#CCU_ECHO Pulmonary-Systemic Shunt Ratio { Qp / Qs }

Pulmonary-Systemic Shunt Ratio { Qp / Qs }
#CCU_ECHO
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can be estimated by using 2-D echo and spectral Doppler measurements in patients who have intra- or extra-cardiac shunts e.g. atrial or ventricular septal defects.

Qp = RVOT VTI ⋅π⋅ (RVOT2) / 2

Qs = LVOT VTI ⋅π⋅ (LVOT2) / 2

Qp/Qs Ratio = Qp / Qs

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Definitions
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LVOT Left ventricular outflow tract diameter (mm)
LVOT VTI LVOT subvalvular velocity time integral (cm)
RVOT Right ventricular outflow tract diameter (mm)
RVOT VTI RVOT subvalvular velocity time integral (cm)
Qp/Qs Pulmonary-Systemic Shunt Ratio
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Example:

You are asked to calculated the shunt fraction
Qp/Qs for a secundum ASD.

For this, you need to obtain the following measurements:

1) The LVOT diameter from the PLAX view.
2) The LVOT VTI from 5 chamber or apical long axis view
3) The RVOT diameter from the PSAX view
4) The RVOT VTI from the PSAX

The calculation here given the numbers above would be:

Qp = 2Pi(2.41/2)^2 X 23

Qs= 2Pi(1.9/2)^2 x 22

= 1.7

Note that for the ratio Qp/Qs the 2Pi terms cancel out.

Important Points
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1) This ratio is highly sensitive to errors in measurement of the LVOT and RVOT diameter, since both of these values are squared.

2) Measuring the RVOT diameter can be challenging without adequate echo windows.
The Diameter should be measured at the RVOT, proximal to the pulmonic valve, which is where the VTI measurement takes place.

3) This formula only works in cases where there is pure left to right shunting.