When and at what dose should IV (intravenous) heparin be started in a patient with myocardial infarction not undergoing PCI (percutaneous coronary intervention)?

IV Heparin in Myocardial Infarction Without PCI

For STEMI patients not undergoing PCI, start IV unfractionated heparin immediately upon diagnosis with a weight-based bolus of 60 U/kg (maximum 4,000 U) followed by continuous infusion at 12 U/kg/hour (maximum 1,000 U/hour), targeting an aPTT of 1.5-2.0 times control (50-70 seconds). 1, 2, 3

Timing of Initiation

• Begin IV heparin as soon as the diagnosis of myocardial infarction is established, regardless of whether fibrinolytic therapy is planned 4, 2
• For patients receiving fibrinolytic therapy, administer heparin concurrently with or immediately after the fibrinolytic agent 4, 3
• Do not delay heparin administration while awaiting laboratory confirmation if clinical and ECG findings strongly suggest MI 2

Dosing Regimen

Initial Bolus Dose

• 60 U/kg IV bolus (maximum 4,000 U) for patients receiving fibrinolytic therapy 4, 3, 5
• 60-70 U/kg IV bolus (maximum 5,000 U) for patients with NSTE-ACS or those not receiving reperfusion therapy 4, 2, 5

Continuous Infusion

• 12 U/kg/hour (maximum 1,000 U/hour) adjusted to maintain therapeutic aPTT 4, 3, 5
• Target aPTT: 50-70 seconds (1.5-2.0 times control) 4, 1, 3

Monitoring Protocol

• Check aPTT at 3,6,12, and 24 hours after initiation 4, 2, 3
• Recheck aPTT 4-6 hours after any dose adjustment 3
• Monitor daily platelet counts to detect heparin-induced thrombocytopenia 2, 3
• aPTT values >70 seconds are associated with increased mortality, bleeding, and reinfarction risk—adjust infusion downward immediately 4

Duration of Therapy

• Continue heparin for minimum 48 hours after fibrinolytic therapy 4, 2, 3
• Preferably extend through duration of hospitalization, up to 8 days 2, 3
• May discontinue earlier if revascularization (delayed PCI) is performed 2, 3
• For patients at high risk for systemic or venous thromboembolism, continuation beyond 48 hours is reasonable 1

Rationale for Heparin Use

With Fibrinolytic Therapy

• Improves coronary artery patency when used with tissue plasminogen activators (alteplase, reteplase, tenecteplase) 4, 1
• Prevents reocclusion after successful fibrinolysis, though benefit must be balanced against bleeding risk 4, 1
• Particularly important with fibrin-specific agents (t-PA, r-PA, TNK-tPA) which have shorter half-lives 2

Without Reperfusion Therapy

• Prevents left ventricular thrombus formation, especially in anterior wall MI 4, 2
• Reduces risk of systemic embolization and stroke 4, 2
• Prevents deep vein thrombosis and pulmonary embolism during prolonged bed rest 4
• Acts as antithrombotic therapy to prevent infarct extension 4

Alternative Anticoagulant Options

Enoxaparin (LMWH)

• For patients <75 years without renal dysfunction: 30 mg IV bolus followed by 1.0 mg/kg subcutaneous every 12 hours 4, 2, 3
• For patients ≥75 years: Omit IV bolus, start with 0.75 mg/kg subcutaneous every 12 hours 4, 2
• Contraindicated if creatinine clearance <30 mL/min 4
• Enoxaparin showed reduced death and reinfarction compared to UFH in the ExTRACT trial, though with increased non-cerebral bleeding 4

Fondaparinux

• 2.5 mg subcutaneous daily for patients not undergoing PCI 4
• Must add additional anticoagulant with anti-IIa activity if patient subsequently undergoes PCI 4
• Not recommended as sole anticoagulant for primary PCI due to catheter thrombosis risk 4

Critical Pitfalls to Avoid

• Never exceed maximum bolus doses (4,000-5,000 U depending on indication) even in obese patients—overdosing increases bleeding risk without improving efficacy 3, 5
• Do not use fixed-dose regimens—weight-based dosing significantly improves outcomes 1, 5
• Avoid aPTT >70 seconds—supratherapeutic anticoagulation paradoxically increases mortality and reinfarction rates 4
• Do not switch between UFH and LMWH—this increases bleeding complications 3
• Never discontinue heparin abruptly in high-risk patients (large anterior MI, atrial fibrillation, severe LV dysfunction)—taper or transition to oral anticoagulation 1, 3
• Do not use LMWH in elderly patients (≥75 years) receiving fibrinolytics without dose adjustment—this significantly increases intracranial hemorrhage risk 4, 2

Adjunctive Antiplatelet Therapy

• Aspirin 150-325 mg should be given concurrently (oral or IV if unable to swallow) 4
• Clopidogrel 75 mg daily (300 mg loading dose if age ≤75 years) should be added 4
• Heparin plus aspirin is the minimum standard of care for all MI patients not undergoing immediate PCI 4