What is the formula for calculating Pulmonary Vascular Resistance (PVR) in a patient with pulmonary hypertension?

Formula for Pulmonary Vascular Resistance (PVR)

The formula for calculating PVR is: PVR = (mean pulmonary artery pressure - pulmonary capillary wedge pressure) / cardiac output, expressed as Wood units. 1

Standard PVR Calculation

• PVR = (mPAP - PCWP) / CO 1

  • mPAP = mean pulmonary artery pressure (mmHg)
  • PCWP = pulmonary capillary wedge pressure (mmHg)
  • CO = cardiac output (L/min)
  • Result expressed in Wood units (preferred) 2, 1

• To convert Wood units to dynes·s·cm⁻⁵, multiply by 80 1

• Normal PVR values are <2-3 Wood units 1

Required Measurements

Mean Pulmonary Artery Pressure (mPAP)

• Must be measured directly via right heart catheterization—this is the gold standard and only validated method for accurate PVR determination 1
• Measurements should be taken at end-expiration during spontaneous breathing, or at end-inspiration if mechanically ventilated 1

Pulmonary Capillary Wedge Pressure (PCWP/PAWP)

• Obtained by wedging a balloon-tipped catheter into a small pulmonary arterial branch with the balloon inflated 1
• The external pressure transducer must be zeroed at the mid-thoracic line 1
• European guidelines use the term PAWP (pulmonary artery wedge pressure) interchangeably with PCWP 2, 1
• PCWP serves as a validated surrogate for left atrial pressure in the absence of pulmonary vein obstruction 1
• A PCWP >15 mmHg excludes the diagnosis of pre-capillary pulmonary arterial hypertension 2, 1

Cardiac Output (CO)

• Measured in L/min, typically via thermodilution or Fick method 1
• In patients with tricuspid regurgitation and right ventricular dilatation, thermodilution measurements can be erroneous, affecting all derived resistance calculations 1
• Direct measurement of oxygen consumption (VO₂) is preferable to estimation, particularly in children <3 years of age, as the LaFarge equation can overestimate VO₂ and lead to underestimation of PVR 1

Indexed PVR (PVRI)

• PVRI = PVR × body surface area (BSA) 1
• Expressed as Wood units·m² (WU·m²) 1
• Particularly important in pediatric populations where body size varies significantly 1

Clinical Thresholds

Diagnostic Criteria

• PVR >3 Wood units defines pre-capillary pulmonary hypertension (2015 guidelines) 2, 1
• PVR >2 Wood units defines pre-capillary pulmonary hypertension (2024 guidelines) 1
• Must be accompanied by mPAP ≥25 mmHg and PAWP ≤15 mmHg for diagnosis of pulmonary arterial hypertension 2

Surgical Decision-Making

• PVR >2.5 Wood units or PVRI >4 WU·m² is a contraindication for congenital heart disease shunt closure 1
• Surgical repair is recommended when PVR is less than one-third of SVR 1
• PVRI >6 WU·m² predicts poor outcomes in single ventricle patients undergoing cavopulmonary surgery 1
• PVRI <6 WU·m² is an indicator for repair in children with structural heart disease (ASD, VSD, PDA) 1

Critical Pitfalls and Caveats

Measurement Timing and Conditions

• Blood pH has a potent effect on pulmonary vascular tone—acidosis causes vasoconstriction while alkalosis causes vasodilation; awareness of arterial blood gas measurements during catheterization is critical 1
• Measurements should be obtained under standardized conditions, as general anesthesia can lower systemic arterial blood pressure and affect resistance calculations 1
• Volume status assessment in pulmonary hypertension patients is notoriously difficult, and non-invasive estimates of central venous pressures may be misleading, requiring direct central line measurement 1

PCWP Accuracy Issues

• PCWP may overestimate the true transmitral gradient due to phase delay when compared to direct left ventricular pressure 1
• Direct left atrial pressure or LVEDP measurement is recommended when left heart disease is the likely etiology, such as in patients with orthopnea or associated risk factors 1
• During mechanical ventilation with high PEEP, calculating the transmural value of PCWP allows estimation of true left ventricular filling pressure 1
• PCWP may be recorded with or without V-wave; this should be documented 1

Alternative Hemodynamic Parameter

• The transpulmonary pressure gradient (TPG = mPAP - PCWP) provides valuable information about pulmonary vascular abnormality independent of calculated PVR 1
• A TPG >6 mmHg suggests high risk for poor outcomes in cavopulmonary anastomosis 1

Echocardiographic Estimation Limitations

• Echocardiographic estimation of pulmonary artery systolic pressure has limited positive predictive value (25-64%) for confirming pulmonary hypertension 1
• While Doppler-derived formulas for PVR estimation show correlation with invasive measurements in research settings 3, 4, 5, echocardiographic estimation should not replace right heart catheterization for PVR calculation in clinical decision-making 1
• Agreement between echocardiographic and invasive PVR is poor in patients with PVR >8 Wood units 4