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
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]
- Adjusting the Nyquist limit (to ensure even medium and low-flow shunts are picked up):
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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