Myocardial infarction (MI) questions aren’t just “STEMI vs NSTEMI.” Step loves the timeline—what’s happening in the myocardium hour-by-hour, day-by-day, and week-by-week, and which complication is most likely right now. If you can map symptoms, labs, histology, and complications onto time after infarct, you’ll pick up easy points across cardiology, pathology, and surgery.
Quick Definition (What is MI?)
Myocardial infarction is ischemic necrosis of cardiac myocytes due to acute reduction/cessation of coronary blood flow, most often from plaque rupture → thrombosis.
First Aid cross-reference (Cardio—Ischemic heart disease / MI):
- “Atherosclerosis → plaque rupture → thrombosis”
- ECG changes in STEMI/NSTEMI
- Cardiac biomarkers (troponin, CK-MB)
- Post-MI complications by time
Pathophysiology: Why the Timeline Matters
After coronary occlusion:
- ATP depletion begins within minutes.
- Reversible injury transitions to irreversible injury after ~20–40 minutes of severe ischemia.
- Necrosis triggers an inflammatory response, then granulation tissue, then scar.
Key Step principle:
Different tissue phases have different mechanical strengths:
- Days 3–7 = weakest wall (macrophages digest dead tissue before collagen is laid down) → highest risk of rupture.
Clinical Presentation (What They’ll Give You)
Classic MI symptoms:
- Crushing substernal chest pain (>20–30 min), radiates to left arm/jaw
- Diaphoresis, nausea, dyspnea
- May be atypical in diabetics, elderly, women (fatigue, dyspnea, epigastric discomfort)
Exam/vignette clues that point to complications:
- New holosystolic murmur (papillary muscle rupture vs VSD)
- Hypotension + JVD + clear lungs (RV infarct)
- Pericardial friction rub (early pericarditis or Dressler)
- Sudden PEA arrest days after MI (free wall rupture → tamponade)
Diagnosis (Step-Style Workflow)
ECG
- STEMI: ST elevation in contiguous leads ± reciprocal changes; later Q waves
- NSTEMI/Unstable angina: ST depression, T-wave inversion, or normal ECG
Biomarkers (High-yield timing)
| Marker | Rises | Peaks | Returns to normal | Why Step cares |
|---|---|---|---|---|
| Troponin I/T | 3–4 hours | 24 hours | 7–10 days | Best overall; stays positive longer |
| CK-MB | 3–6 hours | 18–24 hours | 2–3 days | Best for re-infarction (goes back to baseline) |
First Aid cross-reference: Troponin vs CK-MB timing; re-infarction = CK-MB bump.
Imaging
- Echo: wall motion abnormalities, mechanical complications (papillary rupture, VSD), LV aneurysm, pericardial effusion/tamponade.
Treatment (Core Step 1/2 Management)
Immediate (suspected acute coronary syndrome)
- Aspirin ASAP (chew)
- P2Y12 inhibitor (e.g., clopidogrel/ticagrelor)
- Anticoagulation (heparin)
- High-intensity statin
- Nitroglycerin for pain (avoid in RV infarct or PDE-5 use)
- Oxygen only if hypoxemic (modern Step trend)
Reperfusion (STEMI)
- PCI preferred (door-to-balloon target time)
- If PCI not available quickly: thrombolysis (alteplase/tenecteplase) if no contraindications
Thrombolysis contraindications (Step favorites):
- Prior intracranial hemorrhage
- Ischemic stroke within 3 months
- Active bleeding, suspected aortic dissection, etc.
Secondary prevention (after stabilization)
- Beta-blocker (reduces arrhythmias, mortality)
- ACE inhibitor/ARB (esp. reduced EF, diabetes, CKD)
- Aldosterone antagonist if EF ≤ 35% + symptoms
- Smoking cessation, cardiac rehab
The Post-MI Timeline You Must Know (Hours → Months)
Ultra-early (0–24 hours): “Electrical instability + wavy fibers”
Pathology
- 0–4 hr: early coagulative necrosis may be subtle on light microscopy
- 4–24 hr: wavy fibers, beginning coagulative necrosis, edema/hemorrhage
Complications
- Arrhythmias are the #1 cause of death in the first 24 hours
- Ventricular fibrillation, VT
- Acute LV failure/cardiogenic shock if large infarct
Clinical pearl
- MI patient who suddenly collapses within hours → think VF/VT.
Days 1–3: “Neutrophils arrive”
Pathology
- 1–3 days: coagulative necrosis + neutrophilic infiltrate
Complications
- Continued arrhythmias
- Fibrinous pericarditis (classically 1–3 days)
- Pleuritic chest pain, better leaning forward
- Pericardial friction rub
- Early CHF/pulmonary edema can occur
Step clue
- Post-MI day 2 + sharp chest pain + friction rub → acute fibrinous pericarditis.
Days 3–7: “Macrophages clean up—wall is weakest”
Pathology
- 3–7 days: macrophages remove dead tissue → soft, weak myocardium
This is the highest-yield rupture window.
Know the big 3:
1) Free wall rupture → tamponade (often days 5–14, but overlaps this window)
- Hemopericardium → cardiac tamponade
- Sudden PEA, hypotension, JVD, muffled heart sounds
- Often in first MI, no prior scarring/collateralization
2) Papillary muscle rupture (classically 2–7 days; often ~3–5 days)
- Most often posteromedial papillary muscle (single blood supply: PDA)
- Leads to acute severe mitral regurgitation
- Findings: new loud holosystolic murmur at apex + pulmonary edema
3) Interventricular septal rupture (3–5 days)
- Leads to VSD with left-to-right shunt
- Findings: new harsh holosystolic murmur at left sternal border, ± thrill, acute HF
Step-style differentiation (fast):
| Complication | Timing | New sound | Hemodynamics |
|---|---|---|---|
| Papillary muscle rupture → MR | 2–7 days | Holosystolic at apex | Flash pulmonary edema |
| Septal rupture → VSD | 3–5 days | Harsh holosystolic at LSB + thrill | HF, step-up in O2 sat in RV |
| Free wall rupture → tamponade | ~5–14 days | Often no murmur; may be muffled | PEA, JVD, hypotension |
Weeks 1–2: “Granulation tissue builds”
Pathology
- 7–10 days: early granulation tissue
- 1–2 weeks: well-developed granulation tissue with neovascularization
Complications
- Risk of rupture declines as collagen deposition progresses (but free wall rupture can still occur into week 2)
Weeks 2–8: “Scar forms”
Pathology
- Increasing collagen deposition
- Reduced cellularity
- Progressing to dense fibrous scar
Complications
- True left ventricular aneurysm (late, often weeks to months)
- Due to scarred, thinned myocardium that bulges outward
- Does not rupture (true aneurysm = full-thickness wall)
- Complications: HF, arrhythmias, mural thrombus → emboli
Step clue
- Weeks after MI + persistent ST elevation + ventricular dyskinesis on imaging → LV aneurysm.
Months: “Healed scar + autoimmune pericarditis (Dressler)”
Pathology
- Dense collagenous scar completes by ~2 months (varies)
Complications
- Dressler syndrome (post-MI autoimmune pericarditis)
- Weeks to months after MI
- Fever, pleuritic pain, pericardial effusion, friction rub
- Often responds to NSAIDs (avoid steroids early post-MI unless necessary)
Differentiate pericarditis types:
- 1–3 days: fibrinous pericarditis (direct inflammation)
- Weeks–months: Dressler (autoimmune)
High-Yield Histology Timeline (Step 1 Path Classic)
| Time after MI | Histology | Key clinical risk |
|---|---|---|
| 0–4 hr | Early coagulative necrosis (may be minimal) | Arrhythmias |
| 4–24 hr | Wavy fibers, edema, hemorrhage | Arrhythmias |
| 1–3 days | Neutrophils | Fibrinous pericarditis |
| 3–7 days | Macrophages | Rupture (free wall, septum, papillary) |
| 1–2 weeks | Granulation tissue | Healing, aneurysm begins later |
| >2 weeks–months | Dense scar | True aneurysm, Dressler |
Test-Day “If You See This, Think That”
- Sudden death within 24 hr of MI → ventricular arrhythmia
- Day 2 pleuritic pain + friction rub → fibrinous pericarditis
- Day 4 new holosystolic murmur + pulmonary edema → papillary muscle rupture → acute MR
- Day 4 harsh murmur at LSB + thrill → septal rupture → VSD
- Day 7 sudden PEA + JVD + muffled sounds → free wall rupture → tamponade
- Weeks later persistent ST elevation → LV aneurysm (true)
- Weeks to months fever + pericarditis → Dressler
First Aid Cross-References (Where This Lives Conceptually)
Use these hooks while flipping through FA:
- Ischemic heart disease: plaque rupture → thrombosis; stable vs unstable angina; MI types
- Cardiac biomarkers: troponin vs CK-MB timing; re-infarction
- MI complications by timeline: rupture window days 3–7; Dressler weeks–months; aneurysm weeks–months
- Murmurs after MI: MR (papillary rupture) vs VSD (septal rupture)
Rapid Self-Quiz (2-minute drill)
- Post-MI day 5, sudden hypotension, JVD, clear lungs, PEA arrest → most likely?
- Post-MI day 3, new holosystolic murmur at apex + pulmonary edema → diagnosis?
- Post-MI 6 weeks, persistent ST elevation and a bulging LV wall on echo → complication?
- Post-MI 2 months, fever and pleuritic chest pain → syndrome?