Thread by @jholtzman3

Remember that old joke --

"Is your shunt running? Well you better go catch it!" 😂

No? Well, join me on an adventure to learn about the fascinating world intracardiac shunts and shunt runs!

Along the way, we'll revisit an old case for practice - cardionerds.com/71-case-report…

In this tweetorial, we will

🚨 Define a shunt fraction
🚨 Review the Fick Principle
🚨 Practice how to calculate Qp/Qs
🚨 Discuss non-invasive assessment of intracardiac shunts

Before we get started, how many of you agree with the following statement?

"I feel comfortable with how to calculate the shunt fraction for an intracardiac shunt."

⚠️ Remember this exciting case?

A 78 y/o 👵🏽 with atrial fib and HFpEF presented 🏩🚑 with dyspnea and volume overload, found to have a sinus venosus defect and severe L ➡️ R shunting

The Qp/Qs was ~3. But…what does that mean again…?

cardionerds.com/106-case-repor…

Qp/Qs represents a RATIO of blood flow through the pulmonary to systemic circulations.

Assuming 🚫 intracardiac shunt, pulmonary = system blood flow. Here Qp/Qs ~1.

It is a closed ⚡️ circuit after all!

But what if there is an ASD or VSD or even delayed post-MI complications??

Let's start with the basics --

Remember the Fick Equation?

Recall that you can calculate 🩸 blood flow to an organ by knowing oxygen consumption and ateriovenous oxygen difference.

Most often we are interested in assessing systemic cardiac output.

Here -

VO2 is oxygen consumption

Ca is arterial oxygen content

Cv is venous oxygen content

Remember oxygen CONTENT is not the same as oxygen SATURATION.

Moving forward, let's assume our oxygen consumption, atmospheric pressure, and hemoglobin are constant, ok? 🙌🙌

💡💡💡If we use the difference in oxygen content across the pulmonary and the systemic circulations, we can then calculate the blood flow through the pulmonary and systemic circulations, individually.

If we suspect an intracardiac L ➡️ R shunt, then more blood flows through the pulmonary than the systemic circulation.

In other words, the ratio of blood flow through the pulmonary to the systemic circulation (also known as the Qp/Qs ratio) will be >1.

So how do we collect all these values?

We perform a shunt run via a right heart cath. We measure oxygen saturation at the following stops:

1. Pulmonary artery
2. Right ventricle
3. Right atrium
4. IVC
5. SVC
6. Aorta

When do we suspect a L to R shunt?

If we see a ⚠️ 7% increase in O2 sat between chambers at the level of the RA or great veins

OR

If we see a ⚠️ 5% increase in O2 sat distal to the RA and great arteries

But wait, what about the pulmonary venous sat?

Getting a true pulm venous O2 sat requires transseptal puncture or retrograde cath, so we substitute for the systemic arterial O2 sat (in the proximal aorta) or PCWP sat

Assuming no R ➡️ L shunting, these should be pretty similar

If we do think there is R ➡️ L shunting, we can either assume the pulmonary venous sat is 95%

OR

directly measure the pulmonary venous sat (via a wedge sat, a transseptal approach to measure the LA sat, and compare to Ao sat).

Lastly --

A L to R shunt (e.g., ASD) will affect the mixed venous sat, so don't forget to calculate a pre-shunt MvO2 by Flamm's 🔥 formula.

(3 x SVC O2 + IVC O2) / 4

We use this value for the mixed venous oxygen sat in our shunt ratio equation.

Ok, now we have all the data that we need to calculate our shunt fraction.

You're doing great!

Can you practice using these numbers?

cardionerds.com/71-case-report…

We can use Qp/Qs to determine the hemodynamic significance of shunts.

If Qp/Qs <1, net R ➡️ L shunt.

If Qp/Qs = 1.0 -1.5, small L ➡️ R shunt.

If Qp/ Qs >2.0, very large L ➡️ R shunt. It may warrant surgical intervention to prevent pulmonary vascular disease and RV overload.

Qp/Qs can also be calculated via non-invasive methods, including cardiac MRI.

Cardiac MRI allows for the calculation of velocity-time curves and therefore blood flow.

We can also calculate a shunt ratio using traditional echocardiography. All you need is the LVOT and RVOT diameter, as well as the LVOT and RVOT velocity time integral.

Let's save that math for another day…! ➗ ♾ 🧮

So let's summarize!

Today, we discussed

⚠️ the physiology behind the shunt fraction
⚠️ what values to collect on a RHC shunt run
⚠️ how to measure a Qp/Qs without a cardiac cath

Back to where we started --

Now, how many of you agree with the following statement.

"I feel comfortable with how to calculate the shunt fraction for an intracardiac shunt."

"I learned something in this Tweetorial that may change my clinical practice."

What has been your experience with intracardiac shunts?

What is your preferred method for measuring a Qp/Qs?

Any common pitfalls you see folks fall into?

I can never thank the @CardioNerds leadership enough for the support and feedback! 🔥 🌟🙌

@ThomasMDas @AmitGoyalMD @Dr_DanMD @CarineHamo @karanpdesai
@justinberk @EuniceDuganMD @RichardAFerraro @CBlumenthal2 @EvelynSongMD