Pulmonary Embolism & Venous Thromboembolism

Diagnostic evaluation of suspected acute pulmonary embolism begins with pretest probability stratification rather than reflexive imaging. In patients with a low pretest probability who meet all eight PERC criteria (age <50, HR <100, SpO2 >=95%, no hemoptysis, no estrogen use, no prior DVT/PE, no unilateral leg swelling, no surgery/trauma within 4 weeks), PE is excluded clinically without further testing. In patients with a Wells score <=4.0 ('PE Unlikely') who do not clear PERC, a high-sensitivity D-dimer is ordered: because D-dimer (a fibrin degradation product cross-linked by Factor XIIIa) has a >95% negative predictive value, a normal level (<500 ng/mL, or Wells score is >4.0 ('PE Likely'), D-dimer cannot safely exclude PE and the physician must proceed directly to CT Pulmonary Angiography (CTPA).

CT Pulmonary Angiography (CTPA) is the gold-standard imaging modality for acute PE (sensitivity and specificity >95%), directly visualizing intraluminal pulmonary arterial filling defects and right-to-left ventricular diameter ratio. However, when iodinated IV contrast is contraindicated—specifically in severe contrast anaphylaxis or eGFR <30 mL/min / acute kidney injury—or in young pregnant patients with a normal chest radiograph where minimizing breast radiation is prioritized, a Ventilation-Perfusion (V/Q) nuclear scintigraphy scan is the diagnostic test of choice (a normal chest X-ray is required for an interpretable V/Q scan). In a hemodynamically unstable patient too unstable for transport to the CT scanner, immediate bedside Transthoracic Echocardiography (POCUS) demonstrating acute RV overload (McConnell sign: akinesis of the mid-RV free wall with normal motion of the apex) or a proximal DVT on lower-extremity compression ultrasound justifies emergent systemic thrombolysis.

Although the classical S1Q3T3 pattern (deep S wave in lead I, pathologic Q wave in lead III, and T-wave inversion in lead III) reflects acute right ventricular strain (cor pulmonale), the most common ECG finding in acute PE is sinus tachycardia, followed by nonspecific ST-T wave changes and T-wave inversions in leads V1–V4. On arterial blood gas analysis, tachypnea driven by pulmonary J-receptor stimulation produces an acute respiratory alkalosis (elevated pH, low PaCO2) accompanied by hypoxemia and a widened alveolar-arterial (A-a) oxygen gradient due to dead-space ventilation (V/Q ratio approaching infinity in unperfused alveoli) and V/Q mismatch redistribution.

When thromboembolic obstruction occludes >30% to 50% of the pulmonary vascular bed, combined mechanical obstruction and platelet release of vasoconstrictive thromboxane A2 and serotonin (5-HT) abruptly elevate pulmonary vascular resistance (PVR). Because the thin-walled right ventricle cannot acutely generate systolic pressures above 40 to 50 mmHg, the RV dilates rapidly. Within the rigid pericardial sac, RV dilation shifts the interventricular septum leftward (ventricular interdependence), compressing the left ventricular cavity, impairing LV diastolic filling (preload), dropping systemic cardiac output, and causing right coronary artery ischemia. Consequently, administering large IV fluid boluses (>500 mL) in massive PE worsens RV overdistension and accelerates cardiovascular collapse; instead, support blood pressure with norepinephrine and proceed immediately to systemic thrombolysis (alteplase) or embolectomy.

When an embolus obstructs a pulmonary artery branch, the downstream alveoli continue to be ventilated (V > 0) but receive zero capillary blood flow (Q = 0), creating pathological alveolar dead space (V/Q ratio approaching ∞). However, physiologic dead space alone causes hypercapnia rather than hypoxemia. The profound hypoxemia and widened A-a O₂ gradient in acute PE result from three secondary mechanisms: (1) blood is forcefully redistributed from occluded vessels into non-obstructed capillary beds, causing overperfusion (low V/Q mismatch) that exceeds oxygenation time; (2) platelet release of serotonin (5-HT), thromboxane A₂, and histamine triggers intense local bronchoconstriction and pulmonary vasoconstriction; and (3) loss of alveolar surfactant within 24 hours leads to regional atelectasis and intrapulmonary right-to-left shunting. Tachypnea blows off CO₂ from uninvolved lung units, producing the classic ABG of acute respiratory alkalosis with hypoxemia and an elevated A-a gradient.

Unlike the thick-walled left ventricle, the thin-walled right ventricle (3–4 mm thickness) is designed to pump against a low-impedance pulmonary vascular bed and cannot acutely generate mean pulmonary artery pressures above 40 mmHg. When > 30–50% of the pulmonary vascular bed is occluded by mechanical thrombus and thromboxane/serotonin-mediated vasoconstriction, acute RV afterload surges. The RV dilates via the Law of Laplace, increasing RV wall tension and oxygen demand while simultaneously compressing the right coronary artery during systole and diastole, causing acute RV subendocardial ischemia. Because the pericardium is a rigid sac (ventricular interdependence), the distended RV bows the interventricular septum leftward into the LV cavity (producing a D-shaped left ventricle on short-axis echocardiography), which mechanically impairs diastolic LV filling (preload) and precipitates systemic obstructive shock.

Although the most common chest radiograph in acute PE is completely normal (or shows nonspecific subsegmental atelectasis and small pleural effusion), board exams frequently test three classic radiographic eponyms: (1) Westermark sign—focal oligemia (hyperlucent lung field) distal to a large occluded pulmonary artery due to absent capillary blood volume; (2) Hampton hump—a peripheral, pleural-based, wedge-shaped opacity with its apex pointing toward the hilum, representing hemorrhagic pulmonary infarction (typically occurring when dual bronchial arterial collateral supply is insufficient in distal lobar vessels); and (3) Fleischner sign—prominent enlargement of the central pulmonary artery. On 12-lead ECG, sinus tachycardia is the most common finding, while right-axis deviation, incomplete/complete RBBB, T-wave inversions in V₁–V₄, and the S₁Q₃T₃ pattern (deep S wave in lead I, Q wave and inverted T wave in lead III) indicate acute cor pulmonale.

In emergency department patients with a low clinical gestalt probability (< 15%), the PERC (Pulmonary Embolism Rule-Out Criteria) rule excludes PE without any blood work or imaging if all 8 criteria are negative: Age < 50 years, Heart rate < 100 bpm, Room air SpO₂ ≥ 95%, No prior DVT or PE, No surgery or trauma requiring hospitalization within 4 weeks, No hemoptysis, No exogenous estrogen use, and No unilateral leg swelling. If PERC is positive or probability is intermediate (Modified Wells score ≤ 4), obtain a high-sensitivity quantitative D-dimer (fibrin degradation product generated by plasmin cleavage of cross-linked fibrin). Because baseline D-dimer rises physiologically with aging, for patients over age 50, apply the Age-Adjusted D-Dimer cutoff: Age × 10 ng/mL (or μg/L) (e.g., 750 ng/mL in a 75-year-old) to prevent unnecessary CT scans while maintaining > 99% negative predictive value.

CT Pulmonary Angiography (CTPA) is the diagnostic gold standard for acute PE, directly visualizing intraluminal filling defects down to subsegmental pulmonary arteries (and demonstrating a 'polo mint' or 'railway track' sign). When CTPA is contraindicated due to severe renal insufficiency (eGFR < 30 mL/min/1.73m²) or severe anaphylactic contrast allergy, perform a Ventilation-Perfusion (V/Q) scintigraphy scan, provided the baseline chest X-ray is normal. In pregnant patients presenting with suspected PE, begin with bilateral lower-extremity compression Doppler ultrasound if any leg symptoms exist (if positive for DVT, start LMWH immediately without radiation). If leg ultrasound is negative or leg symptoms are absent, obtain a chest X-ray: if the CXR is normal, perform a V/Q scan (minimizes maternal proliferating breast tissue radiation); if the CXR is abnormal, perform low-dose CTPA.

While transthoracic echocardiography (TTE) has low sensitivity (~50%) for excluding routine low-risk PE, it is the single best immediate bedside diagnostic tool in a hemodynamically unstable patient who is too unstable to travel to the CT scanner. Acute RV pressure overload produces characteristic POCUS findings: RV end-diastolic diameter exceeding LV diameter (RV:LV ratio > 1.0), interventricular septal flattening (D-sign), dilated non-collapsing inferior vena cava, McConnell sign (severe akinesis of the mid-free wall of the right ventricle with preserved hyperdynamic contractility of the RV apex, which is tethered to the hyperdynamic LV apex), and the 60/60 sign (pulmonary artery acceleration time < 60 ms with a tricuspid regurgitation jet gradient < 60 mmHg).

Systemic fibrinolytic therapy (IV Alteplase / tPA 100 mg infused over 2 hours, or 50 mg bolus in cardiac arrest) actively converts plasminogen to plasmin to lyse the obstructing pulmonary arterial thrombus, rapidly unloading right ventricular afterload. It is indicated as first-line reperfusion in Massive (High-Risk) PE (sustained SBP < 90 mmHg or shock) and as rescue therapy in submassive PE patients who deteriorate hemodynamically. Routine systemic thrombolysis is avoided in normotensive submassive PE (PEITHO trial) because intracranial hemorrhage risk outweighs benefit. Absolute contraindications to fibrinolysis include: any prior intracranial hemorrhage, known structural cerebral vascular lesion (AVM) or malignant intracranial neoplasm, ischemic stroke within 3 months, suspected aortic dissection, active bleeding, or closed head/facial trauma within 3 months.

Retrievable Inferior Vena Cava (IVC) filters are deployed percutaneously into the infrarenal IVC (inferior to the renal veins) to mechanically trap large emboli migrating from the iliofemoral veins. On board exams, an IVC filter is indicated in ONLY two strict clinical scenarios: (1) Confirmed acute proximal lower-extremity DVT or acute PE in a patient with an absolute contraindication to therapeutic anticoagulation (e.g., active major GI hemorrhage, recent neurosurgery, hemorrhagic stroke, or severe thrombocytopenia < 50,000/μL), or (2) Recurrent documented PE despite confirmed therapeutic anticoagulation. Once the bleeding contraindication resolves, therapeutic anticoagulation must be resumed and the retrievable filter removed to prevent long-term complications (IVC thrombosis, filter migration, caval perforation, and increased recurrent DVT risk).

In hypotensive patients with Massive PE awaiting reperfusion, resuscitation requires extreme caution with volume loading. A small, cautious fluid challenge (≤ 500 mL of isotonic crystalloid) may help only if central venous pressure is low; further fluid boluses severely overdistend the right ventricle, shifting the interventricular septum into the LV and precipitating cardiac arrest. Instead, immediately initiate Norepinephrine as the first-line vasopressor: its alpha-1 peripheral vasoconstriction raises mean arterial pressure and restores right coronary artery perfusion pressure, while its beta-1 activity augments RV contractility without increasing pulmonary vascular resistance. Dobutamine or inhaled nitric oxide / epoprostenol (selective pulmonary vasodilators) can be added, and intubation should be avoided if possible because positive-pressure ventilation acutely drops venous return and increases RV afterload.

Determining the duration of anticoagulation after a first episode of DVT or PE depends on whether the event was provoked by a reversible risk factor. For a first VTE provoked by a transient major or minor risk factor (e.g., recent orthopedic surgery, trauma, hospitalization ≥ 3 days, prolonged immobilization, or exogenous estrogen therapy that has been discontinued), treat with therapeutic anticoagulation for a finite duration of exactly 3 months. Conversely, for an unprovoked proximal DVT or PE, a recurrent VTE, or a VTE provoked by a persistent irreversible risk factor (such as active malignancy, antiphospholipid syndrome, or high-risk inherited thrombophilia), indefinite (extended/lifelong) anticoagulation is recommended unless bleeding risk is prohibitive.

Board examinations heavily test specific pharmacological antidotes for anticoagulants: (1) Unfractionated Heparin (UFH) is completely reversed by Protamine sulfate (a positively charged arginine-rich polypeptide that ionically binds negatively charged heparin; partially ~60% reverses LMWH); (2) Warfarin in life-threatening bleeding (e.g., intracranial hemorrhage) requires immediate 4-Factor Prothrombin Complex Concentrate (4F-PCC, containing Factors II, VII, IX, X, Protein C, and Protein S) plus IV Vitamin K₁ (phytonadione); (3) Apixaban and Rivaroxaban (Factor Xa inhibitors) are reversed by Andexanet alfa (a recombinant catalytically inactive Factor Xa decoy protein) or 4F-PCC; and (4) Dabigatran (direct thrombin inhibitor) is reversed by Idarucizumab (a humanized monoclonal antibody fragment).

Type II Heparin-Induced Thrombocytopenia (HIT) is an immune-mediated prothrombotic disorder caused by IgG autoantibodies directed against complexes of Heparin and Platelet Factor 4 (PF4). The Fc domain of the IgG-heparin-PF4 complex binds platelet FcγRIIa receptors, triggering massive platelet activation, microparticle release, splenic platelet clearance, and explosive arterial and venous thrombosis ('white clot syndrome'). Evaluated by the 4Ts score, HIT presents with: (1) Thrombocytopenia (> 50% drop in platelet count, nadir typically 20,000–100,000/μL), (2) Timing 5–10 days after starting UFH or LMWH (or < 24 hours if exposed within the past 100 days), (3) Thrombosis (DVT/PE, skin necrosis, limb gangrene), and (4) No other cause. Immediately STOP all heparin (including flushes and LMWH), start a Direct Thrombin Inhibitor (Argatroban [hepatic clearance] or Bivalirudin) or Fondaparinux, and confirm with anti-PF4 ELISA and Serotonin Release Assay (gold standard).