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 · Sources & educational scope

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

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 · 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

All echo guides · Learning paths