How to Measure Pulmonary Vascular Resistance (PVR)
PVR is calculated using the formula: PVR = (mPAP - PAWP) / CO, where mPAP is mean pulmonary artery pressure, PAWP is pulmonary artery wedge pressure, and CO is cardiac output, all measured during right heart catheterization. 1
Required Measurements
To calculate PVR, you need three hemodynamic values obtained during right heart catheterization:
1. Mean Pulmonary Artery Pressure (mPAP)
- Measured directly through the pulmonary artery catheter (PAC) when positioned in the main pulmonary artery
- The PAC is a 110 cm length, 6-F to 8-F diameter catheter with a balloon tip that can be inflated with 1.5 mL of air 1
2. Pulmonary Artery Wedge Pressure (PAWP)
- Obtained by advancing the PAC with balloon inflated until it wedges in a distal pulmonary artery branch
- This reflects left atrial pressure in the context of normally functioning mitral and aortic valves 1
3. Cardiac Output (CO)
Use thermodilution (TD) method over indirect Fick methods, as TD is superior as a predictor of mortality. 1 The direct Fick method is the gold standard but requires additional equipment to measure oxygen consumption (VO₂), making it less universally available 1.
Thermodilution Technique:
- Rapidly inject saline into the proximal port of the PAC
- The integrated thermistor measures temperature change as injectate passes through the pulmonary artery
- Quicker temperature normalization = higher cardiac output 1
Perform measurements in triplicate; if measurements vary by >10%, obtain five measurements and average after discarding the highest and lowest values 1
The Calculation
PVR (Wood Units) = (mPAP - PAWP) / CO
Where:
- mPAP and PAWP are in mm Hg
- CO is in L/min
- Result is expressed in Wood Units (WU)
Clinical Significance
The current diagnostic threshold for elevated PVR is >2 WU (lowered from the previous 3 WU threshold by the 2024 Seventh World Symposium on Pulmonary Hypertension), as even mild elevations in PVR are not benign 1
Critical Pitfalls to Avoid
Cardiac Output Measurement Errors
Inaccurate CO values result in clinically important misclassification of pulmonary hypertension 1. Be aware that:
- Significant tricuspid regurgitation causes backward flow of injectate into the right atrium, affecting TD accuracy
- Significant intracardiac shunting can affect TD accuracy 1
Indirect Fick Method
Avoid using estimated VO₂ values for indirect Fick calculations, as this introduces clinically significant error 1
Intrathoracic Pressure Effects
Severe emphysema, obstructive lung disorders, and obesity can increase intrathoracic pressures, affecting the accuracy of PAP and PAWP measurements 1