What is the equation for calculating the QP (pulmonary flow)/QS (systemic flow) ratio in cardiac shunt hemodynamics?

Calculating QP/QS Ratio in Cardiac Shunt Hemodynamics

The equation for calculating the QP/QS ratio (pulmonary to systemic flow ratio) in cardiac shunt hemodynamics is: QP/QS = pulmonary blood flow divided by systemic blood flow. 1, 2

Standard Measurement Methods

Invasive Cardiac Catheterization Method (Oximetry)

• The reference standard method uses the Fick principle during cardiac catheterization, measuring oxygen saturations in the pulmonary and systemic circulation 2
• The equation using the Fick principle is:
  • QP/QS = (SaO₂ - SvO₂) / (SpvO₂ - SpaO₂)
  • Where:
    • SaO₂ = Systemic arterial oxygen saturation
    • SvO₂ = Mixed systemic venous oxygen saturation
    • SpvO₂ = Pulmonary venous oxygen saturation
    • SpaO₂ = Pulmonary arterial oxygen saturation 2

Non-invasive Measurement Methods

• Cardiac MRI (CMR) - Now considered the non-invasive gold standard:

  • Direct flow measurement in the pulmonary trunk and aorta using phase-contrast CMR 1
  • QP/QS = Flow in pulmonary artery / Flow in ascending aorta 1, 3
  • Alternative CMR methods include measuring flow in pulmonary veins and vena cavae, or using ventricular stroke volumes 1

• Doppler Echocardiography:

  • QP/QS = (Aortic VTI × Aortic valve area) / (Pulmonary VTI × Pulmonary valve area)
  • Where VTI = velocity time integral 2, 4

Clinical Interpretation

• QP/QS = 1.0: Normal (no shunt) 2
• QP/QS > 1.0: Left-to-right shunt 2
• QP/QS < 1.0: Right-to-left shunt 2
• QP/QS ≥ 1.5: Considered hemodynamically significant, often warranting intervention 1, 2

Special Considerations

• When using ventricular stroke volumes to calculate QP/QS, the shunt location must be considered for accurate calculation 1
• In complex congenital heart disease, CMR can quantify additional flows such as collateral flow 1
• For ventricular septal defects, a more complex relationship exists between QP/QS and the ratio of pulmonary to systemic vascular resistance (RP/RS) 5:
  • RP/RS = QS/QP + log(AV/AA) × (1-QS/QP)
  • Where AV = defect area and AA = aortic cross-sectional area 5

Common Pitfalls

• Background velocity offsets in phase-contrast CMR can introduce errors in traditional two-acquisition methods 3
• Doppler echocardiography is prone to inaccuracy due to inadequate data acquisition and invalid assumptions about vessel geometry 2
• In Eisenmenger syndrome, the QP/QS ratio becomes < 1 as blood flows from right-to-left, contraindicating closure 6, 1
• Shunt closure should not be performed when pulmonary arterial systolic pressure is greater than two-thirds systemic, or pulmonary vascular resistance is greater than two-thirds systemic 1

Recent Advances

• Single-acquisition phase-contrast CMR techniques have been developed that encompass both the pulmonary trunk and ascending aorta in one plane, potentially improving accuracy by reducing background velocity offset errors 3