Ultimate Guide To Qp:Qs And How To Calculate This Ratio

The Qp:Qs ratio can be a very useful calculation to help determine how significant a left to right intracardiac shunt is.  But what is Qp/Qs and how do you calculate it? Keep reading to find out.

The “Q” in the Qp/Qs ratio stands for blood flow.  The “p” stands for pulmonary and the “s” stands for systemic.  

Q = Blood Flow

P = Pulmonary

S = Systemic

So when we use the term Qp:Qs, what we’re saying it is the ratio of pulmonary blood flow to systemic blood flow.

Qp:Qs ratio is used to determine the ratio of the pulmonary to systemic blood flow across an intracardiac shunt, such as a PFO or a VSD.

How To Calculate Qp:Qs In Patients With Atrial Septal Defects  

The Qp:Qs ratio can be calculated by using Doppler echo measurements of stroke volume at two locations and cross sectional area measurements from 2D echo.

To calculate the Qp:Qs in patients with ASD’s, follow these steps:

#1. Cross Sectional Area Of The RVOT

The first thing you need to do is determine the cross sectional area of the right ventricular outflow tract (RVOT).  You want to do this during early to mid ventricular systole.

The diameter of the RVOT should be measured from inner edge to inner edge at the base of the pulmonary valve leaflet. Be sure to use the best optimized 2D image you can obtain. I prefer to make this measurement from the parasternal short axis (PSAX) view and zoom in close to the pulmonic valve.

Cross Sectional Area Calculation

CSA_RVOT= .785 x RVOT diameter²   

#2.  Obtain RVOT VTI

Next, using your well optimized image, you need to obtain the VTI of the right ventricular outflow tract.

Place your PW sample volume at the level of the RVOT to obtain this measurement.

#3.  Calculate The RVOT Stroke Volume

With the information you obtained in the first two steps, you can now calculate the stroke volume (SV) of the RVOT.

RVOT Stroke Volume Calculation

SV_RVOT = CSA_RVOT  x VTI_RVOT   

Note: The SV is in cc’s, the CSA is in cm2 and the VTI is in cm.

#4.  Cross Sectional Area Of The LVOT

From the parasternal long axis view, measure the LVOT diameter in early to mid systole. Measure the same way you did when determining the RVOT diameter, from inner edge to inner edge.

Cross Sectional Area Calculation

CSA_LVOT = .785 x LVOT diameter²

#5.  Obtain LVOT VTI

Next, using your well optimized image, you need to obtain the VTI of the left ventricular outflow tract.

Place your PW sample volume at the level of the LVOT to obtain this measurement.  A closing valve click should be included in the PW Doppler tracing. This is how you know you are in the right place.

#6. Calculate The RVOT Stroke Volume

With the information you obtained in the last two steps, you can now calculate the stroke volume (SV) of the LVOT.

LVOT Stroke Volume Calculation

SV_LVOT = CSA_LVOT  x VTI_LVOT    

Note: The SV is in cc’s, the CSA is in cm2 and the VTI is in cm.

#7. Calculate Qp:Qs

Now that you’ve completed the previous steps, you have all the information you need to calculate the Qp:Qs ratio.

Qp:Qs Calculation

SV_RVOT / SV_LVOT 

How To Calculate Qp:Qs In Echocardiograms With Ductus Arteriosis

When you are trying to calculate Qp:Qs in patients who have a ductus arteriosis, there is a very important difference in the calculation.  

In echocardiograms where the patient is known to have a ductus arteriosis, it is important to make the following changes to your Qp:Qs ratio calculation:

For Patent Ductus Arteriosis:

SV_LVOT =  Qp

SV_RVOT =  Qs

Qp:Qs = SV_LVOT / SV_RVOT

Tips For Calculating Qp:Qs

• Always make sure you have good quallity images. The information you obtain is only as good as the quality of images you can get.  If your guessing, don’t try to report it.
• Always use a good Doppler angle when obtaining VTI’s for Qp:Qs. Again, if you’re unable to obtain a good Doppler signal at a good angle, don’t try to report the data.  Simply report it as suboptimal image quality.