What are the cut-offs for QP (pulmonary flow)/QS (systemic flow) ratio in deciding shunt closure?

QP/QS Cut-offs for Shunt Closure Decision-Making

A QP/QS ratio ≥1.5:1 is the primary threshold for recommending shunt closure in congenital heart defects, provided that pulmonary artery systolic pressure is less than 50% of systemic pressure and pulmonary vascular resistance is less than one-third of systemic resistance. 1

Understanding QP/QS Ratio

The QP/QS ratio represents the relationship between pulmonary blood flow (QP) and systemic blood flow (QS):

• QP/QS > 1: Indicates a left-to-right shunt (higher QP/QS indicates larger shunt)
• QP/QS < 1: Indicates a right-to-left shunt (closer to zero indicates larger shunt)
• QP/QS = 1: No significant shunt

Decision Algorithm for Shunt Closure

Primary Criteria for Closure (Class I Recommendation)

• QP/QS ratio ≥1.5:1 (hemodynamically significant shunt)
• PA systolic pressure <50% of systemic pressure
• Pulmonary vascular resistance <1/3 of systemic resistance
• Evidence of right heart enlargement/volume overload

Secondary Considerations (Class IIa/IIb Recommendation)

• QP/QS ratio ≥1.5:1 with PA systolic pressure ≥50% of systemic pressure and/or pulmonary vascular resistance >1/3 of systemic resistance may be considered for closure (Class IIb) 1
• For VSD specifically, QP/QS ratio ≥2.0:1 with clinical evidence of LV volume overload is a strong indication (Class I) 1

Contraindications for Closure (Class III Harm)

• PA systolic pressure >2/3 of systemic pressure
• Pulmonary vascular resistance >2/3 of systemic resistance
• Net right-to-left shunt (Eisenmenger physiology) 1

Measurement Methods

The QP/QS ratio can be determined through several methods:

1. Cardiac MRI (CMR): Now considered the non-invasive reference standard using phase-contrast CMR 1, 2

   • High accuracy and reproducibility
   • Sensitivity of 93% and specificity of 100% for detecting QP/QS ≥1.5:1 2

2. Cardiac Catheterization with Oximetry: Traditional invasive reference method 1

   • Particularly important when assessment of pulmonary pressures and resistance is also required
   • Direct measurement of oxygen saturations in pulmonary and systemic circulation

3. Doppler Echocardiography: Commonly used but less accurate 1, 3

   • Recent studies show poor correlation with catheterization-derived values
   • Tends to overestimate the degree of left-to-right shunting 3

Clinical Pearls and Pitfalls

• Measurement Variability: Different measurement techniques may yield different QP/QS values. A recent study showed poor correlation between echocardiography and catheterization-derived values 3

• Timing Considerations: For accurate comparison, measurements should ideally be performed within a short timeframe, as shunt dynamics can change

• Technical Challenges: Invasive oximetry requires multiple sampling sites during steady state, which can lead to error propagation 1

• Special Scenarios: In certain conditions (e.g., systemic-pulmonary arterial collaterals), quantification by oximetry may not be possible 1

• Decision-Making: While QP/QS ≥1.5:1 is the general threshold, the decision for closure should also consider chamber enlargement, symptoms, and other physiological parameters

For patients with borderline values or complex anatomy, CMR is particularly valuable as it provides both anatomical and functional assessment without assumptions about flow patterns that can affect other measurement techniques 1, 2.