UMEM Educational Pearls

Hypoxemic respiratory failure is a common presentation of critically ill patients. If the degree of hypoxemia is severe and disproportionate to the patient's radiographic findings and not responding to increasing FiO2, a right-to-left shunt should be considered. To evaluate for an anatomic shunt, an intravenous agitated saline contrast (ASC) echocardiographic evaluation can be conducted by an ED provider at the bedside.

 

Technique:

1. Use two operators, nursing can perform the ASC with the physician obtaining the echo views
2. Set-up:
   • 20 gauge (or larger) PIV in the AC fossa or more proximal is sufficient (does not have to be a CVC)
   • Flush PIV aggressively prior to attempt to make sure it won't blow
   • Obtain 3-way stopcock and 2 10 cc syringes
   • One port is connected to the PIV, and a second port to an empty 10cc syringe with the plunger fully depressed 
   • Third port connected to a syringe filled with 9ccs of saline and 1cc of air (eject 1cc of saline from the syringe of normal saline (NS) and replace it with air)
3. Echo technique:
   • Any view where the RA, LA, and IAS can be seen will suffice
   • Apical 4-chamber view is favored, with a focus on the bilateral atria (can also do sub-xiphoid)
4. Procedure:
   • With the equipment connected to the PIV, bubbles are created by turning the stopcock valve to “off” toward the patient and alternately depressing the plungers on the 2 syringes to send the air/NS mixture back-and-forth between them (should be done forcefully)
   • Push ASC completely into one of the syringes and quickly turn the stopcock “off” toward the other, and inject the ASC into patient while maintaining echo view and actively recording

Interpretation:

1. Quality control check:
   •  A vigorous injection should result in dense opacification of the RA
     • If the chamber is not densely opacified, likely technique issue and the exam should not be interpreted
2. The LA should be examined for a period of at least 10 full beats
3. Timing when microbubbles are seen in the LA:
   • Immediately (within 3-6 beats is a typically used cutoff):  likely to be intracardiac (most likely PFO)
     • Under ideal circumstances, bubbles can be seen to transit across the septum in real time
   • After the 3-6 beat cutoff: more likely to be due to a transpulmonary shunt, either an AVM or hepatopulmonary syndrome, depending on the clinical circumstances
     • Further workup might include a CT angiogram of the chest or workup for cirrhosis
4. Rough qualitative interpretation
   • no bubbles
   • a small number (roughly <10)
   • a large number (roughly >10)
   • enough to completely opacify the LA
   • (Significant continuous hypoxemia requires significant continuous right-to-left shunting, and thus the ongoing passage of many ASC bubbles)

 

References