Qp/Qs Ratio Calculator (Cardiac Shunt Fraction)

What Is the Qp/Qs Ratio?

The $Q_p/Q_s$ ratio compares pulmonary blood flow ($Q_p$) to systemic blood flow ($Q_s$). In a healthy cardiovascular system without anatomical communications, pulmonary blood flow is identical to systemic blood flow, producing a normal $Q_p/Q_s$ ratio of $1.0 : 1$.

When an intracardiac or extracardiac shunt exists, such as an atrial septal defect (ASD), ventricular septal defect (VSD), patent ductus arteriosus (PDA), or anomalous pulmonary venous return, blood crosses between the pulmonary and systemic circulations. The $Q_p/Q_s$ ratio quantifies the magnitude and direction of this shunt to guide medical, catheter-based, or surgical interventions.

How the Qp/Qs Ratio Is Calculated

Two primary diagnostic modalities are used in clinical cardiology to calculate $Q_p/Q_s$:

1. Oximetry (Cardiac Catheterization / Fick Principle)

During right and left heart catheterization, blood samples are drawn from various cardiac chambers and vessels to measure oxygen saturation ($S\text{O}_2$). Applying the Fick principle yields the oximetric ratio:

$$\frac{Q_p}{Q_s} = \frac{S_a\text{O}_2 - S_v\text{O}_2}{S_{pv}\text{O}_2 - S_{pa}\text{O}_2}$$

Where:

$S_a\text{O}_2$: Systemic arterial oxygen saturation (measured in the aorta or peripheral artery).
$S_v\text{O}_2$: Mixed venous oxygen saturation entering the right heart. When superior and inferior vena cava samples are measured separately, Flamm's equation is commonly applied: $$S_v\text{O}_2 = \frac{3 \times S_{svc}\text{O}_2 + 1 \times S_{ivc}\text{O}_2}{4}$$
$S_{pv}\text{O}_2$: Pulmonary venous oxygen saturation (often assumed to be $98\%$ to $100\%$ or measured directly in the pulmonary vein or left atrium).
$S_{pa}\text{O}_2$: Pulmonary artery oxygen saturation (reflecting any oxygen step-up from a left-to-right shunt).

2. Echocardiography (Doppler Volumetric Flow)

Transthoracic echocardiography calculates stroke volume ($SV$) across the right ventricular outflow tract (RVOT) and left ventricular outflow tract (LVOT) using Doppler Velocity-Time Integral ($VTI$) and cross-sectional area ($CSA$):

$$SV = CSA \times VTI = \frac{\pi \times D^2}{4} \times VTI$$ $$\frac{Q_p}{Q_s} = \frac{SV_{\text{RVOT}}}{SV_{\text{LVOT}}} = \frac{D_{\text{RVOT}}^2 \times VTI_{\text{RVOT}}}{D_{\text{LVOT}}^2 \times VTI_{\text{LVOT}}}$$

Shunt Fraction Calculation

The percentage of pulmonary blood flow derived from the shunt (Left-to-Right Shunt Fraction) is computed as:

$$\text{Shunt Fraction (\%)} = \frac{Q_p - Q_s}{Q_p} \times 100\%$$

Clinical Classification and Intervention Criteria

Qp/Qs Ratio	Shunt Direction & Size	Shunt Fraction	Clinical Recommendation
1.0 : 1	Normal / Balanced	0%	No pathological shunt.
1.0 to 1.49 : 1	Small Left-to-Right	< 33%	Generally hemodynamically insignificant; periodic echocardiographic follow-up.
1.5 to 1.99 : 1	Moderate Left-to-Right	33% to 50%	Hemodynamically significant; evaluate for catheter-based or surgical closure.
≥ 2.0 : 1	Large Left-to-Right	≥ 50%	Severe volume overload; defect closure strongly indicated prior to irreversible pulmonary vascular remodeling.
< 1.0 : 1	Right-to-Left (Cyanotic)	Variable	Deoxygenated blood enters systemic circulation. Evaluate for Eisenmenger syndrome or cyanotic congenital defects.

Check related hemodynamic tools such as our Pulmonary Vascular Resistance Calculator and Doppler Echo Cardiac Output Calculator.

Frequently Asked Questions

What is considered a significant Qp/Qs shunt ratio?

A $Q_p/Q_s$ ratio of $1.5 : 1$ or greater is considered clinically and hemodynamically significant in adults and children with left-to-right shunts (such as ASD or VSD), often meeting criteria for percutaneous device closure or surgical repair.

Why is Flamm's formula used for mixed venous saturation?

Blood returning from the superior vena cava (SVC) and inferior vena cava (IVC) has different oxygen saturations because abdominal organs and kidneys extract oxygen differently than the brain and upper body. Flamm's weighted formula ($\frac{3 \times \text{SVC} + 1 \times \text{IVC}}{4}$) accounts for these physiological mixing dynamics in the right atrium.

What does a Qp/Qs ratio below 1.0 indicate?

A ratio below $1.0$ indicates a right-to-left shunt where deoxygenated systemic venous blood bypasses the pulmonary capillary bed and enters the systemic circulation directly, causing hypoxemia and cyanosis (seen in conditions such as Tetralogy of Fallot or Eisenmenger syndrome).

Can Qp/Qs be measured accurately if there is significant valvular regurgitation?

Doppler echocardiography relies on forward laminar stroke volume measurements. Severe aortic or pulmonary regurgitation can overestimate flow at the regurgitant valve; in such cases, oximetric catheterization or cardiac MRI phase-contrast flow analysis is preferred.

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Qp/Qs Ratio Calculator