Welcome to the Qp/Qs calculator, a tool that will help you quickly calculate and interpret the Qp/Qs ratio for patients with cardiac shunts. Along the way, we'll give you a short refresher on the circulatory system and insight into the causes and effects of cardiac shunts.

Speaking of cardiac shunts, what exactly are they? Why should we even quantify them? What is the Qp/Qs ratio, how to calculate Qp/Qs, and what are normal Qp/Qs values? For the answers to these questions and more, read on!

The circulatory system

If a heart had a slogan, it would be: "your left or my left?" That's because the left and right sides of the heart were named from a "dorsal" view, meaning from the perspective of the person whose heart it is. The heart has four chambers: a left atrium, a right atrium, a left ventricle, and a right ventricle. The normal circulation of blood is as follows:

  1. Blood enters the right upper atrium.
  2. Blood is pumped from the right upper atrium to the right lower ventricle.
  3. Blood is carried from the heart to the lungs via the pulmonary artery (this is known as pulmonary circulation).
  4. Oxygenated blood returns to the heart from the lungs via the pulmonary vein by entering the left upper atrium.
  5. Blood is then pumped to the left lower ventricle.
  6. Blood exits the heart via the aorta and starts making its way around the body (this is called systemic circulation).

See the diagram below of a labeled human heart for a visual representation of the heart blood flow.

diagram of the human heart

What is a shunt in the heart?

A cardiac shunt is a pattern of blood flow that deviates from the regular circulatory system blood flow. Depending on the blood flow pattern, the cardiac shunt is described as:

  • Right to left cardiac shunt;
  • Left to right cardiac shunt;
  • Bidirectional;
  • Systemic-to-pulmonary; or
  • Pulmonary-to-systemic.

The severity of a shunt depends on the volume of shunted blood.

Causes of cardiac shunts

Now that we know what a shunt in the heart is, let's talk about its causes. A cardiac shunt can be congenital (from birth) or acquired (either biologically or mechanically):

Congenital shunt

  • Congenital heart defects (CHD) - The most common CHDs that cause cardiac shunting are atrial septal defects (ASD), patent foramen ovale (PFO), ventricular septal defects (VSD), and patent ductus arteriosus (PDA).

Acquired shunt

  • Biological - Some acquired shunts are just modifications of congenital ones. For example, a balloon catheter is used to modify CHD-related shunts so that a larger amount of oxygenated blood can enter systemic circulation.
  • Mechanical - In some cases of CHD, using a mechanical shunt, like the Blalock-Taussig shunt, can be used to control blood flow or blood pressure.

What is Qp/Qs? The Qp/Qs formula

A shunt can be quantified by measuring the flow ratio of the pulmonary cardiac output (Qp) to the systemic cardiac output (Qs). This flow ratio is the Qp/Qs ratio, otherwise known as the pulmonary-systemic shunt ratio. To calculate Qp and Qs, we can use the following shunt fraction equations:

  • Qp = RVOT VTI * π * (RVOT / 2)2
  • Qs = LVOT VTI * π * (LVOT / 2)2


  • RVOT - Right ventricular output diameter;
  • RVOT VTI - RVOT subvalvular velocity time integral;
  • LVOT - Left ventricular output diameter;
  • LVOT VTI - LVOT subvalvular velocity time integral; and
  • π - Pi, a constant that is approximately equal to 3.14

Finally, the Qp/Qs ratio calculation is as simple as dividing the two values.

How to use the Qp/Qs calculator?

Here is how to use the Qp/Qs calculator:

  1. Enter the RVOT, RVOT VTI, LVOT, and LVOT VTI values in the input section.
  2. We do the calculations automatically according to the Qp/Qs formula.
  3. You can find the Qp/Qs ratio underneath in the output section.
  4. If you would like to know the values for both Qp and Qs separately, click on advanced mode in our Qp/QS calculator.

How to calculate Qp/Qs?

We've now got the Qp/Qs formula from the above shunt fraction equation, and we know what all the variables in the formula stand for, but how do we get these values? That can be done by some method of echocardiography, such as Doppler echocardiography.

An echocardiogram uses high-frequency sound waves to create an image of the heart.

Doppler ultrasonography is used to determine the speed and the direction of blood flow using the Doppler effect, which is the change in the frequency of a sound (or light) wave that occurs when the source of its is moving in relation to the observer. Huh? Here's an example: when an ambulance is approaching you, the pitch of the sirens is very high, but as the ambulance drives away, you notice that the pitch abruptly drops. That's the Doppler effect!

If you're interested in behavior of waves, make sure to check out the speed of sound in air calculator.

Qp/Qs values

Back to the Qp/Qs ratio calculation. What does the final ratio value tell us? It tells us the magnitude of the cardiac shunt.

In normal conditions after birth, the pulmonary output is practically identical to the systemic output, so: Qp/QS = 1.

In people with cardiac shunts, this ratio changes. If Qp/Qs values are high, i.e.,Qp/Qs > 1, the pulmonary flow is greater than the systemic one, indicating a left-to-right shunt. If Qp/Qs < 1, the systemic flow is greater than the pulmonary flow, indicating a right-to-left shunt.

It's also possible to have both directions of shunting in one patient. In that case, if the left-to-right shunt = right-to-left shunt, so the Qp/Qs ratio would be exactly 1:1! Being able to quantify the shunt is important in informing how independent the two circulations are and also in studying the associated complications of the shunt.


Are cardiac shunts found in other animals?

Cardiac shunts can be found in other mammals, as well as in birds and even in reptiles! In birds and mammals, cardiac shunts are detrimental, but in reptiles, shunts are considered to be a derived trait.

How common are congenital heart defects?

Congenital heart defects are the most common congenital disorders. It is estimated that congenital heart defects affect between 4 and 75 per 1,000 live births depending on how they are diagnosed. Usually, a diagnosis occurs in childhood, but sometimes a congenital heart defect is not diagnosed until adulthood.

What are common adverse effects of a cardiac shunt?

Common effects of cardiac shunts include increased strain on the ventricles, reduced oxygen delivery to tissues, and limited work capacity of the muscles.

Can congenital heart defects be prevented?

Congenital heart defects can be partially prevented through a rubella vaccination. Other prevention strategies include adding iodine to salt and fortifying foods with folic acid (vitamin B9).

Alexa Ruiz
Pulmonary-systemic shunt ratio (Qp/Qs)
People also viewed…

Clomid ovulation

Our Clomid ovulation calculator allows you to painlessly compute the day of your ovulation and fertility window during your treatment with Clomiphene citrate.


Do you feel like you could be doing something more productive or educational while on a bus? Or while cleaning the house? Well, why don't you dive into the rich world of podcasts! With this podcast calculator, we'll work out just how many great interviews or fascinating stories you can go through by reclaiming your 'dead time'!


With this calculator you will be able to assess the mortality risk of your patient with infection using qSOFA score.
main background