Management of mi under pharmacology
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Pharmacological Management of Myocardial Infarction (MI)
Myocardial infarction occurs when coronary artery obstruction (most commonly from plaque rupture and thrombus) causes ischemia severe enough to produce myocardial necrosis. Management is divided by MI type (STEMI vs. NSTEMI/UA) and by phase (acute vs. post-MI maintenance).
Classification
| Type | ECG | Mechanism |
|---|---|---|
| STEMI | ST elevation | Complete occlusion - requires urgent reperfusion |
| NSTEMI | ST depression / T-wave changes, elevated troponin | Partial/intermittent occlusion |
| Unstable Angina (UA) | ECG changes without troponin rise | Recurrent platelet-rich nonocclusive thrombus |
A. Acute Phase Management (STEMI)
1. Antiplatelet Agents
The cornerstone of acute MI therapy:
- Aspirin - given immediately (loading dose 300 mg, then 75-100 mg daily). Inhibits COX-1, blocks TXA2-mediated platelet aggregation.
- P2Y12 receptor blockers - Clopidogrel, ticagrelor, or prasugrel. Block ADP-mediated platelet activation. Used in combination with aspirin (dual antiplatelet therapy, DAPT).
- Glycoprotein IIb/IIIa inhibitors (abciximab, eptifibatide, tirofiban) - for patients undergoing primary PCI/stenting. Block the final common pathway of platelet aggregation.
2. Anticoagulants
Prevent further thrombus propagation:
- Unfractionated heparin (UFH) - IV infusion, short-acting, reversible with protamine.
- Low-molecular-weight heparin (LMWH) - Enoxaparin, more predictable pharmacokinetics, subcutaneous route.
- Fondaparinux - Factor Xa inhibitor, preferred in NSTEMI when PCI not planned.
- Bivalirudin - Direct thrombin inhibitor, used during PCI; reduces bleeding vs. heparin + GP IIb/IIIa.
3. Nitrates
- Nitroglycerin (GTN) - sublingual for acute pain; IV infusion for ongoing ischemia or hypertension.
- Mechanism: releases nitric oxide → activates guanylyl cyclase → increases cGMP → smooth muscle relaxation → vasodilation. Reduces preload (venous dilation) and, at higher doses, afterload (arterial dilation), reducing myocardial oxygen demand.
- Bioavailability of sublingual form is high (avoids first-pass); therapeutic effect in minutes.
- Avoid if hypotension (SBP < 90 mmHg), suspected right ventricular MI, or recent PDE-5 inhibitor use.
4. Beta-Blockers
- Agents: metoprolol, atenolol, carvedilol.
- Mechanism: reduce heart rate, contractility, and blood pressure → lower myocardial O2 demand. Also antiarrhythmic (reduce risk of VF).
- Oral beta-blockers should be started within 24 hours in patients without contraindications (acute heart failure, severe bradycardia, hypotension, reactive airway disease).
- IV beta-blockers are used selectively for tachyarrhythmias.
- Significantly reduce both infarct size and mortality.
5. Oxygen
- Supplemental oxygen only if SpO2 < 90%; routine oxygen in normoxic patients is not indicated and may be harmful.
6. Opioids
- Morphine IV for pain not relieved by nitroglycerin. Reduces anxiety, preload (venodilation), and pain-related sympathetic activation. Use cautiously - may delay/reduce absorption of oral antiplatelet agents.
B. Reperfusion Therapy
Reperfusion is the most time-critical intervention:
Primary PCI (Preferred)
- Stent placement is the standard of care for ACS.
- Target: door-to-balloon time < 90 minutes.
- Superior to thrombolysis: lower mortality, lower risk of intracranial hemorrhage.
- Both PCI and thrombolysis are most effective within 12 hours of symptom onset.
Thrombolysis (Fibrinolytics)
Used when PCI is not available within 120 minutes:
- Alteplase (tPA), reteplase, tenecteplase - recombinant tissue plasminogen activators.
- Streptokinase - older agent; alteplase shown to have significantly lower mortality than streptokinase in head-to-head trials.
- Given within 12 hours of symptom onset; contraindicated if recent surgery, stroke, active bleeding, or uncontrolled hypertension.
(Katzung's Basic and Clinical Pharmacology, 16th Ed., pp. 321-324)
C. NSTEMI / Unstable Angina Management
"In patients with unstable angina with recurrent ischemic episodes at rest, recurrent platelet-rich nonocclusive thrombus formation is the principal mechanism. Aggressive antiplatelet therapy with a combination of aspirin and clopidogrel is indicated."
-
Katzung's Basic and Clinical Pharmacology, 16th Ed.
-
Dual antiplatelet therapy (aspirin + P2Y12 blocker)
-
Anticoagulation - LMWH or fondaparinux preferred
-
GP IIb/IIIa inhibitors if PCI is planned (abciximab)
-
Nitroglycerin + beta-blockers for ischemia
-
Calcium channel blockers (diltiazem or verapamil) in refractory cases when beta-blockers are contraindicated. Avoid in heart failure.
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AHA/ACC guidelines recommend early invasive strategy (PCI within 24 hours) for high-risk NSTEMI.
D. Post-MI Maintenance Pharmacotherapy
These agents are proven to reduce mortality and recurrent events:
| Drug Class | Agent | Benefit |
|---|---|---|
| Antiplatelet | Aspirin + P2Y12 blocker (DAPT 1 year post-stent) | Prevent stent thrombosis, recurrent MI |
| Beta-blocker | Metoprolol succinate, carvedilol | Reduce mortality, prevent remodeling |
| ACE inhibitor / ARB | Ramipril, enalapril, lisinopril | Reduce cardiac remodeling, mortality especially in reduced EF or diabetes |
| Statin (high-intensity) | Atorvastatin 80 mg, rosuvastatin 40 mg | Target LDL < 70 mg/dL; reduce cardiovascular events |
| Mineralocorticoid antagonist | Eplerenone, spironolactone | Post-MI with reduced EF (<40%) and heart failure or diabetes |
"Specific pharmacologic therapy to prevent myocardial infarction and death consists of antiplatelet agents (aspirin, ADP receptor blockers) and lipid-lowering agents, especially statins. Aggressive therapy with statins has been shown to reduce the incidence and severity of ischemia... ACE inhibitors also reduce the risk of adverse cardiac events in patients at high risk for CAD."
- Katzung's Basic and Clinical Pharmacology, 16th Ed., p. 174
E. Drug Summary Table
| Drug | Class | Key Mechanism | Use in MI |
|---|---|---|---|
| Aspirin | Antiplatelet | COX-1 inhibition → ↓TXA2 | Acute + long-term |
| Clopidogrel/Ticagrelor | Antiplatelet (P2Y12) | ADP receptor block | Acute + post-PCI DAPT |
| Abciximab | GP IIb/IIIa inhibitor | Final platelet aggregation block | During PCI |
| Heparin/LMWH | Anticoagulant | Antithrombin activation | Acute phase |
| Nitroglycerin | Nitrate | NO → cGMP → vasodilation | Acute ischemia, pain |
| Metoprolol | Beta-blocker | ↓ HR, ↓ contractility | Acute + maintenance |
| Alteplase/Tenecteplase | Fibrinolytic | Converts plasminogen → plasmin | STEMI when no PCI |
| Atorvastatin | Statin | HMG-CoA reductase inhibition | Post-MI, LDL < 70 |
| Ramipril | ACE inhibitor | ↓ angiotensin II, ↓ cardiac remodeling | Post-MI + EF < 40% |
| Eplerenone | Aldosterone antagonist | Blocks aldosterone receptor | Post-MI + HF + EF < 40% |
| Morphine | Opioid analgesic | CNS pain modulation, venodilation | Refractory chest pain |
Key Memory Aid: MONA-BAH (Acute STEMI)
- M - Morphine (for refractory pain)
- O - Oxygen (if SpO2 < 90%)
- N - Nitroglycerin (sublingual or IV)
- A - Aspirin (300 mg stat)
- B - Beta-blocker
- A - Anticoagulant (heparin/LMWH)
- H - Heparin + reperfusion (primary PCI preferred)
Sources: Katzung's Basic and Clinical Pharmacology 16th Ed., pp. 301-324; Symptom to Diagnosis: An Evidence-Based Guide 4th Ed., pp. 174-176
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