When calculating Pulmonary Vascular Resistance (PVR), is it correct to use Pulmonary Capillary Wedge Pressure (PCWP) or Pulmonary Artery Wedge Pressure (PAWP)?

Calculating Pulmonary Vascular Resistance: PCWP, PAWP, and PAOP Are Interchangeable Terms

When calculating PVR, PCWP (Pulmonary Capillary Wedge Pressure), PAWP (Pulmonary Artery Wedge Pressure), and PAOP (Pulmonary Artery Occlusion Pressure) are all correct and refer to the same measurement—they are interchangeable terms for the same hemodynamic parameter. 1

Standard PVR Calculation Formula

The universally accepted formula for calculating pulmonary vascular resistance is:

PVR = (mPAP - PCWP) / CO

Where:

• mPAP = mean pulmonary artery pressure (mmHg) 1
• PCWP/PAWP/PAOP = pulmonary capillary wedge pressure (mmHg) 1
• CO = cardiac output (L/min) 1

The result is expressed in Wood units, with normal values <2-3 Wood units 1. To convert to dynes·s·cm⁻⁵, multiply Wood units by 80 1.

Terminology Clarification

All three terms describe the same measurement obtained during right heart catheterization:

• PCWP (Pulmonary Capillary Wedge Pressure): The most commonly used term in clinical practice 2, 1
• PAWP (Pulmonary Artery Wedge Pressure): Used interchangeably with PCWP, particularly in European guidelines 2
• PAOP (Pulmonary Artery Occlusion Pressure): The technically precise term describing the measurement obtained when the balloon catheter occludes a pulmonary arterial branch 2, 3

The measurement is obtained by advancing a balloon-tipped catheter into a small pulmonary arterial branch until the inflated balloon "wedges" or occludes flow, allowing measurement of downstream left atrial pressure 2.

Critical Measurement Technique

Proper technique is essential for accurate PVR calculation:

• The external pressure transducer must be zeroed at the mid-thoracic line (halfway between anterior sternum and bed surface in a supine patient), representing the level of the left atrium 2
• The balloon catheter should be inflated in the right atrium and advanced until it reaches the wedge position 2
• Repeated balloon deflations and inflations in distal pulmonary arteries should be avoided due to risk of arterial rupture 2
• Right heart catheterization should be performed only in expert centers due to technical demands and potential complications 2

Important Clinical Caveats

PCWP May Not Equal True Left Ventricular Filling Pressure

A critical pitfall: PCWP can significantly underestimate or overestimate true left ventricular end-diastolic pressure (LVEDP), leading to misclassification of pulmonary hypertension type. 4

• In one large study of 3,926 patients with pulmonary hypertension, 53.5% of patients classified as having pulmonary arterial hypertension based on PCWP ≤15 mmHg actually had LVEDP >15 mmHg, indicating pulmonary venous hypertension 4
• PCWP showed poor calibration to LVEDP with Bland-Altman limits of agreement ranging from -15.2 to 9.5 mmHg 4
• When PCWP is unexpectedly low or discordant with clinical findings, direct measurement of LVEDP should be obtained 2, 4

Context-Dependent Accuracy Issues

Several clinical scenarios compromise PCWP accuracy:

• Mechanical ventilation with high PEEP: Calculate transmural PCWP to estimate true left ventricular filling pressure 2, 5
• Diuretic therapy: In patients with left heart disease, PCWP may be reduced to <15 mmHg despite underlying elevated filling pressures 2
• Tricuspid regurgitation with RV dilatation: Cardiac output measurements by thermodilution become erroneous, affecting all derived resistance calculations 1
• Single ventricle physiology: PCWP consistently overestimates left atrial pressure by an average of 0.6 Wood units in PVR calculations, potentially underestimating true PVR by up to 33% 6

When to Measure Direct Left Atrial Pressure

Direct left atrial pressure measurement via transseptal catheterization is indicated when:

• PCWP and clinical findings are discordant 2
• Evaluating single ventricle patients for Fontan completion (if PCWP-derived PVR >2.3 Wood units, obtain direct LAP to determine if true PVR >3 Wood units) 6
• Distinguishing pulmonary arterial hypertension from pulmonary venous hypertension when PCWP is borderline (12-18 mmHg) 4

Clinical Decision Thresholds Using PVR

Key PVR thresholds for clinical decision-making:

• PVR >3 Wood units: Required for diagnosis of pulmonary arterial hypertension (precapillary pulmonary hypertension) 2, 1
• PVR >2.5 Wood units or >4 Wood units·m² indexed: Contraindication for congenital heart disease shunt closure 1
• PVR >6 Wood units·m²: Predicts poor outcomes in single ventricle patients undergoing cavopulmonary surgery 1
• PVR >8 Wood units·m²: Associated with poor surgical outcomes in children with ventricular septal defects 1

Alternative Approach: Transpulmonary Gradient

When concerns exist about PCWP accuracy or in the setting of high PEEP ventilation, the transpulmonary pressure gradient (TPG = mPAP - PCWP) provides valuable information about pulmonary vascular abnormality independent of calculated PVR 2. A TPG >6 mmHg suggests high risk for poor outcomes in cavopulmonary anastomosis 1.