Acute Coronary Syndrome (ACS): Full Detailed Explanation

What is ACS?

• STEMI - ST-Elevation Myocardial Infarction (complete occlusion)
• NSTEMI - Non-ST-Elevation Myocardial Infarction (subtotal occlusion)
• Unstable Angina - ischemia without myocyte necrosis (troponin negative)

Section 1: Pathophysiology - The 12-Step Sequence

Step 1 - Plaque Rupture / Erosion

• Atherosclerotic plaques that are vulnerable have a large lipid core, thin fibrous cap (≤65 µm), high macrophage density, and matrix metalloproteinase (MMP) activity
• MMPs secreted by macrophages and foam cells degrade the fibrous cap, causing it to rupture or erode
• This exposes two critical pro-thrombotic substances beneath: Collagen and Tissue Factor (TF, Factor III)
• This is the triggering event for the entire cascade

Step 2 - Platelet Adhesion

• Exposed subendothelial collagen binds von Willebrand Factor (vWF), which acts as a molecular bridge
• Circulating platelets recognize this collagen-vWF complex and bind to it via their GP Ib receptors (Glycoprotein Ib)
• This is the initial, reversible attachment of platelets to the damaged vessel wall

Step 3 - Platelet Activation

• Adhered platelets become activated and release a burst of chemical signals:
  • ADP (Adenosine Diphosphate) - recruits more platelets
  • Thromboxane A2 (TXA2) - potent vasoconstrictor and platelet recruiter (synthesized via COX-1)
  • Serotonin - promotes vasoconstriction
  • Ca2+ - intracellular signaling mediator
• These signals cause conformational changes in the platelet and recruit more platelets in a positive feedback loop

This is where Aspirin and P2Y12 inhibitors act - by blocking TXA2 production and ADP signaling respectively.

Step 4 - Platelet Aggregation (Platelet Plug)

• Activated platelets expose GP IIb/IIIa receptors on their surface
• Fibrinogen molecules bridge adjacent platelets by binding GP IIb/IIIa receptors simultaneously on two platelets
• This forms the primary hemostatic plug (soft, unstable platelet plug)

This is where GP IIb/IIIa inhibitors act - blocking the final common pathway of aggregation.

Step 5 - Tissue Factor Activates Coagulation (Extrinsic Pathway)

• Exposed Tissue Factor (TF/Factor III) from the ruptured plaque binds with circulating Factor VIIa to form the TF-VIIa complex
• This complex activates Factor X → Factor Xa
• Factor Xa combines with Factor Va (prothrombinase complex) to convert Prothrombin to Thrombin
• This is the start of the coagulation cascade (extrinsic pathway)

Step 6 - Thrombin Generation

• Prothrombin (Factor II) → Thrombin (Factor IIa) via the prothrombinase complex
• Thrombin is the central enzyme of coagulation - it does three things simultaneously:
  1. Converts fibrinogen to fibrin
  2. Amplifies coagulation by activating more upstream factors (V, VIII, XI)
  3. Directly activates platelets via PAR-1 receptors (further fueling aggregation)

Heparins and Fondaparinux act at steps 5-6 by inhibiting Factor Xa and/or Thrombin (IIa).

Step 7 - Fibrin Formation

• Thrombin cleaves soluble Fibrinogen (Factor I) into insoluble Fibrin monomers
• Factor XIIIa (activated by Thrombin) cross-links these fibrin monomers into a stable fibrin mesh
• This reinforces the platelet plug, forming a stable, firm thrombus

Step 8 - Coronary Thrombus

• The combined platelet plug + cross-linked fibrin = coronary thrombus
• In STEMI: complete occlusion
• In NSTEMI/UA: subtotal (partial) occlusion

Step 9 - Coronary Occlusion → Ischemia → Necrosis

• Blocked coronary blood flow → ischemia → if prolonged, irreversible myocyte necrosis (infarction)
• Troponin I and T are released from dying myocytes into the bloodstream
• CK-MB rises within 3-24 hours; LDH-1 is a late marker

Step 10 - Remodeling (Weeks to Months)

• Infarcted myocardium is replaced by scar tissue
• Adverse remodeling: decreased contractility, ventricular dilatation, arrhythmias, heart failure, cardiogenic shock
• ACE inhibitors, Beta-blockers, and Aldosterone antagonists are given to prevent/slow remodeling

Step 12 - Long-Term Prevention

• Statins stabilize remaining plaques and reduce LDL
• DAPT (Dual Antiplatelet Therapy), ACE inhibitors, and beta-blockers continue for secondary prevention

Section 2: Drugs - Where They Act

A. Antiplatelet Drugs

1. Aspirin (COX Inhibitor) - Acts at Step 3

Key point: aspirin is "irreversible" - this is why it must be stopped 7-10 days before surgery, not just 24 hours.

2. P2Y12 Inhibitors - Acts at Step 3

3. GP IIb/IIIa Inhibitors - Acts at Step 4

B. Anticoagulants - Act at Steps 5-6

4. Unfractionated Heparin (UFH)

• Dose: 60 U/kg IV bolus (max 4000 U), then 12 U/kg/hr infusion
• Mechanism: Activates Antithrombin III, which then inhibits both Factor Xa AND Factor IIa (Thrombin)
• Monitoring: aPTT maintained at 1.5-2.5 times control
• Advantage: Reversible with protamine sulfate; adjustable by infusion

5. Low Molecular Weight Heparin (LMWH) - Enoxaparin

• Dose: 1 mg/kg SC every 12 hours (once daily if CrCl <30 mL/min)
• Mechanism: Predominantly inhibits Factor Xa (less Thrombin inhibition than UFH)
• Monitoring: Anti-Xa level only when needed (e.g., renal impairment, obesity, pregnancy); no routine aPTT needed
• Advantage: Predictable pharmacokinetics, subcutaneous dosing, less HIT risk

6. Fondaparinux

• Dose: 2.5 mg SC OD
• Mechanism: Selective, direct Factor Xa inhibitor (via Antithrombin III)
• Advantage: No HIT (Heparin-Induced Thrombocytopenia) risk; preferred in NSTEMI/UA where PCI is not immediately planned

C. Fibrinolytics (Thrombolytics) - Act at Steps 7-8

• FDP/D-dimer rises (fibrin breakdown products released)
• Fibrinogen falls (consumed by plasmin)

Section 3: Other Anti-Ischemic & Cardioprotective Drugs (Do NOT dissolve clot)

Nitrates (Nitroglycerin)

• SL: 0.4 mg every 5 min, max 3 doses
• IV: 5-10 µg/min, titrate up
• MOA: Nitric oxide (NO) donor → increases cGMP → venodilation > arterial dilation → reduces preload → decreases LV wall stress → reduces myocardial O2 demand
• Also: Dilates coronary arteries (relieves spasm)
• Avoid: RV infarction (preload-dependent), hypotension (SBP <90), recent PDE-5 inhibitor use (sildenafil/tadalafil - can cause fatal hypotension)

Beta-Blockers (Metoprolol)

• IV: 5 mg every 5 min × 3 doses; Oral: 25-50 mg every 6-12 hours
• MOA: Beta-1 blockade → decreased HR, decreased contractility, decreased BP → reduced O2 demand + anti-arrhythmic effect
• Benefit: Reduces infarct size, prevents ventricular arrhythmias, reduces mortality
• Avoid: Acute decompensated HF, cardiogenic shock, bradycardia (<60 bpm), AV block (>1st degree), severe asthma/COPD

Morphine

• Dose: 2-4 mg IV, repeat 2-8 mg every 5-15 min if needed
• MOA: Analgesia + venodilation (reduces preload) + reduces sympathetic drive (lowers HR and BP)
• Effect: Relieves pain and anxiety, reduces O2 demand
• Note: Recent data suggests morphine may delay absorption of oral P2Y12 inhibitors (due to slowed gastric emptying) - use cautiously

Statins (Atorvastatin)

• Dose: 80 mg stat (high-intensity loading dose)
• MOA: HMG-CoA reductase inhibition → decreased LDL + plaque stabilization + anti-inflammatory effects
• Effect: Prevents plaque recurrence, stabilizes vulnerable plaques
• Lab: Monitor liver disease, myopathy (CK if symptomatic)

ACE Inhibitors (Ramipril)

• Dose: 2.5 mg daily, titrate up
• MOA: Blocks angiotensin II and aldosterone → decreased afterload → prevents adverse cardiac remodeling
• Benefit: Improves survival, especially in anterior MI, EF <40%, DM, hypertension
• Avoid: Hypotension, renal artery stenosis, hyperkalemia

Aldosterone Antagonist (Eplerenone)

• Dose: 25 mg OD, titrate to 50 mg OD
• Indication: EF ≤40% + Heart Failure OR post-MI
• MOA: Blocks aldosterone receptor → reduces myocardial fibrosis and remodeling
• Avoid: Hyperkalemia, renal failure

Section 4: Coagulation Cascade (Extrinsic Pathway) and Drug Targets

Tissue Factor (TF, from damaged plaque)
          ↓
       Factor VII
          ↓
    Factor VIIa + TF complex
          ↓
    Factor X → Factor Xa  ←── LMWH inhibits here
          ↓                ←── Fondaparinux inhibits here
    Prothrombin (II)       ←── Heparin (UFH) inhibits BOTH Xa AND IIa
          ↓
    Thrombin (IIa)         ←── UFH also inhibits here
          ↓
    Fibrinogen (I)
          ↓
        Fibrin
          ↓
    Cross-linked Fibrin (stable clot) ← via Factor XIIIa

• UFH: Inhibits both Xa and IIa (via Antithrombin III)
• LMWH: Predominantly inhibits Xa
• Fondaparinux: Selectively inhibits Xa ONLY

Section 5: Reperfusion Strategy

STEMI

1. Primary PCI (Percutaneous Coronary Intervention) is always preferred - balloon/stent opens the blocked artery mechanically
2. Target: FMC (First Medical Contact) to device time ≤120 minutes
3. If PCI not achievable within 120 min AND symptoms <12 hours → Fibrinolysis (Tenecteplase)
4. After fibrinolysis → Pharmacoinvasive strategy: transfer to PCI center for angiography within 2-24 hours

NSTEMI / Unstable Angina

• No fibrinolytics (the clot is subtotal; lysis carries bleeding risk without proven benefit)
• DAPT + Anticoagulation (medical stabilization)
• Early invasive strategy based on risk stratification (GRACE score, TIMI score)

Section 6: Dual Antiplatelet Therapy (DAPT)

• Ticagrelor is preferred in most ACS (faster, more potent, better outcomes in PLATO trial)
• Duration: Continue for 12 months post-ACS (unless high bleeding risk)
• After 12 months, aspirin monotherapy continues indefinitely

Section 7: Coagulation Monitoring Summary

Section 9: Common Lab Changes in ACS

Key Takeaway Messages (Bottom of Diagram)

1. ACS is triggered by plaque rupture → thrombosis → ischemia
2. Aspirin blocks TXA2 → prevents platelet activation
3. P2Y12 inhibitors block ADP pathway → prevents platelet aggregation
4. Heparin/LMWH/Fondaparinux prevent thrombin generation
5. Fibrinolytics dissolve the clot (plasmin → fibrin breakdown) - only in STEMI when PCI unavailable
6. Nitrates, Beta-blockers, Morphine reduce O2 demand - do NOT dissolve clot
7. Statins and ACE inhibitors improve long-term outcomes
8. Primary PCI is the best reperfusion strategy for STEMI