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.
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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).

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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]

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 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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