What is the purpose and management of QP (pulmonary blood flow) QS (systemic blood flow) shunt calculation in assessing cardiac shunts?

QP/QS Shunt Calculation: Purpose and Management in Cardiac Shunts

The Qp/Qs ratio calculation is vital for determining appropriate management of patients with cardiac shunt lesions, with a ratio >1.5 generally considered hemodynamically significant and warranting intervention in left-to-right shunts, while a ratio <1 indicates a right-to-left shunt that may contraindicate closure. 1, 2

Definition and Clinical Significance

• Qp/Qs represents the ratio between pulmonary blood flow (Qp) and systemic blood flow (Qs), which normally equals 1:1 in healthy individuals without cardiac shunts 2
• A left-to-right shunt results in greater blood flow to the pulmonary vasculature compared to systemic circulation (Qp/Qs > 1), with higher values indicating larger shunts 1
• A right-to-left shunt results in greater flow to the systemic vasculature (Qp/Qs < 1), with values closer to zero indicating larger shunts 1
• Quantifying Qp/Qs is essential for clinical decision-making regarding intervention in patients with shunt lesions 1

Measurement Methods

• The reference standard method is invasive oximetry during cardiac catheterization, measuring oxygen saturations in pulmonary and systemic arterial and venous systems 1
• Cardiac Magnetic Resonance (CMR) has become the non-invasive reference standard using phase-contrast CMR (PC-CMR), allowing highly accurate and reproducible quantification without assumptions 1
• CMR offers several advantages over invasive oximetry:
  • Direct measurement of flow without assumptions 1
  • Multiple measurement methods providing internal quality assurance 1
  • Ability to measure flow in alternative vessels when standard measurements are not possible 1
  • Comprehensive anatomical evaluation of shunts 1
• Doppler echocardiography can be used but is prone to inaccuracy due to inadequate data acquisition and invalid assumptions 1, 3

Clinical Interpretation and Management

• A Qp/Qs ratio > 1.5:1 is generally considered hemodynamically significant for left-to-right shunts and may warrant intervention 2
• Surgical or device closure is recommended for atrial septal defects when Qp/Qs ≥ 1.5:1 with right atrial and RV enlargement, provided pulmonary vascular resistance is less than one-third of systemic resistance 2
• Closure should not be performed when:
  • Pulmonary arterial systolic pressure is greater than two-thirds systemic 2
  • Pulmonary vascular resistance is greater than two-thirds systemic 2, 4
  • Net right-to-left shunting is present (Qp/Qs < 1) 4
• In Eisenmenger syndrome, the Qp/Qs ratio becomes < 1 as blood flows from right-to-left, contraindicating closure 2, 4

Advanced Measurement Considerations

• CMR can measure Qp/Qs using multiple methods:
  • Direct flow measurement in the pulmonary trunk and aorta 1
  • Flow measurement in pulmonary veins and vena cavae 1
  • Ventricular stroke volumes (with caution regarding shunt location) 1
• When using ventricular stroke volumes to calculate Qp/Qs, shunt location must be considered:
  • In ASD: RV stroke volume will be twice the LV stroke volume with Qp/Qs of 2:1 1
  • In VSD: LV stroke volume will be twice the RV stroke volume with Qp/Qs of 2:1 1
• Single-acquisition phase-contrast CMR techniques can improve accuracy by reducing background velocity offset errors 5

Clinical Applications Beyond Simple Shunts

• CMR can quantify collateral flow in complex congenital heart disease:
  • Systemic-to-pulmonary collaterals in single ventricle patients 1
  • Veno-venous collaterals 1
  • Aortic collaterals in coarctation 1
• CMR provides comprehensive evaluation of complex shunts like sinus venosus defects and anomalous pulmonary venous connections that are difficult to image with echocardiography 1
• In patients with pulmonary hypertension associated with congenital heart disease, the Qp/Qs ratio helps determine whether repair is feasible or contraindicated 2, 4

Common Pitfalls and Caveats

• Invasive oximetry has limitations:
  • Invasive nature with associated morbidity and cost 1
  • Error propagation from multiple sampling sites 1
  • Inability to sample distal to some extracardiac shunts 1
  • Challenges in single ventricle physiology 1
• Ventricular stroke volume method for Qp/Qs calculation requires:
  • Knowledge of shunt location 1
  • Competence of atrioventricular and semilunar valves 1
• Echocardiographic evaluation may be inaccurate due to inadequate data acquisition and invalid assumptions 1