Pediatric Echo: A Step-by-Step Guide to Quantifying Shunt Flow (Qp/Qs) in ASD/VSD

Johnson Francis | January 22, 2026 | General Cardiology | No Comments

In pediatric cardiology, the Qp/Qs ratio is a fundamental metric used to quantify the hemodynamic significance of left-to-right shunts like Atrial Septal Defects (ASD) and Ventricular Septal Defects (VSD).

The ratio is defined as:

Qp/Qs = Pulmonary Blood Flow (Qp)/Systemic Blood Flow (Qs)

Step 1: Obtain the Master Formula

The calculation relies on the continuity equation, where flow (Stroke Volume) equals the cross-sectional area (CSA) of the vessel multiplied by the Velocity Time Integral (VTI) of blood flow through that vessel.

SV = Area x VTI = (πr²) x VTI = 0.785 x d² x VTI

Therefore:

Qp/Qs = [(0.785 x (RVOT diameter)² x PV VTI] / [(0.785 x (LVOT diameter)² x AV VTI]

(The constant 0.785 cancels out, simplifying the ratio to d² x VTI for both sides).

Step 2: Systemic Flow (Qs) Measurements

In patients with ASD or VSD, the Left Ventricular Outflow Tract (LVOT) represents the systemic output.

LVOT Diameter (d_LVOT):
- View: Parasternal Long Axis (PLAX).
- Timing: Mid-systole (when the aortic valve is fully open).
- Technique: Measure from inner edge to inner edge at the insertion of the aortic valve leaflets.
LVOT VTI (VTI_LVOT):
- View: Apical 5-Chamber (A5C).
- Technique: Place the Pulsed Wave (PW) Doppler sample volume approximately 0.5–1 cm proximal to the aortic valve. Trace the envelope of the spectral waveform.

Step 3: Pulmonary Flow (Qp) Measurements

The Right Ventricular Outflow Tract (RVOT) represents the pulmonary output.

RVOT Diameter (d_RVOT):
- View: Parasternal Short Axis (PSAX) at the level of the aortic valve.
- Timing: Mid-systole.
- Technique: Measure the diameter of the RVOT just proximal to the pulmonary valve.
RVOT VTI (VTI_RVOT):
- View: Parasternal Short Axis (PSAX).
- Technique: Place the PW Doppler sample volume just proximal to the pulmonary valve. Trace the spectral envelope.

Step 4: Calculate and Interpret

Plug your measurements into the ratio:

Qp/Qs = [(d_RVOT)² x VTI_RVOT] / [(d_LVOT)² x VTI_LVOT]

Critical Technical Tips

The “Squared” Error: Because diameters are squared in the formula, even a 1mm measurement error can significantly skew the final Qp/Qs ratio. Always take the average of 3–5 measurements.
Sample Placement: Ensure the Doppler sample volume is placed at the exact location where the diameter was measured to maintain the validity of the continuity equation.