What is the formula to calculate Pulmonary Vascular Resistance Index (PVRI)?

How to Calculate PVRI (Pulmonary Vascular Resistance Index)

PVRI is calculated by dividing the transpulmonary pressure gradient (mean pulmonary artery pressure minus pulmonary capillary wedge pressure) by cardiac index, expressed in Wood units·m² (WU·m²). 1

The Formula

PVRI = (mPAP - PCWP) / CI

Where:

• mPAP = mean pulmonary artery pressure (mmHg)
• PCWP = pulmonary capillary wedge pressure (mmHg)
• CI = cardiac index (L/min/m²)
• Result = Wood units·m² (WU·m²)

2, 1

Key Distinction: PVRI vs PVR

PVRI differs from non-indexed PVR by incorporating body surface area adjustment, making it particularly important in pediatric populations where body size varies significantly. 1

• PVR = (mPAP - PCWP) / CO, expressed in Wood units (WU) 1
• PVRI = (mPAP - PCWP) / CI, expressed in Wood units·m² (WU·m²) 1
• Cardiac Index (CI) = Cardiac Output (CO) / Body Surface Area (BSA) 1

Required Measurements

Mean Pulmonary Artery Pressure (mPAP)

• Must be measured via right heart catheterization 2, 1
• Measurements should be taken at end expiration if breathing spontaneously, or at end inspiration if mechanically ventilated 2

Pulmonary Capillary Wedge Pressure (PCWP)

• 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 2
• Critical pitfall: If wedge pressures appear unusually high or low, measure LV end-diastolic pressure simultaneously to verify accuracy 2
• Avoid repeated balloon deflations and inflations in distal pulmonary arteries 2

Cardiac Index (CI)

• Calculated by dividing cardiac output by body surface area (L/min/m²) 1
• Cardiac output typically measured via thermodilution or Fick method 1
• Important caveat: In patients with tricuspid regurgitation and right ventricular dilatation, thermodilution measurements can be erroneous, affecting all derived resistance calculations 1

Clinical Thresholds for PVRI

PVRI >6 WU·m² predicts poor prognosis in children with congenital heart disease, regardless of lung morphology. 1

Surgical Decision-Making

• PVRI <6 WU·m²: Repair should be considered in children with structural heart disease (ASD, VSD, PDA) 2
• PVRI ≥6 WU·m²: Repair is not indicated unless acute vasodilator testing demonstrates reversibility (absolute PVRI <6 WU·m² and PVR/SVR <0.3) 2
• PVRI <7-8 WU·m² in response to vasodilator challenge predicts good surgical outcomes in patients with simple shunts 1

Single Ventricle Physiology

• PVRI >6 WU·m² predicts poor outcomes in single ventricle patients undergoing cavopulmonary surgery 1
• A transpulmonary gradient >6 mmHg (mPAP - PCWP) suggests high risk for poor outcomes in cavopulmonary anastomosis 1

Critical Measurement Considerations

Anesthesia Effects

• Most children <15 years require conscious sedation or general anesthesia for cardiac catheterization, which can alter hemodynamic measurements. 2
• Fentanyl, ketamine, and propofol have minimal effects on PA pressure and PVRI 2
• General anesthesia can lower systemic arterial blood pressure and affect resistance calculations 1
• Avoid systemic hypotension, particularly in patients with marked elevation of PAP and low cardiac output 2

Blood Gas Management

• 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
• Hypoxia causes vasoconstriction; hyperoxia causes vasodilation 2

Oxygen Consumption in Fick Method

• Direct measurement of oxygen consumption (VO₂) is preferable to estimation, particularly in children <3 years of age. 1
• The LaFarge equation can overestimate VO₂ and lead to underestimation of PVR 1

Common Pitfalls to Avoid

1. Inaccurate wedge pressure: Always verify PCWP accuracy, especially if values seem unusual; measure LV end-diastolic pressure simultaneously if needed 2

2. Thermodilution errors: In patients with significant tricuspid regurgitation and RV dilatation, thermodilution cardiac output measurements are unreliable 1

3. Timing of measurements: Ensure pressures are recorded at appropriate points in the respiratory cycle (end expiration for spontaneous breathing, end inspiration for mechanical ventilation) 2

4. Snapshot limitation: Acute hemodynamic evaluation provides only a snapshot and may not represent overall clinical status 1

5. Volume status: Assessment of volume status in pulmonary hypertension patients is notoriously difficult; non-invasive estimates of central venous pressures may be misleading, requiring direct central line measurement 1

Relationship to Systemic Vascular Resistance

In pulmonary arterial hypertension patients, SVR must be maintained greater than PVR to prevent right ventricular ischemia. 1, 3

• Right ventricular coronary perfusion occurs during both systole and diastole 1, 3
• If systolic pulmonary arterial pressure exceeds systolic systemic arterial pressure (PVR > SVR during systole), right ventricular ischemia results 1, 3