How is mean pulmonary artery pressure measured to calculate Pulmonary Vascular Resistance (PVR)?

Measurement of Mean Pulmonary Artery Pressure for PVR Calculation

Mean pulmonary artery pressure (mPAP) must be measured directly via right heart catheterization (RHC) to calculate pulmonary vascular resistance (PVR), as this is the gold standard and only validated method for accurate PVR determination. 1, 2

Direct Invasive Measurement via Right Heart Catheterization

Catheter Placement and Technique

• A balloon-tipped pulmonary artery catheter is advanced through the right atrium and right ventricle into the pulmonary artery 2
• The external pressure transducer must be zeroed at the mid-thoracic line to ensure accurate measurements 2, 3
• Measurements should be obtained at end-expiration during spontaneous breathing, or at end-inspiration if the patient is mechanically ventilated 2

The PVR Calculation Formula

PVR = (mPAP - PCWP) / CO 2

Where:

• mPAP = mean pulmonary artery pressure (measured directly in the pulmonary artery)
• PCWP = pulmonary capillary wedge pressure (measured by wedging the balloon-tipped catheter into a small pulmonary arterial branch with the balloon inflated) 2
• CO = cardiac output in L/min (typically measured via thermodilution or Fick method) 2

Units of Measurement

• PVR is expressed in Wood units (WU), which is the preferred unit for clinical decision-making 2
• To convert to dynes·s·cm⁻⁵, multiply Wood units by 80 2
• Normal PVR values are <2-3 Wood units 2

Critical Technical Considerations

Timing of Measurements

• Blood pH has a potent effect on pulmonary vascular tone—acidosis causes vasoconstriction while alkalosis causes vasodilation 2
• Awareness of arterial blood gas measurements during catheterization is critical for accurate interpretation of baseline hemodynamics 2
• General anesthesia can lower systemic arterial blood pressure and affect resistance calculations 2

Common Pitfalls to Avoid

• Inaccurate wedge pressure: Verify PCWP accuracy, especially if values seem unusual; measure LV end-diastolic pressure simultaneously if needed 2
• Tricuspid regurgitation with RV dilatation: Cardiac output measurements by thermodilution can be erroneous in this setting, affecting all derived resistance calculations 2, 3
• High PEEP ventilation: Calculate the transmural value of PCWP to estimate true left ventricular filling pressure 2, 3
• Repeated balloon inflations: Avoid repeated balloon deflations and inflations in distal pulmonary arteries during PCWP measurement 2

Noninvasive Estimation Methods (Not Validated for Clinical Decision-Making)

Phase-Contrast MRI

• Phase-contrast imaging techniques can measure average blood flow velocity of the main pulmonary artery, which correlates with mean pulmonary arterial pressure 1
• Decreased pulmonary artery blood flow velocity correlates to increased vascular resistance 1
• A small study suggested that estimation of mPAP from high-temporal-resolution phase-contrast MRI is possible, but further investigation in a larger population is still needed 1

Echocardiographic Estimation

• Doppler echocardiography can estimate systolic pulmonary artery pressure using tricuspid regurgitation velocity, but has limited positive predictive value (25-64%) for confirming pulmonary hypertension 1
• Echocardiographic estimation of pulmonary artery systolic pressure has limited accuracy and should not replace RHC for PVR calculation in clinical decision-making 1

Clinical Thresholds for PVR

Diagnostic Criteria

• PVR >3 Wood units defines precapillary pulmonary hypertension (2018 guidelines) 1, 2
• PVR >2 Wood units defines precapillary pulmonary hypertension (2024 updated guidelines) 2
• PAWP ≤15 mm Hg must accompany elevated PVR for precapillary PH diagnosis 1

Surgical Decision Thresholds

• PVR >2.5 WU or >4 WU·m² indexed is a contraindication for congenital heart disease shunt closure 2
• PVRI >6 WU·m² predicts poor prognosis in children with congenital heart disease undergoing cavopulmonary surgery 2
• Transpulmonary gradient >6 mm Hg (mPAP - PCWP) suggests high risk for poor outcomes in cavopulmonary anastomosis 2, 3

Critical Hemodynamic Relationship

• In pulmonary arterial hypertension patients, systemic vascular resistance (SVR) must be maintained greater than PVR to prevent right ventricular ischemia 2
• If systolic pulmonary arterial pressure exceeds systolic systemic arterial pressure (PVR > SVR during systole), right ventricular ischemia results 2