How do you calculate pulmonary vascular resistance (PVR) versus systemic vascular resistance (SVR) and what factors do you consider?

Calculation of Pulmonary and Systemic Vascular Resistance

Both pulmonary vascular resistance (PVR) and systemic vascular resistance (SVR) are calculated using the same fundamental principle: the pressure gradient across the vascular bed divided by cardiac output, with PVR using transpulmonary gradient and SVR using the systemic arterial-to-venous gradient. 1

PVR Calculation Formula

PVR is calculated as (mean pulmonary artery pressure minus mean pulmonary capillary wedge pressure) divided by cardiac output. 1

• The formula is: PVR = (mPAP - PCWP) / CO 1
• Units are expressed as Wood units or dynes·s·cm⁻⁵ 1
• To convert Wood units to dynes·s·cm⁻⁵, multiply by 80 1
• Normal PVR values are typically <2-3 Wood units 1

Required Measurements for PVR

• Mean pulmonary artery pressure (mPAP) measured via right heart catheterization 1
• Pulmonary capillary wedge pressure (PCWP) or pulmonary artery wedge pressure (PAWP), measured by wedging a pulmonary catheter with an inflated balloon into a small pulmonary arterial branch 1
• Cardiac output (CO) measured in L/min, typically via thermodilution or Fick method 1

SVR Calculation Formula

SVR is calculated as the systemic mean arterial blood pressure minus right atrial pressure divided by cardiac output. 1, 2

• The formula is: SVR = (MAP - RAP) / CO 1, 2
• Units are expressed as dynes·s·cm⁻² 1
• Normal SVR values are approximately 800-1200 dynes·s·cm⁻² 2

Required Measurements for SVR

• Mean arterial pressure (MAP) from systemic arterial line or calculated as (systolic BP + 2 × diastolic BP) / 3 1, 2
• Right atrial pressure (RAP) or central venous pressure (CVP) measured via central venous catheter 1
• Cardiac output (CO) measured in L/min 1

Critical Factors to Consider

Cardiac Output Measurement Accuracy

• 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
• In patients with tricuspid regurgitation and right ventricular dilatation, cardiac output measurements by thermodilution can be erroneous, affecting all derived resistance calculations 3
• Cardiac index (CI) should be calculated by dividing CO by body surface area (L/min/m²) for size-adjusted comparisons 1

Pressure Measurement Considerations

• 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, 2
• During mechanical ventilation with high PEEP, calculating the transmural value of PCWP allows estimation of true left ventricular filling pressure 3
• PCWP may be recorded with or without V-wave; this should be documented 1

Clinical Context for PVR Interpretation

• In congenital heart disease, surgical repair is recommended when PVR is less than one-third of SVR. 2
• A PVR >6 Wood units·m² predicts poor outcomes in single ventricle patients undergoing cavopulmonary surgery 1
• A transpulmonary gradient >6 mm Hg (mPAP - PCWP) suggests high risk for poor outcomes in cavopulmonary anastomosis 1
• PVR >8 Wood units·m² in children with ventricular septal defects has been associated with poor surgical outcomes, though positive acute vasodilator testing may predict benefit 1

Critical Hemodynamic Relationship

• In pulmonary arterial hypertension patients, SVR must be maintained greater than PVR to prevent right ventricular ischemia, as right ventricular coronary perfusion occurs during both systole and diastole. 1, 2
• If systolic pulmonary arterial pressure exceeds systolic systemic arterial pressure (PVR > SVR during systole), right ventricular ischemia results 1, 2

Common Pitfalls and Caveats

Theoretical Limitations of PVR Calculation

• The traditional PVR equation assumes Poiseuille resistance with laminar flow of a Newtonian fluid, but blood is non-Newtonian and pulmonary flow is pulsatile, making calculated PVR only partially representative of true pulmonary vascular mechanics. 4
• The equation ignores vessel distension, recruitment, and the existence of critical closing pressure in different lung zones 4
• Different perfusion models exist in various lung regions (Poiseuille vs. Starling resistor behavior), making single-equation calculations theoretically inadequate 4

Measurement Timing and Conditions

• Measurements should be obtained under standardized conditions, as general anesthesia can lower systemic arterial blood pressure and affect resistance calculations. 1
• Acute hemodynamic evaluation provides only a snapshot and may not represent overall clinical status 1
• In patients with elevated PCWP (>15 mm Hg), PVR calculations may not accurately reflect pulmonary vascular disease alone 5

Non-Invasive Estimation Limitations

• Doppler echocardiography can estimate PVR using the ratio of tricuspid regurgitation velocity to right ventricular outflow tract velocity time integral, but accuracy varies 5, 6, 7
• The formula PVR = (PASP/RVOT VTI) + 3 (if notch present) correlates well with catheterization (r = 0.80) but should not replace invasive measurement for critical clinical decisions 6
• Echocardiographic estimation of pulmonary artery systolic pressure has limited positive predictive value (25-64%) for confirming pulmonary hypertension 1