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]


  • 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.


1. Mayo PH, Beaulieu Y, Doelken P, Feller-Kopman D, Harrod C, Kaplan A, Oropello J, Vieillard-Baron A, Axler O, Lichtenstein D, others. American College of Chest Physicians/La Societe de Reanimation de Langue Francaise statement on competence in critical care ultrasonography. CHEST J 2009;135:1050–1060.
2. Mayo PH, Narasimhan M, Koenig S. Critical Care Transesophageal Echocardiography. CHEST 2015;148:1323–1332.
3. Silvestry FE, Cohen MS, Armsby LB, Burkule NJ, Fleishman CE, Hijazi ZM, Lang RM, Rome JJ, Wang Y. Guidelines for the Echocardiographic Assessment of Atrial Septal Defect and Patent Foramen Ovale: From the American Society of Echocardiography and Society for Cardiac Angiography and Interventions. J Am Soc Echocardiogr 2015;28:910–958.
4. Mulvagh SL, Rakowski H, Vannan MA, Abdelmoneim SS, Becher H, Bierig SM, Burns PN, Castello R, Coon PD, Hagen ME, Jollis JG, Kimball TR, Kitzman DW, Kronzon I, Labovitz AJ, Lang RM, Mathew J, Moir WS, Nagueh SF, Pearlman AS, Perez JE, Porter TR, Rosenbloom J, Strachan GM, Thanigaraj S, Wei K, Woo A, Yu EHC, Zoghbi WA. American Society of Echocardiography Consensus Statement on the Clinical Applications of Ultrasonic Contrast Agents in Echocardiography. J Am Soc Echocardiogr 2008;21:1179–1201.
5. Expert Round Table on Echocardiography in ICU. International consensus statement on training standards for advanced critical care echocardiography. Intensive Care Med 2014;40:654–666.
6. Lau V, Priestap F, Landry Y, Ball I, Arntfield R. Diagnostic Accuracy of Critical Care Transesophageal Echocardiography vs Cardiology-Led Echocardiography in ICU Patients. CHEST 2019;155:491–501.
7. Douglas PS, Garcia MJ, Haines DE, Lai WW, Manning WJ, Patel AR, Picard MH, Polk DM, Ragosta M, Ward RP, Weiner RB. ACCF/ASE/AHA/ASNC/HFSA/HRS/SCAI/SCCM/SCCT/SCMR 2011 Appropriate Use Criteria for Echocardiography: A Report of the American College of Cardiology Foundation Appropriate Use Criteria Task Force, American Society of Echocardiography, American Heart Association, American Society of Nuclear Cardiology, Heart Failure Society of America, Heart Rhythm Society, Society for Cardiovascular Angiography and Interventions, Society of Critical Care Medicine, Society of Cardiovascular Computed Tomography, and Society for Cardiovascular Magnetic Resonance Endorsed by the American College of Chest Physicians. J Am Coll Cardiol 2011;57:1126–1166.
8. Douglas PS, Khandheria B, Stainback RF, Weissman NJ, Brindis RG, Patel MR, Khandheria B, Alpert JS, Fitzgerald D, Heidenreich P, Martin ET, Messer JV, Miller AB, Picard MH, Raggi P, Reed KD, Rumsfeld JS, Steimle AE, Tonkovic R, Vijayaraghavan K, Weissman NJ, Yeon SB, Brindis RG, Douglas PS, Hendel RC, Patel MR, Peterson E, Wolk MJ, Allen JM. ACCF/ASE/ACEP/ASNC/SCAI/SCCT/SCMR 2007 Appropriateness Criteria for Transthoracic and Transesophageal Echocardiography: A Report of the American College of Cardiology Foundation Quality Strategic Directions Committee Appropriateness Criteria Working Group, American Society of Echocardiography, American College of Emergency Physicians, American Society of Nuclear Cardiology, Society for Cardiovascular Angiography and Interventions, Society of Cardiovascular Computed Tomography, and the Society for Cardiovascular Magnetic Resonance Endorsed by the American College of Chest Physicians and the Society of Critical Care Medicine. J Am Soc Echocardiogr 2007;20:787–805.
9. Porter TR, Abdelmoneim S, Belcik JT, McCulloch ML, Mulvagh SL, Olson JJ, Porcelli C, Tsutsui JM, Wei K. Guidelines for the Cardiac Sonographer in the Performance of Contrast Echocardiography: A Focused Update from the American Society of Echocardiography. J Am Soc Echocardiogr 2014;27:797–810.