Pulmonary Vascular Resistance (PVR) represents the opposition encountered by blood flowing through the lungs’ circulatory network. This resistance measures the impedance presented by the pulmonary vessels to blood pumped by the heart’s right ventricle. Since the lungs must accommodate the entire cardiac output, PVR must remain low to facilitate massive blood flow without high pressure. Calculating PVR provides medical professionals with a valuable assessment for diagnosing and managing various cardiopulmonary conditions.
The Foundational Principle of Vascular Resistance
The calculation of pulmonary vascular resistance is derived from a physiological application of Ohm’s Law, a principle originally used to describe electrical circuits. In this analogy, the flow of blood through the pulmonary circuit is compared directly to the flow of electrical current. The rate of blood flow is represented by the Cardiac Output (CO), which is the volume of blood the heart pumps each minute.
Resistance (PVR) is calculated by dividing the pressure difference across the circuit by the flow. This pressure difference, known as the pressure gradient, is the driving force that pushes blood through the pulmonary circulation. The resistance value restricts this flow, determining how easily the heart can move blood through the lungs.
The pressure gradient is found by subtracting the pressure at the end of the circuit from the pressure at the beginning. In the pulmonary system, the pressure begins at the mean pulmonary artery pressure and ends at the left atrial pressure. This establishes the physical relationship between the heart’s pumping action, the pressure generated, and the resistance offered by the lung’s blood vessels.
Measuring the Necessary Hemodynamic Variables
Calculating PVR requires obtaining three hemodynamic variables that cannot be measured externally, necessitating an invasive procedure called Right Heart Catheterization (RHC). This method involves inserting a specialized device, often a Swan-Ganz catheter, into a large vein and guiding it through the right side of the heart into the pulmonary artery. RHC is the standard procedure for accurately assessing these pressures and flows.
The first variable measured is the Mean Pulmonary Artery Pressure (mPAP), which represents the average pressure exerted by the blood on the walls of the pulmonary artery. This measurement serves as the inlet pressure for the pulmonary circuit, representing the force with which the right ventricle is pushing blood into the lungs. A healthy mPAP value typically ranges between 9 and 16 mmHg.
The second measurement is the Pulmonary Artery Wedge Pressure (PAWP), which estimates the pressure in the left atrium, the chamber that receives blood from the lungs. The catheter balloon is briefly inflated to block a small branch of the pulmonary artery, allowing the transducer to measure pressure past the pulmonary capillaries.
PAWP is used as the outlet pressure in the PVR calculation because it provides a surrogate for the pressure opposing the flow of blood leaving the pulmonary circulation. Normal PAWP measurements typically fall between 4 and 12 mmHg.
The final variable is the Cardiac Output (CO), the volume of blood flowing through the pulmonary system per minute. This measurement is commonly determined during the RHC using the thermodilution technique or the Fick method. A typical CO value for a healthy adult ranges from 4 to 8 liters per minute. These three measurements are essential inputs for determining the vascular resistance calculation.
The Core Calculation and Unit Conversion
With the pressure and flow variables determined, the pulmonary vascular resistance calculation follows the logic of Ohm’s Law, using the pressure gradient divided by the cardiac output. The formula is expressed as PVR = (mPAP – PAWP) / CO. This relationship defines the impedance to blood flow across the lungs, reflecting the combined resistance of all pulmonary arteries, capillaries, and veins.
The initial result of this calculation is expressed in Wood units (WU). This unit is a convenient simplification for medical professionals since it uses the clinical units of millimeters of mercury (mmHg) for pressure and liters per minute (L/min) for flow. The Wood unit represents the pressure drop in mmHg required to maintain a flow of one liter per minute through the pulmonary vasculature.
For instance, if a patient has a mean pulmonary artery pressure of 25 mmHg and a wedge pressure of 10 mmHg, the pressure gradient is 15 mmHg. If that patient’s cardiac output is measured at 5 L/min, the calculation is (25 – 10) / 5, which yields a PVR of 3.0 Wood units. This value represents the resistance in the pulmonary circuit using readily available hemodynamic measurements.
While Wood units are used clinically, the standard metric unit for vascular resistance is the dyne-second per centimeter to the fifth power (dynes·s/cm⁻⁵). To ensure standardization and comparison, the PVR value calculated in Wood units must be converted to this absolute resistance unit. The full formula includes a constant conversion factor of 80, which is multiplied by the result of the initial calculation.
This factor of 80 converts the pressure from mmHg to the metric unit of dynes/cm² and the flow from L/min to cm³/s. Therefore, a PVR of 3.0 Wood units multiplied by 80 yields a final value of 240 dynes·s/cm⁻⁵. Applying this conversion factor ensures that all measured PVR values are reported in a universally comparable format for diagnostic purposes.
Interpreting the Clinical Significance
PVR provides a direct measure of the vascular health of the lungs. A low PVR is desirable because it means the right side of the heart does not need to exert excessive force to push blood through the pulmonary vessels. Normal PVR values for adults are typically less than 3 Wood units, corresponding to less than 240 dynes·s/cm⁻⁵. While some sources suggest a tighter range, the upper threshold of 3 Wood units is widely accepted for defining disease.
An elevated PVR is the defining characteristic used to diagnose Pulmonary Arterial Hypertension (PAH), a condition characterized by high blood pressure in the arteries of the lungs. When resistance consistently exceeds the 3 Wood unit threshold, pulmonary hypertension is diagnosed, provided the mean pulmonary artery pressure is also elevated. This increased resistance is caused by the progressive narrowing, stiffening, or obstruction of the small pulmonary arterioles.
The physiological consequence of a high PVR is an increased workload on the right ventricle of the heart. The right ventricle must overcome this greater opposition to maintain adequate blood flow. Over an extended period, this sustained effort can lead to compensatory thickening and enlargement of the right ventricular muscle, known as hypertrophy. Eventually, the muscle may become overwhelmed and dilate, causing the right ventricle to weaken and fail.
Physicians use the PVR value for initial diagnosis, to classify disease severity, and to monitor therapy effectiveness. Tracking changes in PVR over time helps determine the success of medications designed to relax the pulmonary vessels and reduce impedance. A reduction in PVR following treatment suggests a positive response, while a persistently high or rising value indicates the disease is advancing or requires a different approach.