Pulmonary-to-Systemic Flow Ratio (Qp:Qs) Calculation in Atrial Septal Defects
The pulmonary-to-systemic flow ratio (Qp:Qs) in patients with atrial septal defects is calculated by dividing the pulmonary blood flow by the systemic blood flow, with values ≥1.5:1 indicating a hemodynamically significant shunt that typically warrants closure. 1
Formula and Calculation Method
The Qp:Qs ratio is calculated using the Fick principle during cardiac catheterization:
Qp:Qs = (SaO₂ - MvO₂) / (PvO₂ - PaO₂)
Where:
- SaO₂ = Systemic arterial oxygen saturation
- MvO₂ = Mixed venous oxygen saturation (from pulmonary artery)
- PvO₂ = Pulmonary venous oxygen saturation
- PaO₂ = Pulmonary arterial oxygen saturation
Alternative Calculation Method
Qp:Qs can also be calculated using cardiac output measurements:
Qp:Qs = Pulmonary Cardiac Index / Systemic Cardiac Index
Clinical Example
A 24-year-old female with a large secundum ASD (15×15 mm) underwent right heart catheterization with the following findings 1:
- Pulmonary arterial pressure: 57 mmHg
- Pulmonary capillary wedge pressure: 3 mmHg
- Pulmonary vascular resistance: 880 dyn·s·cm⁻⁵
- Pulmonary cardiac index: 3.2 L·min⁻¹·m⁻²
- Systemic cardiac index: 2.6 L·min⁻¹·m⁻²
Calculation: Qp:Qs = 3.2 / 2.6 = 1.22
This ratio of 1.22:1 indicates a modest left-to-right shunt, but with the patient's elevated pulmonary vascular resistance, repair surgery was contraindicated.
Clinical Significance and Decision-Making
The Qp:Qs ratio is critical for clinical decision-making in ASD management:
- Qp:Qs ≥1.5:1: Indicates a hemodynamically significant shunt that typically warrants closure 1
- Qp:Qs <1.5:1: Generally considered a small shunt that may not require intervention unless the patient is symptomatic
Additional Hemodynamic Parameters to Consider:
- Pulmonary artery systolic pressure (PASP): Should be <50% of systemic pressure for safe closure 1
- Pulmonary vascular resistance (PVR): Should be <1/3 of systemic vascular resistance for safe closure 1
Clinical Application Algorithm
Measure Qp:Qs ratio during cardiac catheterization
- If ≥1.5:1: Consider closure if other criteria are met
- If <1.5:1: Closure may still be considered if symptomatic
Assess pulmonary pressures and resistance:
- PASP <50% systemic and PVR <1/3 systemic: Proceed with closure
- PASP ≥50% systemic or PVR >1/3 but <2/3 systemic: Careful evaluation needed
- PASP >2/3 systemic or PVR >2/3 systemic: Closure contraindicated (Eisenmenger physiology) 1
Evaluate for right heart enlargement (echocardiography)
- Present: Strengthens indication for closure
- Absent: Reassess clinical significance of shunt
Pitfalls and Caveats
- Oxygen consumption assumptions can lead to inaccuracies in Fick calculations
- Bidirectional shunts may complicate measurements; careful sampling is required
- Exercise testing may reveal latent pulmonary hypertension not apparent at rest 2
- Fenestrated device closure may be considered in high-risk patients with pulmonary hypertension to allow pressure relief if needed 3, 4
By accurately calculating and interpreting the Qp:Qs ratio alongside other hemodynamic parameters, clinicians can make informed decisions about ASD management to improve morbidity, mortality, and quality of life outcomes.