This indexes pulmonary vascular resistance to a person's body surface area, allowing fairer comparison across people of different sizes — particularly useful in pediatric patients.
A resistance value that would be abnormal in a small child could be entirely normal in a large adult, so indexing to body surface area allows a fairer comparison across patients of different sizes.
By multiplying pulmonary vascular resistance (in Wood units) by body surface area (in m²), giving Wood units · m².
In pediatric pulmonary hypertension, where body size varies enormously and raw resistance values are hard to interpret without indexing.
PVRI is simply PVR scaled by body surface area — the same underlying resistance, expressed in a size-independent way.