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Myocardial Infarction - Drug Management & Precautions

(2nd MBBS Pharmacology)

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Overview

1. Relieve pain and ischemia
2. Restore blood flow (reperfusion)
3. Prevent re-occlusion
4. Limit infarct size
5. Prevent complications and recurrence

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A. ACUTE PHASE MANAGEMENT (First 12-24 hours)

1. Morphine (Opioid Analgesic)

• Use: Pain relief, reduces anxiety and sympathetic activation
• Dose: 2-4 mg IV, repeated every 5-15 min as needed
• Mechanism: Opioid receptor agonist - analgesia + venodilation (reduces preload)
• Precautions:
  • Causes respiratory depression - have naloxone ready
  • Nausea/vomiting - give with antiemetic (metoclopramide)
  • Avoid in hypotension or bradycardia
  • May mask worsening pain
  • Note: Some guidelines now de-emphasize routine morphine in NSTEMI due to potential interaction with antiplatelet absorption

2. Oxygen

• Use: Correct hypoxia (O2 saturation < 90%)
• Precaution: Do NOT give routinely to all MI patients - excess O2 increases mortality in non-hypoxic patients (causes vasoconstriction)

3. Nitrates (Nitroglycerin / Isosorbide dinitrate)

• Use: Chest pain relief, reduces preload and afterload
• Route: Sublingual (0.4 mg) initially, then IV infusion if needed
• Mechanism: Releases NO → venodilation → reduced preload → reduced O2 demand
• Precautions:
  • Contraindicated if SBP < 90 mmHg (causes severe hypotension)
  • Contraindicated with sildenafil/tadalafil (PDE5 inhibitors) - severe hypotension
  • Avoid in right ventricular infarction (patient is preload-dependent)
  • Tachyphylaxis with prolonged use (nitrate-free interval required)

4. Aspirin (Antiplatelet)

• Use: Given IMMEDIATELY on suspicion of MI - reduces mortality by ~20%
• Dose: 300-325 mg (loading, chewable/crushed) then 75-100 mg daily
• Mechanism: Irreversible inhibition of COX-1 → inhibits TXA2 → prevents platelet aggregation
• Precautions:
  • Caution in peptic ulcer disease (give with PPI)
  • Avoid in aspirin hypersensitivity / asthma triggered by NSAIDs
  • GI bleeding risk

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B. REPERFUSION THERAPY

5. Thrombolytic / Fibrinolytic Agents

• Prior intracranial hemorrhage (any time)
• Ischemic stroke within 3 months
• Active internal bleeding
• Suspected aortic dissection
• Significant closed head trauma within 3 months
• Intracranial neoplasm / AVM
• Severe uncontrolled hypertension (SBP > 180 mmHg)

• Severe hypertension (SBP 160-180 mmHg)
• History of ischemic stroke > 3 months
• Traumatic CPR, major surgery < 3 weeks
• Pregnancy
• Streptokinase: Do NOT repeat within 12 months (antibodies form - resistance + anaphylaxis risk)

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C. ANTIPLATELET THERAPY (Dual Antiplatelet - DAPT)

6. P2Y12 Receptor Inhibitors (Thienopyridines / others)

• Contraindicated in prior TIA/stroke
• Not recommended in age ≥ 75 years or weight < 60 kg
• Higher bleeding risk than clopidogrel

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D. ANTICOAGULANT THERAPY

7. Heparin (Unfractionated Heparin - UFH)

• Use: Prevents extension of thrombus; adjunct to PCI and fibrinolysis
• Mechanism: Activates antithrombin III → inhibits thrombin (IIa) and Xa
• Monitoring: aPTT (target 50-75 sec) / ACT during PCI
• Precautions: Bleeding, HIT (heparin-induced thrombocytopenia) - monitor platelets

8. Low Molecular Weight Heparin (LMWH) - Enoxaparin

• Advantages over UFH: Predictable pharmacokinetics, no monitoring, SQ injection
• Dose: 1 mg/kg SQ twice daily
• Precaution: Avoid in severe renal failure (accumulates)

9. Fondaparinux

• Mechanism: Selective anti-Xa (indirect, via antithrombin III)
• Advantage: Does not cause HIT; preferred in NSTEMI if no immediate PCI planned
• Precaution: Avoid if eGFR < 20 mL/min

10. Bivalirudin

• Direct thrombin inhibitor (DTI); used during PCI as UFH alternative
• Less bleeding risk

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E. BETA-BLOCKERS

11. Metoprolol, Atenolol, Carvedilol

• Use: Reduce heart rate, BP, myocardial O2 demand; reduce infarct size; prevent ventricular arrhythmias; reduce mortality
• Timing: Start early (within 24 hours) in hemodynamically stable patients; continue long-term
• Mechanism: β1 blockade → reduced HR, contractility, BP → reduced O2 demand

• Cardiogenic shock / acute heart failure
• Bradycardia (HR < 50 bpm) or heart block (PR > 0.24 sec)
• Severe bronchospasm / asthma
• Hypotension (SBP < 90 mmHg)
• Cocaine-induced MI (paradoxical hypertension due to unopposed α stimulation)

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F. ACE INHIBITORS / ARBs

12. Ramipril, Enalapril, Captopril (ACEI); Valsartan, Losartan (ARB)

• Use: Start within 24-48 hours; especially in anterior STEMI, LV dysfunction (EF <40%), heart failure, diabetes
• Mechanism: Inhibit RAAS → reduce ventricular remodeling, reduce afterload, cardioprotective
• Benefit: Reduce mortality and prevent HF progression

• Avoid in hypotension (SBP < 100 mmHg)
• Avoid in bilateral renal artery stenosis
• Contraindicated in pregnancy (teratogenic)
• Monitor: renal function, serum K+ (risk of hyperkalemia)
• Dry cough with ACEI → switch to ARB

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G. STATINS (HMG-CoA Reductase Inhibitors)

13. Atorvastatin (80 mg), Rosuvastatin

• Use: Start HIGH-DOSE statin immediately (regardless of cholesterol level) and continue long-term
• Mechanism: Inhibit HMG-CoA reductase → reduce LDL → plaque stabilization + anti-inflammatory effects
• Precautions:
  • Myopathy / rhabdomyolysis - monitor CK if symptoms
  • Hepatotoxicity - monitor LFTs
  • Drug interactions (macrolides, azole antifungals increase statin levels)
  • Avoid in liver disease / pregnancy

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H. SECONDARY PREVENTION (Long-term, post-discharge)

14. Aldosterone Antagonists (Eplerenone)

• Use: Post-MI patients with EF ≤40% + symptomatic HF or diabetes
• Precaution: Hyperkalemia (monitor K+), avoid if K+ > 5 mEq/L or eGFR < 30

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Summary Table: Drug Mnemonic for MI

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Key Contraindications to Remember (Exam-Focused)

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