Pulmonary vascular resistance (PVR), when expressed in Woods units, provides a standardized measure of resistance to blood flow within the pulmonary vasculature. It is derived by dividing the mean pulmonary artery pressure (in mmHg) minus the mean pulmonary capillary wedge pressure (also in mmHg) by the cardiac output (in liters per minute). The resulting value is typically expressed as Woods units, where 1 Woods unit is equivalent to approximately 80 dyns/cm5. As an example, if a patient has a mean pulmonary artery pressure of 20 mmHg, a mean pulmonary capillary wedge pressure of 10 mmHg, and a cardiac output of 5 L/min, the calculated resistance would be (20-10)/5 = 2 Woods units.
Determining the pressure gradient across the pulmonary circulation and normalizing it for cardiac output is crucial in assessing pulmonary hypertension and right ventricular function. Elevated resistance may indicate underlying pulmonary vascular disease, such as pulmonary arterial hypertension, chronic thromboembolic pulmonary hypertension, or left heart failure. Historically, this metric has been instrumental in guiding clinical decisions related to medical and surgical management of these conditions. The standardization afforded by Woods units allows for comparison of this resistance across different patients and institutions, improving the reliability of clinical decision-making.
