Canonical: https://echosuite.app/guides/qpqs/

# Qp/Qs calculator: pulmonary to systemic flow ratio

Review the Doppler stroke-volume measurements used for pulmonary and systemic flow in the Qp/Qs tool.

Created by [Mohammed Kamareddine, MD](https://echosuite.app/about/) · [Sources & educational scope](https://echosuite.app/about/)

[Open interactive calculator →](https://echosuite.app/calculators/#qpqs)

## Method

Qp/Qs = (RVOT CSA × RVOT VTI) ÷ (LVOT CSA × LVOT VTI)

Use the measurement units shown below. Some fields apply only to a selected method or condition; follow the interactive tool’s method choices. The interactive tool keeps missing measurements incomplete and provides its method-specific results and reference context.

## Measurements and acquisition

### RVOT maximal-systolic diameter at PW site  cm

2D echo · inner-edge measurement

- Use the specified view and cardiac phase.
- Align the calipers perpendicular to the structure’s long axis.
- Measure inner edge to inner edge and avoid oblique foreshortening.

### RVOT VTI  cm

Pulsed-wave Doppler

- Place the sample volume at the specified annulus or outflow tract.
- Align the Doppler beam with flow and record a clean envelope.
- Trace the modal velocity envelope and use the reported VTI.

### LVOT maximal-systolic diameter at PW site  cm

2D echo · inner-edge measurement

- Use the specified view and cardiac phase.
- Align the calipers perpendicular to the structure’s long axis.
- Measure inner edge to inner edge and avoid oblique foreshortening.

### LVOT VTI  cm

Pulsed-wave Doppler

- Place the sample volume at the specified annulus or outflow tract.
- Align the Doppler beam with flow and record a clean envelope.
- Trace the modal velocity envelope and use the reported VTI.

## Sources and method context

ASE / SCAI · 2015 · Echocardiographic assessment of atrial septal defect and patent foramen ovale · Pulsed Doppler shunt-flow quantification · pp. 929–930

[Primary reference](https://www.asecho.org/wp-content/uploads/2025/04/2015_ASD-PFO.pdf)

- Pulmonary-to-systemic flow ratio · pp. 929–930 · Doppler shunt quantification

Each outflow stroke volume is its cross-sectional area times matched systolic VTI. Qp/Qs is RV outflow stroke volume divided by LV outflow stroke volume. Circular-area assumptions and semilunar regurgitation limit the estimate.

- [Adult shunt context · AHA/ACC 2025](https://doi.org/10.1161/CIR.0000000000001402) · ACHD guideline · §4.1 / Table 25

A Qp/Qs of at least 1.5 is interpreted with chamber loading, PVR and the full congenital assessment. The app’s greater-than-2 interval is a flow-reporting descriptor, not a universal defect severity or closure rule.

Catalogue review status: Exact guideline locator mapped · clinical sign-off pending

These pages describe the educational tool. Interpret measurements with the complete clinical context and the method’s acquisition limitations.

## Keep exploring

- [Parasternal long axis acquisition guide](https://echosuite.app/guides/views/plax/) · [Open in Probe Lab](https://echosuite.app/#plax)
- [Apical five chamber acquisition guide](https://echosuite.app/guides/views/apical-five-chamber/) · [Open in Probe Lab](https://echosuite.app/#a5c)
- [RV outflow acquisition guide](https://echosuite.app/guides/views/rv-outflow/) · [Open in Probe Lab](https://echosuite.app/#rvOutflow)
- [Parasternal short axis, aortic valve level acquisition guide](https://echosuite.app/guides/views/psax-aortic-valve/) · [Open in Probe Lab](https://echosuite.app/#psaxAV)
- [Atrial septal defect: shunt and Doppler calculator](https://echosuite.app/guides/asd/)
- [VSD · size & right-heart pressure calculator](https://echosuite.app/guides/vsd-pressure/)
- [PDA · shunt & PA pressure calculator](https://echosuite.app/guides/pda-pressure/)
- [Coarctation · anatomy & gradients calculator](https://echosuite.app/guides/coarctation/)

[All echo guides](https://echosuite.app/guides/) · [Learning paths](https://echosuite.app/learn/)