Calculating Qp/Qs in Atrial Septal Defect
The gold standard for calculating Qp/Qs in ASD is cardiac magnetic resonance (CMR) using phase-contrast imaging, which has replaced invasive oximetry as the reference standard due to superior accuracy and reproducibility. 1
Primary Calculation Methods
CMR Phase-Contrast Imaging (Preferred Method)
CMR directly measures blood flow without assumptions and provides multiple internal quality checks, making it the most accurate non-invasive method. 1
- Direct flow measurement: Measure flow in the pulmonary trunk (Qp) and aorta (Qs), then calculate the ratio 1
- Alternative vessel measurements: When standard measurements are limited by artifact, measure flow in pulmonary veins (for Qp) and vena cavae (for Qs) 1, 2
- Ventricular stroke volume method: Calculate from RV and LV stroke volumes, but shunt location is critical—in ASD, RV stroke volume will be twice LV stroke volume when Qp/Qs = 2:1 (opposite of VSD) 1
- Internal quality assurance: Perform multiple calculations using different vessel combinations in the same patient to verify accuracy 1
Invasive Oximetry (Traditional Reference)
Calculate using oxygen saturations from cardiac catheterization: 2, 3
Qp/Qs = (Arterial O₂ - Mixed Venous O₂) / (Pulmonary Vein O₂ - Pulmonary Artery O₂)
- Simplified formula using SVC instead of mixed venous: Qp/Qs = (Arterial O₂ - SVC O₂) / (Pulmonary Vein O₂ - PA O₂) shows high correlation (r = 0.98) 3
- Key diagnostic thresholds: PA O₂ saturation >78% is highly suspicious for ASD (sensitivity/specificity 97.5%) 3
- Saturation step-up criteria: PA O₂ - SVC O₂ ≥7.4% (sensitivity 98.3%, specificity 96.2%) or PA O₂ - IVC O₂ ≥2.0% (sensitivity/specificity 100%) 3
Doppler Echocardiography (Bedside Method)
Calculate stroke volumes at the pulmonary and aortic valves: 4, 5
- RSV/LSV ratio: Measure right ventricular stroke volume (flow velocity integral × cross-sectional area at pulmonary valve) divided by left ventricular stroke volume (flow velocity integral × cross-sectional area at aortic valve) 4
- Normal RSV/LSV = 0.99 ± 0.05; ASD patients = 2.26 ± 0.63 4
- Correlation with oximetry: r = 0.92 (y = 1.11x - 0.30) 4
- Alternative transatrial velocity method: Mean transatrial septal velocity correlates with Qp/Qs (r = 0.89) when conventional measurements cannot be performed due to turbulence 5
Critical Pitfalls and Limitations
Echocardiography Limitations
- Semilunar valve regurgitation modifies RSV/LSV values and must be accounted for 4
- Tricuspid or mitral regurgitation affects accuracy but correlation remains acceptable 4
- Poor acoustic windows in adults limit accuracy 1
Oximetry Limitations
- Fails in patients with systemic-to-pulmonary collaterals where CMR is superior 1
- Requires accurate mixed venous sampling from multiple sites 3
- High inter-method variability: Only 33% concordance among all three methods (oximetry, nuclear, Doppler) 6
CMR Limitations
- Metallic prosthetic valves create artifact obscuring great vessels 1
- Requires competent atrioventricular and semilunar valves for ventricular stroke volume method 1
Clinical Interpretation Thresholds
Qp/Qs ≥1.5:1 with RV enlargement indicates hemodynamically significant shunt requiring intervention (provided PA systolic pressure <50% systemic and PVR <1/3 systemic resistance) 1
- Qp/Qs 1.5-2.0: Moderate shunt, intervention reasonable if symptomatic or RV enlargement present 1
- Qp/Qs >2.0: Large shunt, intervention strongly recommended 1
- Qp/Qs <1.0: Right-to-left shunt (Eisenmenger physiology), closure contraindicated 1, 7
Practical Algorithm
- First-line: Perform transthoracic echocardiography with Doppler to estimate Qp/Qs and assess RV size/function 1
- If echocardiography inadequate or borderline findings: Obtain CMR with phase-contrast imaging for definitive Qp/Qs measurement 1, 2
- If CMR contraindicated or unavailable: Proceed to cardiac catheterization with oximetry 2
- Always correlate Qp/Qs with RV size and function—RV volume overload is the key physiological consequence in ASD 1