Which scan is typically the first-line test for pulmonary embolism?

The first-line test for diagnosing pulmonary embolism is the ventilation-perfusion (V/Q) scan. This imaging technique evaluates the distribution of air flow (ventilation) and blood flow (perfusion) in the lungs. When assessing for pulmonary embolism, the V/Q scan can identify mismatches between ventilation and perfusion; specifically, areas of the lung that are well-ventilated but poorly perfused suggest the presence of an embolism obstructing blood flow.

The V/Q scan is particularly useful in patients who may have contraindications to other imaging modalities, such as a computed tomography pulmonary angiography (CTPA), for example, in those with renal impairment or allergies to contrast dye.

In essence, the V/Q scan helps in identifying areas of the lung that have effective ventilation but inadequate perfusion due to blocked pulmonary arteries, which is indicative of a pulmonary embolism. Hence, it serves as a crucial initial diagnostic tool in evaluating patients with suspected pulmonary embolism.