How this calculator works
The Doppler method multiplies outflow area by velocity-time integral to estimate stroke volume, then takes the pulmonary/systemic ratio. A common heart rate cancels. The Fick method is fundamentally an oxygen-content balance. Saturations approximate content differences only when haemoglobin is shared and dissolved oxygen is negligible; high inspired oxygen, sampling errors, regurgitation or complex anatomy can invalidate that approximation. Mixed venous saturation must come from an appropriate assessment, not an arbitrary peripheral sample. The result is an estimate for information only and is not medical advice.
Formula
Doppler: Qp/Qs = (RVOT diameter² x RVOT VTI)/(LVOT diameter² x LVOT VTI). Stroke volume = pi x diameter² x VTI/4. Saturation approximation: Qp/Qs = (Sa - Sv)/(Spv - Spa). Direct: Qp/Qs = Qp/Qs with both flows in the same units.
Worked example
RVOT 2.5 cm, LVOT 2 cm and both VTIs 20 cm give Qp/Qs = 2.5²/2² = 1.5625. Saturations Sa 98%, Sv 65%, Spv 98% and Spa 76% give 33/22 = 1.5.
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Frequently asked questions
Why is the outflow diameter squared?
Cross-sectional area is pi times diameter squared divided by four. A diameter measurement error therefore has a large effect on estimated flow.
Can I assume pulmonary venous saturation is 100%?
The tool requires an entered value. Lung disease and measurement circumstances may make an assumed value inappropriate.
Does a ratio alone determine treatment?
No. It is one haemodynamic measurement that must be combined with anatomy, pressures, symptoms and the reliability of the sampling method.