V/Q scan

Last revised by Rob Foley on 20 Jan 2023

V/Q (ventilation/perfusion) scan is a scintigraphic examination of the lung that evaluates pulmonary vasculature perfusion and segmental bronchoalveolar tree ventilation.

A chest x-ray should be performed within 12 to 24 hours and reviewed prior to lung scintigraphy 2 as there are other causes of perfusion defects such as atelectasis

The ventilation scan performed using radio-isotope labeled aerosols like technetium-99m DTPA delivered to the patient through a non-rebreathing mask, with the patient supine. The micro-aerosol particles are small enough to reach the distal tracheobronchial tree and reflect regional ventilation 1. The patient is then imaged in the upright position in three phases: initial breath, equilibrium and washout.

The perfusion lung scan involves injecting Tc-99m MAA intravenously in the supine position. The MAA particles are just small enough to get lodged in the pre-capillary arterioles. A high resolution, large field of view gamma camera is used to image the lungs.

Interpretive criteria developed from the PIOPEDPIOPED II, or PISAPED trials can be used to determine the probability of a pulmonary embolism on a V/Q scan. For the diagnosis of PE, a V/Q scan has a reported sensitivity of 77.4% and specificity of 97.7% when compared to CT pulmonary angiography (CTPA) or digital subtraction angiography (DSA) 3.

A normal ventilation scan (even distribution of radionuclide throughout the lung fields) with multiple, bilateral perfusion defects are the classical findings of pulmonary embolism 1.

Perfusion defects will be present in regions of acute or chronic airway obstruction due to reflex vasoconstriction in the same region. As result, abnormalities of perfusion may be classed as ventilation matched (airway obstruction) or mismatched (unlikely to be due to airway obstruction) 4

Causes of ventilation/perfusion mismatch include:

The presence of a right-to-left shunt can be assessed by searching for evidence of renal or brain activity.

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