How VTE is tested on Step 1

Expect a postoperative, immobilized, pregnant, or cancer patient with sudden dyspnea, and questions about the next diagnostic step, the blood gas, or anticoagulant choice.

workspace_premium Probability first, then test

Use pretest probability to choose the test. A negative D-dimer rules out PE only when probability is low or intermediate; high probability goes straight to CT pulmonary angiography.

Risk factors and pathophysiology

Any factor in Virchow triad raises risk; many patients have several.

Virchow elementExamples
StasisImmobilization, long flights, surgery, heart failure, atrial fibrillation
Endothelial injuryTrauma, surgery (especially orthopedic), central venous catheters
HypercoagulabilityCancer, pregnancy and postpartum, estrogen therapy, factor V Leiden, prothrombin mutation, antiphospholipid syndrome, nephrotic syndrome

Factor V Leiden (resistance to activated protein C) is the most common inherited thrombophilia in people of European ancestry.

Leg DVTProximalveinsEmbolusLodges inpulmonaryarteryDeadspaceVentilated,not perfusedHypoxemia↑ A–agradient; ↓PaCO₂RV strain↑ PApressure;shock ifmassive
PE physiology. Blood is redirected to the remaining vessels, overperfusing them relative to ventilation; bronchoconstriction and atelectasis add shunt.

Diagnosis

Clinical prediction rules guide testing.

1. PretestWells score (or PERC rule if very low risk)2. Low/intermediateD-dimer: negative → PE excluded; positive → imaging3. High probabilityCT pulmonary angiography directly4. CT not possibleV/Q scan (pregnancy, renal failure, contrastallergy)5. UnstableBedside echo for RV strain; treat
Diagnostic pathway. Compression ultrasound diagnoses DVT and, if positive in suspected PE, justifies treatment.
FindingTypical result in PE
Arterial blood gasHypoxemia, hypocapnia, respiratory alkalosis, ↑ A–a gradient
ECGSinus tachycardia most common; S1Q3T3, right heart strain, new RBBB (uncommon)
Chest x-rayOften normal; Hampton hump (wedge infarct), Westermark sign (oligemia)
Troponin, BNPElevated with RV strain — worse prognosis
EchocardiogramRV dilation, McConnell sign

A normal chest x-ray in a hypoxemic patient should raise suspicion for PE.

Treatment

Treatment depends on hemodynamic stability and bleeding risk.

Stable PE or DVTAnticoagulate promptlyDOAC (apixaban, rivaroxaban) preferredLMWH in pregnancy and many cancersWarfarin with heparin bridge if neededIVC filter only if anticoagulationimpossibleMassive PE (hypotension)Systemic thrombolysis (alteplase)Unfractionated heparinCatheter-directed therapy or embolectomyif lysis contraindicatedSupportive: oxygen, cautious fluids,vasopressorsHigh short-term mortalityvs
Treat by stability. Submassive PE (RV strain without hypotension) is anticoagulated and monitored closely.

Clinical application

The blood gas is a frequent test of PE physiology.

functions Worked example — the postoperative dyspnea

Five days after a total knee replacement, a 66-year-old develops sudden dyspnea and pleuritic chest pain. HR 118, RR 28, SpO₂ 89%. ABG: pH 7.49, PaCO₂ 30 mm Hg, PaO₂ 62 mm Hg on room air. Chest x-ray is clear. Explain the gas and the next step.

  1. Recent orthopedic surgery and immobility give high pretest probability.
  2. Low PaO₂ with a clear x-ray suggests V/Q mismatch from dead space.
  3. Hypoxemia and anxiety drive hyperventilation, lowering PaCO₂ and causing respiratory alkalosis.
  4. A–a gradient = (150 − 30/0.8) − 62 ≈ 50 mm Hg, clearly elevated.

Answer. Pulmonary embolism with V/Q mismatch. With high probability, go directly to CT pulmonary angiography (skip D-dimer) and start anticoagulation if no contraindication.