Terminology for Calculating Pulmonary Vascular Resistance
All three terms—PCWP (Pulmonary Capillary Wedge Pressure), PAWP (Pulmonary Artery Wedge Pressure), and PAOP (Pulmonary Artery Occlusion Pressure)—are interchangeable and refer to the same measurement used in the PVR calculation formula. 1
Why These Terms Are Equivalent
The measurement technique is identical for all three terms: a balloon-tipped catheter is advanced into a small pulmonary arterial branch until the inflated balloon "wedges" or occludes flow, allowing measurement of downstream left atrial pressure. 1 This single measurement has simply been given different names in different guidelines and publications:
- PCWP is the traditional American terminology used predominantly in U.S. guidelines 2
- PAWP is the preferred European terminology, particularly in ESC/ERS guidelines 2, 1
- PAOP is an alternative descriptor emphasizing the occlusion mechanism 1
The Standard PVR Formula
The formula for calculating PVR is: PVR = (mPAP - PCWP) / CO, where mPAP is mean pulmonary artery pressure and CO is cardiac output. 1 This can equivalently be written as:
The result is expressed in Wood units, with normal values <2-3 Wood units. 1 To convert to dynes·s·cm⁻⁵, multiply by 80. 1
Clinical Application Thresholds
Pre-capillary pulmonary hypertension is defined by PVR >3 Wood units in 2015 guidelines and >2 Wood units in 2024 guidelines, regardless of which terminology you use for the wedge pressure measurement. 1 For congenital heart disease, shunt closure is contraindicated when PVR >2.5 Wood units. 1
Critical Measurement Considerations
Accurate PCWP/PAWP/PAOP measurement is essential for correct PVR calculation. 2 The measurement must be obtained with proper technique, including zeroing the external pressure transducer at the mid-thoracic line and avoiding repeated balloon deflations and inflations in distal pulmonary arteries. 1 A PCWP/PAWP >15 mmHg excludes the diagnosis of pre-capillary pulmonary arterial hypertension. 2
Common Pitfall: PCWP vs LVEDP Discrepancy
Roughly half of patients presumed to have pulmonary arterial hypertension based on PCWP ≤15 mmHg may actually have pulmonary venous hypertension when LVEDP (left ventricular end-diastolic pressure) is directly measured. 3 In patients with heart failure and normal ejection fraction with diastolic dysfunction, PCWP may be mildly elevated or at the higher end of normal at rest, and exercise hemodynamics or volume challenge can show disproportionate increases in PCWP. 2 In rare cases, left heart catheterization may be required for direct assessment of LVEDP to accurately distinguish pulmonary arterial hypertension from left heart disease. 2
Documentation Recommendation
When documenting hemodynamic measurements, note whether PCWP was recorded with or without V-wave, as this should be specified. 1 Use whichever terminology (PCWP, PAWP, or PAOP) is standard in your institution or guideline framework, but recognize that all three terms represent the same measurement and yield identical PVR calculations.