Pulmonary-to-Systemic Flow Ratio (Qp/Qs) Calculator

Pulmonary-to-Systemic Flow Ratio (Qp/Qs): Explanation and Clinical Context The Pulmonary-to-Systemic Flow Ratio (Qp/Qs) is a hemodynamic index derived from the Fick principle that quantifies the relative blood flow between the pulmonary and systemic circulations. It is essential for the assessment of cardiac shunts such as atrial septal defect (ASD), ventricular septal defect (VSD), and patent ductus arteriosus (PDA). The formula is: Qp/Qs = (SaO₂ – SvO₂) / (PVO₂ – PAO₂) Where: SaO₂ = Systemic arterial oxygen saturation (usually from aorta) SvO₂ = Systemic venous oxygen saturation (from superior or mixed venous sample) PVO₂ = Pulmonary venous oxygen saturation (typically assumed to be near 100%) PAO₂ = Pulmonary arterial oxygen saturation (from pulmonary artery sample) A normal Qp/Qs is approximately 1.0, indicating balanced flow between the two circulations.

A value greater than 1.5 typically indicates a significant left-to-right shunt that may warrant surgical or percutaneous intervention, especially if associated with right ventricular volume overload. Conversely, a Qp/Qs less than 1.0 indicates a right-to-left shunt, which can be seen in cyanotic congenital heart disease or advanced pulmonary hypertension. Clinical Significance: Understanding Qp/Qs helps in determining shunt severity, guiding therapeutic decisions, and predicting long-term outcomes.

For instance, in secundum ASD, closure is generally recommended when Qp/Qs ≥ 1.5 with evidence of right heart dilation. Clinical Interpretation Summary: Qp/Qs ≈ 1.0 → Normal flow balance Qp/Qs > 1.5 → Significant left-to-right shunt Qp/Qs < 0.8 → Right-to-left shunt / Pulmonary hypertension