Thrombolytics — tPA and Streptokinase

Thrombolytics are fibrinolytic agents that activate the endogenous fibrinolytic cascade to dissolve thrombi, primarily used in acute myocardial infarction (AMI), acute ischemic stroke, and pulmonary embolism. Tissue plasminogen activator (tPA) and streptokinase are the most commonly used agents, with tPA being preferred in contemporary practice due to superior efficacy and reduced immunogenicity. The "golden window" for maximum benefit is within 12 hours of symptom onset for AMI and within 4.5 hours for acute ischemic stroke, with greatest benefit in the first 3-6 hours. These agents represent a cornerstone of reperfusion therapy, competing with percutaneous coronary intervention (PCI) as primary reperfusion strategies in acute coronary syndromes. Understanding the pharmacology, indications, contraindications, and complications of thrombolytics is essential for board certification and clinical practice in emergency and critical care settings.

The fibrinolytic system operates through coordinated activation of plasminogen to plasmin, the serine protease responsible for degrading fibrin matrices and dissolving pathologic thrombi.

Tissue plasminogen activator (tPA) and streptokinase both function as serine protease activators but through distinct molecular mechanisms. tPA (alteplase), a naturally occurring tissue enzyme, directly binds fibrin-bound plasminogen with high specificity, catalyzing conversion to plasmin. This fibrin-specific mechanism minimizes systemic plasminogen depletion and theoretically reduces the bleeding complications associated with non-selective activators. In contrast, streptokinase, a streptococcal protein, forms a 1:1 stoichiometric complex with plasminogen; this complex then activates additional plasminogen molecules. Streptokinase activates both circulating and fibrin-bound plasminogen non-selectively, resulting in systemic fibrinolytic activity and greater consumption of plasminogen and fibrinogen. Both pathways ultimately generate plasmin, which cleaves fibrin cross-links and degrades fibrin polymers, dissolving the thrombotic matrix.

Once generated, plasmin degrades fibrin through endopeptidase cleavage, producing fibrin degradation products (FDPs) and D-dimers. The high local concentration of plasmin within the thrombus preferentially acts on fibrin rather than circulating fibrinogen (particularly with tPA due to its fibrin-specificity). Degradation products are soluble and cleared systemically. This fibrinolytic cascade restores blood flow through the occluded vessel, reperfusing downstream myocardium or brain tissue. The time-dependent nature of reperfusion is critical: early reperfusion (within 1-3 hours) salvages substantial myocardium or neural tissue, while delayed treatment shows diminishing benefit-to-risk ratios.

Streptokinase and, to a lesser degree, tPA cause systemic fibrinolysis, activating circulating plasminogen and degrading fibrinogen and other plasma proteins. This produces a hypofibrinogenemic state and elevation of FDPs, which themselves have anticoagulant properties by inhibiting fibrin polymerization and platelet aggregation. Additionally, FDPs cross-react with platelet surface receptors, impairing platelet function. These systemic anticoagulant effects contribute to both the therapeutic benefit (reduced propagation of thrombosis) and the major liability (systemic bleeding risk). The magnitude of systemic effect differs: streptokinase causes more profound systemic fibrinolysis, while tPA maintains greater fibrin-specificity (though still producing detectable systemic effects at therapeutic doses).

tPA demonstrates rapid thrombolytic activity; plasmin generation begins within minutes, with peak fibrinolytic activity achieved within 5-10 minutes of infusion. The half-life of tPA is approximately 5 minutes, but the thrombolytic effect persists longer due to plasmin generation and binding to fibrin within the thrombus. Streptokinase has a slower onset (15-20 minutes to peak effect) due to the requirement for formation of the plasminogen-streptokinase complex, but achieves more sustained systemic fibrinolysis. Both agents are cleared hepatically (tPA) or by antibody neutralization and reticuloendothelial clearance (streptokinase).

Streptokinase, derived from Group A streptococci, is immunogenic; patients frequently develop neutralizing antibodies after initial exposure, rendering subsequent streptokinase treatment ineffective if administered within 6-12 months. This limits its re-use and necessitates alternative thrombolytics (such as tPA) in patients with prior streptokinase exposure. tPA, being a recombinant human protein, is non-immunogenic and can be re-administered without loss of efficacy. Additionally, tPA's fibrin-specificity theoretically preserves circulating fibrinogen better than streptokinase, though clinical fibrinogen levels still decline with tPA.

Thrombolytics are not etiologic agents; rather, they are therapeutic interventions deployed in response to thrombotic occlusive disease. The conditions necessitating thrombolytic therapy are listed below:

STEMI results from acute thrombotic occlusion of an epicardial coronary artery, typically precipitated by atherosclerotic plaque rupture, platelet aggregation, and tissue factor activation. Risk factors include smoking, hyperlipidemia, hypertension, diabetes, male sex, family history, and thrombophilic states. Thrombolytics are indicated as primary reperfusion therapy in STEMI when PCI is not available within 120 minutes (or 90 minutes with high-risk features).

AIS caused by thrombotic or embolic occlusion of a cerebral artery results in neuron death due to ischemia. Risk factors include atrial fibrillation, prior stroke, hypertension, diabetes, and age. Intravenous tPA is the standard thrombolytic agent for AIS within 4.5 hours of symptom onset (FDA approval within 3 hours, though Class IIb evidence supports use up to 4.5 hours). Streptokinase is contraindicated in stroke due to increased hemorrhagic transformation risk.

PE results from thromboembolism to the pulmonary circulation, typically originating from deep venous thrombosis (DVT) in the lower extremities. Risk factors include immobility, malignancy, hypercoagulable states, surgery, and trauma. Thrombolytics are reserved for hemodynamically significant (massive) PE or proximal DVT with limb-threatening venous gangrene.

Mechanical valve thrombosis is a prosthetic valve complication managed with anticoagulation escalation, thrombolytics, or surgical intervention. Thrombolytics are considered in hemodynamically compromised patients when surgery is not immediately available.

Acute arterial thrombosis or embolism causes limb ischemia. Thrombolytics are considered in selected cases with viable limb within the treatment window.

The clinical presentation of conditions warranting thrombolytic therapy reflects the organ system affected and the acuity of vascular occlusion. Thrombolytics themselves do not produce distinctive symptoms; rather, they are deployed to reverse the ischemic manifestations of thrombotic disease.

STEMI presents with acute-onset chest pain (classically substernal, crushing, radiating to left arm or jaw), often accompanied by autonomic symptoms (diaphoresis, nausea, dyspnea). The pain is typically unresponsive to nitrates and lasts >20 minutes. Patients may present with atypical symptoms, particularly diabetics and women, with complaints of epigastric discomfort, dyspnea alone, or fatigue. Physical examination may reveal an anxious, diaphoretic patient with tachycardia, tachypnea, and hemodynamic instability (hypotension, pulmonary edema).

AIS presents with sudden-onset focal neurological deficits corresponding to the vascular territory occluded. Common presentations include unilateral weakness or numbness (motor cortex or internal capsule distribution), aphasia (dominant hemisphere), hemineglect (nondominant parietal lobe), visual field defects (optic radiations), or ataxia/vertigo (brainstem or cerebellum). The National Institutes of Health Stroke Scale (NIHSS) quantifies severity; higher scores (>15) indicate extensive stroke burden and potentially higher thrombolytic risk.

PE presents with acute dyspnea, chest pain (pleuritic if peripheral PE with infarction), hemoptysis, or syncope (if massive PE with acute right ventricular failure). Tachycardia and hypoxemia are cardinal findings. Massive PE may present with cardiogenic shock and hemodynamic collapse, representing the primary indication for thrombolytics in PE.

Acute limb ischemia presents with sudden pain, pallor, pulselessness, paresthesias, and paralysis (the "6 P's"). Sensory and motor deficits progress over hours if reperfusion is not achieved, eventually leading to tissue necrosis.

The diagnosis of conditions warranting thrombolytics is established through clinical presentation combined with confirmatory testing. The urgency of diagnosis necessitates rapid decision-making algorithms to minimize time-to-treatment.

12-lead electrocardiography (ECG) is the gold standard for STEMI diagnosis and should be obtained within 10 minutes of presentation. STEMI is defined by ST-segment elevation ≥1 mm in two contiguous leads (≥2 mm in V1-V3) in the appropriate clinical context. ECG identifies the infarct-related artery territory. Cardiac troponin (high-sensitivity troponin I or T) rises within 2-4 hours and confirms myocardial injury; troponin elevation alone is not specific for acute MI (chronic elevation occurs in chronic kidney disease, heart failure, sepsis). Serial troponins establish rising/falling pattern diagnostic of acute MI. CK-MB was historically used but is now rarely employed due to inferior specificity. Myoglobin rises rapidly (within 1 hour) but lacks cardiac specificity. In STEMI, troponin and CK-MB rise concordantly. Natriuretic peptides (BNP, NT-proBNP) are nonspecific markers of ventricular stress, useful for prognosis but not diagnosis of acute MI.

Non-contrast CT brain must be obtained urgently to exclude hemorrhagic stroke (absolute contraindication to IV tPA). CT brain is normal in early AIS; ischemic changes (loss of gray-white differentiation, hypodensity) may appear after 6-12 hours. Diffusion-weighted imaging (DWI) MRI is the most sensitive imaging for acute ischemia, detecting cytotoxic edema within minutes of symptom onset. Perfusion imaging (perfusion-weighted imaging, PWI) may show perfusion deficits exceeding DWI signal abnormalities, identifying the "penumbra" (tissue at risk). CT angiography (CTA) or MR angiography (MRA) identifies large vessel occlusions, informing thrombectomy candidacy. The NIHSS quantifies stroke severity (scores 0-42; higher scores indicate larger strokes); baseline NIHSS predicts outcome and risk of hemorrhagic transformation with thrombolytics.

D-dimer (a fibrin degradation product) is elevated in acute PE but lacks specificity; normal D-dimer effectively excludes PE in low-risk patients. CT pulmonary angiography (CTPA) is the confirmatory gold standard, demonstrating filling defects in pulmonary arteries; sensitivity approaches 95% for subsegmental PE. Ventilation-perfusion (V/Q) scan is an alternative in patients with renal insufficiency or contrast allergy. Echocardiography demonstrates right ventricular dilatation and dysfunction in massive PE but is not diagnostic. ECG may show sinus tachycardia, right heart strain (S1Q3T3 pattern, though nonspecific), or atrial fibrillation. Troponin and BNP elevation correlate with adverse outcomes in PE.

Duplex ultrasound with color and pulsed-wave Doppler identifies arterial thrombosis and assesses flow direction. CT or MR angiography provides anatomic detail and vessel visualization. Digital subtraction angiography is the reference standard and allows simultaneous mechanical or catheter-based thrombolytic intervention.

For STEMI, door-to-balloon time (primary PCI) and door-to-needle time (thrombolytics) determine appropriateness. Thrombolytics are preferred if PCI is not available within 120 minutes (or 90 minutes in high-risk STEMI). For AIS, the time from last known normal determines tPA eligibility; IV tPA is FDA-approved within 3 hours and has Class IIb evidence to 4.5 hours. The NIHSS score predicts hemorrhagic transformation risk; very high NIHSS (>25) may warrant mechanical thrombectomy over thrombolytics. For PE, hemodynamic status determines thrombolytic indication: massive PE (hypotension, shock) warrants immediate thrombolytics or thrombectomy; submassive PE (right ventricular dysfunction without hypotension) remains controversial.

Treatment with thrombolytics requires rapid assessment for eligibility, swift administration, and vigilant monitoring for efficacy and complications. Treatment regimens differ by indication and thrombolytic agent selected.

tPA is preferred over streptokinase in contemporary STEMI management due to superior patency rates and reduced mortality. tPA dosing in STEMI employs a weight-based regimen: 15 mg intravenous (IV) bolus, followed by 0.75 mg/kg infusion over 30 minutes (maximum 50 mg), then 0.5 mg/kg infusion over 60 minutes (maximum 35 mg). Total dose typically ranges from 80-100 mg. Administration should occur within 30 minutes of hospital arrival (door-to-needle goal). tPA achieves TIMI (Thrombolysis in Myocardial Infarction) grade 3 flow in approximately 50-65% of arteries at 90 minutes. The benefit is time-dependent: mortality reduction is greatest in the first 6 hours (absolute reduction ~10 lives per 1000 treated) and diminishes progressively, becoming negligible after 12 hours.

Streptokinase is less frequently used in contemporary practice due to lower patency rates (TIMI 3 flow ~40-50% at 90 minutes) compared to tPA and higher rates of allergic reactions and reperfusion arrhythmias. When used, the standard dose is 1.5 million units IV infused over 60 minutes. Streptokinase is contraindicated in patients with prior exposure within 6-12 months (due to neutralizing antibodies) or allergies to strept

Contraindications (ACC/AHA acute coronary syndromes and AHA/ASA acute ischemic stroke guidance): any prior intracranial hemorrhage, known cerebral AVM or malignant intracranial neoplasm, ischemic stroke within 3 months, significant closed head or facial trauma within 3 months, intracranial/intraspinal surgery, suspected aortic dissection, active bleeding or bleeding diathesis, and severe uncontrolled hypertension. For stroke, blood pressure must be lowered below 185/110 mm Hg before treatment (labetalol or nicardipine) and kept below 180/105 mm Hg for 24 hours after.