Shunts & Bubble Studies - Saving Lives With Sound

Shunt detection in the critically ill

Are you worried about an intra-cardiac or intra-pulmonary right-to-left shunt causing refractory hypoxemia in your patient that is not responding to increases in FiO2? What is the role of an agitated-saline bubble study to help diagnose these etiologies?

Bubble studies

Agitated-saline intravenous (IV) contrast (aka “bubble studies”) can be used to determine the presence of right-to-left shunts.

Equipment required:

IV access (e.g. antecubital fossa or central line)
3-way Luer lock
10cc saline syringe
empty 10cc syringe [with 0.5cc of air]

Normal bubble studies

Agitated-saline contrast bubbles go through the IV, which travel to the right side of the heart (opacifies right side), and then through to intra-pulmonary vessels.

Normally, the lungs filter out the saline bubbles – which means bubbles normally do not reach the left side of the heart.

Bubble studies can be performed using either TTE Apical 4-chamber or TEE mid-esophageal 4-chamber. However, TEE has better sensitivity and specificity for assessing for patent foramen ovales (PFO) or atrial septal defects (ASD)

intra-atrial septal interrogation using tee

With the transducer in the mid-esophageal bicaval position (ME Bicaval) with the omni-plane angle ~90 degrees and the transducer rotated clockwise to the right (to focus on right-sided structures). This view provides further interrogation of the: superior vena cava (SVC), inferior vena cava (IVC), intra-atrial septum (IAS), left atrium (LA) and right atrium (RA), and occasionally the right ventricle (RV) and tricuspid valve (TV).

We interrogate the IAS for PFO/ASDs by looking at the:

2D structures
- At the thinnest point of the IAS possible (where the fossa ovalis would be)
Colour Doppler
- Adjusting the Nyquist limit (to ensure even medium and low-flow shunts are picked up):
  - High [~67cm/s]
  - Medium [~45cm/s]
  - Low [~20-23cm/s]

Note:

Remember to sweep through the IAS and zoom-in on the septum
The IAS is also noted to be bowing into the RA from the LA (indicating that left atrial pressure is greater than right atrial pressure) – indicating that a shunt would most likely be shunting from left-to-right
- This would predict that if the patient did an IAS defect, the bubble study will likely still be negative
There are is a left-to-right shunt (notably a blue jet) – noting blood moving from the left atrium to the right atrium
This is an example of a PFO

Detection of right-to-left shunts

Using the ME4C and ME Bicaval, we can see:

Septal bowing of IAS from RA to LA (indicating right atrial pressure is higher than left atrial pressure)
Colour Doppler evidence of flow from RA to LA

Timing of bubbles

During the bubble study (either TTE or TEE):

Video clip is initiated before the injection of the agitated saline (at least 2 cardiac cycles before injection)
Once bubbles start entering the right side – we also count how many cycles it takes for bubbles to cross to the left-side
- Intra-cardiac shunt: usually crosses within 1-2 cardiac cycles
- Intra-pulmonary shunt: usually crosses within 4-8 cardiac cycles
This is an example of an atrial septal defect (ASD)

Shunt and bubble study tutorial

In this tutorial, we will breakdown performance and interpretation bubble studies looking for shunts.

References

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