How is pulmonary vascular resistance (PVR) calculated and what does it indicate?

• A. PVR = mean PA pressure / CO; indicates systemic vascular resistance.
• B. PVR = (mean PA pressure − PCWP) / CO; measures systemic vascular resistance.
• C. PVR = (mean PA pressure − PCWP) / CO; reflects resistance in the pulmonary vasculature.
• D. PVR = mean PA pressure − PCWP; indicates venous return.

Answer: (D)

Pulmonary vascular resistance measures the afterload the right ventricle faces as blood moves through the lungs. It is calculated from the pressure gradient across the pulmonary circulation divided by flow, specifically PVR = (mean pulmonary arterial pressure − pulmonary capillary wedge pressure) / cardiac output. This puts the resistance in the pulmonary bed into a flow-adjusted context, so the units are Wood units when CO is in liters per minute.

Interpreting PVR helps distinguish where the problem lies. If PVR is elevated with a normal wedge pressure, that points to a primary pulmonary vascular issue (pre-capillary pulmonary hypertension). If the wedge pressure is high due to left-heart dysfunction, PVR may be normal or only mildly elevated, since the elevated left-sided pressure is the main driver of the pulmonary pressure.

Normal PVR is about 1–3 Wood units; higher values indicate increasing pulmonary vascular resistance and greater right ventricular afterload. The idea that PVR is just a simple pressure difference or that it indicates venous return isn’t correct—the division by CO makes it a true resistance measure reflecting the pulmonary vasculature’s resistance to flow.