ventilation-perfusion scan

**Technique** - Perfusion: IV Tc-99m macroaggregated albumin (MAA) particles lodge in pulmonary capillaries and arterioles in proportion to blood flow. Gamma camera images show the distribution of flow. - Ventilation: the patient inhales a radiotracer (Xe-133 gas, or Tc-99m DTPA or Technegas aerosol). Images show the distribution of airflow. - The two sets of images are compared segment by segment. A chest X-ray is always obtained to help interpret the results.

**Interpretation** - Normal: homogeneous uptake on both ventilation and perfusion. This effectively rules out PE (high negative predictive value). - High probability (PE): one or more segmental or larger **mismatched** defects. Perfusion is absent but ventilation is preserved, so V/Q is high in that region (dead-space-like physiology). - Matched defects (both ventilation and perfusion reduced): seen in COPD, pneumonia, atelectasis, pulmonary edema, effusion, and mucus plugging. Hypoxic vasoconstriction reduces perfusion in poorly ventilated areas. These are low or intermediate probability for PE. - Indeterminate: intermediate-probability results need further testing (CTPA, lower extremity duplex, clinical pretest probability).

**Pathophysiology** - PE blocks blood flow to ventilated alveoli, which raises dead space and the V/Q ratio (V/Q → ∞ with complete obstruction). This causes hypoxemia and a widened A–a gradient. - Airway obstruction or consolidation lowers V/Q toward 0 (shunt). This is the opposite of PE and gives matched defects. - In the normal upright lung, V/Q is highest at the apex (~3) and lowest at the base (~0.6). Both ventilation and perfusion are greater at the base, but perfusion is more so.

**Clinical use** - Alternative to CTPA, which is the first-line imaging test for suspected PE. Use V/Q in pregnancy (lower breast radiation than CT), renal failure, contrast allergy, and young patients. - Best in patients with a normal chest X-ray and no significant underlying lung disease. - Diagnostic work-up: use Wells or PERC criteria for pretest probability. If low probability, get a D-dimer, and if it is negative, PE is excluded. If moderate or high probability, get CTPA (or V/Q if CTPA is contraindicated). If the patient is unstable, treat empirically and get bedside echocardiography. - Other uses: quantifying split lung function before lung resection or lung volume reduction surgery, and evaluating chronic thromboembolic pulmonary hypertension (CTEPH). V/Q is more sensitive than CT for CTEPH, so it is the preferred screening test.

**Board pearls** - Classic stem: a pregnant woman or a patient with renal failure and sudden dyspnea, pleuritic chest pain, and tachycardia leads to a V/Q scan showing a mismatch. - Ventilation is normal and perfusion is decreased in PE. - A normal V/Q scan excludes PE. - Pulmonary angiography is the gold standard but is rarely used now. - Related findings of PE: S1Q3T3 on ECG, Hampton hump, Westermark sign, and elevated D-dimer (sensitive, not specific). - Treatment: anticoagulation (heparin, then DOAC or warfarin). Use thrombolytics for massive PE with hemodynamic instability.