cardiac marker is most specific for myocardial necrosis?
Cardiac troponin I and T; troponin I rises within 3-4 hours and remains elevated for 7-10 days.
What is Dressler syndrome?
An immune-mediated pericarditis with fever, pleuritic pain and a friction rub appearing two to ten weeks after infarction, treated with non-steroidal anti-inflammatory drugs.
Which coronary artery is most often occluded in fatal myocardial infarction?
The left anterior descending artery, accounting for roughly 40-50% of cases and supplying the anterior wall, apex and anterior two-thirds of the septum.