Heparin Therapy in Acute to Early Subacute Myocardial Infarction
Yes, you should start heparin therapy in acute to early subacute myocardial infarction, with the specific regimen depending on whether the patient receives thrombolytic therapy and which agent is used. 1, 2
Treatment Algorithm Based on Reperfusion Strategy
If Patient Receives Alteplase (tPA)
- Administer intravenous heparin for at least 48 hours 1
- Initial bolus: 60 U/kg (maximum 4000 U) 2, 3
- Continuous infusion: 12 U/kg/hour (maximum 1000 U/hour) 2, 3
- Target aPTT: 50-75 seconds (1.5-2.0 times control) 1, 2
- The evidence shows higher infarct-related artery patency rates with this combination, though clinical outcome data show increased bleeding risk 1
If Patient Receives Non-Selective Thrombolytics (Streptokinase, Anistreplase, Urokinase)
- Do NOT give routine intravenous heparin within 6 hours if patient is not at high risk for systemic embolism 1
- These agents cause systemic fibrinolysis with depletion of coagulation factors, making additional anticoagulation less necessary and potentially harmful 1
- Exception: Give heparin if high embolic risk (see below) 1
If Patient Undergoes Primary PCI
- Administer high-dose intravenous heparin 1
- Target ACT: 300-350 seconds 1
- This recommendation comes from general angioplasty data showing lower complication rates at these ACT levels 1
If Patient Does NOT Receive Reperfusion Therapy
- Administer subcutaneous heparin 7,500-12,500 U twice daily until fully ambulatory 1
- Alternative: Intravenous heparin is acceptable 1
- Historical data (pre-aspirin era) showed 17% mortality reduction and 22% reduction in reinfarction risk 1
High-Risk Patients Requiring Heparin Regardless of Thrombolytic Choice
Always give heparin if any of the following are present: 1
- Left ventricular thrombus visualized on echocardiography 1
- Atrial fibrillation 1
- History of previous embolic event 1
- Large anterior wall infarction 1
For these patients, the SCATI trial showed in-hospital mortality of 4.6% with heparin versus 8.8% without, plus reduced stroke rates 1
Critical Monitoring Requirements
Check aPTT every 4 hours initially when using IV heparin, then at appropriate intervals 4
- The relationship between aPTT and outcomes is U-shaped: both subtherapeutic and supratherapeutic levels increase adverse events 1, 3
- Death, stroke, reinfarction, and bleeding are lowest in the 50-75 second aPTT range 1
Monitor platelet counts, hematocrit, and stool occult blood throughout therapy 4
Duration of Therapy
Continue heparin infusion for 48 hours after thrombolysis with alteplase 1
Continuation beyond 48 hours should be restricted to patients at high risk for systemic or venous thromboembolism 1
Important Caveats and Contraindications
Do not use low molecular weight heparin in patients >75 years receiving thrombolytics due to increased bleeding risk 2
Avoid LMWH in significant renal dysfunction with thrombolytics 2
In right ventricular infarction with hypotension, delay aggressive anticoagulation until hemodynamic stabilization with IV fluids 2
Never administer intramuscularly due to frequent hematoma formation 4
The evidence base for heparin in MI is complex and somewhat contradictory. While older pre-thrombolytic era trials showed mortality benefit 1, modern data with concurrent aspirin use show the benefit is less clear 5. The strongest contemporary evidence supports heparin use with fibrin-specific agents like alteplase to maintain vessel patency 1, and in high-risk embolic situations regardless of reperfusion strategy 1. The narrow therapeutic window (aPTT 50-75 seconds) requires vigilant monitoring, as both inadequate and excessive anticoagulation worsen outcomes 1, 3.