Saline Shunt Study AKA “Bubble Study,” for the Evaluation of Atrial-Septal Defect (ASD), Patent Foramen Ovale (PFO) or Pulmonary Atriovenous Malformations (AVM).
©Walter Rasmussen, R.D.C.S.
An IV injection of agitated saline is used for the detection of right to left shunts when there are no obvious septal defects seen on two-dimensional and color Doppler. During this procedure, an authorized staff member agitates and injects five to ten ml of saline in a peripheral vein while the sonographer records a video clip of the micro-bubbles entering and opacifying the right atrium and right ventricle.
With good beam penetration, properly agitated and injected saline will completely opacify the right atrium and right ventricle. If however, the two-dimensional image is of poor quality, or the saline is not adequately mixed, the sensitivity of the exam will be greatly reduced and a negative result cannot definitively rule out a shunt. A positive shunt study for an ASD or PFO demonstrates micro-bubbles appearing on the left side of the heart within four cardiac cycles of their appearance in the right heart chambers. In the case of pulmonary AVM’s, so called “late bubbles” will appear within about 8 or more cardiac cycles.
The success of the saline shunt study is dependent upon many factors, often related to the mixing of the saline, the clarity of the ultrasound image and effective coordination between the sonographer and the person making the injection.
- Transducer Frequency: Image quality is of great importance to the success of the saline shunt study therefore, using the highest frequency that will pick up the greatest quantity of micro-bubbles without compromising beam penetration should be the goal. If the apical four-chamber two-dimensional image is of good to excellent quality, then the detection of micro-bubbles will not be difficult as long as the solution is agitated and injected effectively. If however, there is a poor two-dimensional image with a lot of “noise,” requiring a low frequency setting, then micro-bubbles on the left side of the heart may go undetected.
- Image Adjustments: The image contrast should be adjusted by the use of dynamic range, image compression and reject so that the micro-bubbles will be easier to identify and distinguish from the background noise, which looks very similar to micro-bubbles. A colorized image can aid in visual perception of the micro-bubbles.
- Transducer Position: Since the transducer position can have a great influence on the clarity of the image, a clear off-axis apical four-chamber image is better than an image that is obtained from a technically correct imaging plane, but poor in image quality. The ideal image demonstrates the right and left atria, including the right upper pulmonary vein, the inter-atrial septum and at least the mid portions of the right and left ventricles. A zoomed image covering the area from the pulmonary veins to the mid ventricular level will allow for more careful review of the video clip.
- Clip Timing and Retrospective Acquisition: Current ultrasound machines have good options for recording long video clips. A very simple clip acquisition mode is to simply freeze the image once the microbubbles have been observed entering the right atrium and circulating for at least 10 cardiac cycles. The cineloop is then played and edited to start just before the bubbles enter the right atrium, and then to the last desired cardiac cycle. Once edited, the clip is stored. It is advisable to review the clip once before the IV is discontinued in order to assure that it was properly stored.
- Prospective Clip Acquisition: Another option is to pre-determine the number of seconds the clip will record and then start the clip as soon as the injection has commenced. This is the only way to capture long clips on older machines, but is certainly less desirable because the clip often ends acquisition before the result is fully recorded.
- Alternative View: If an apical window does not provide the image clarity that is desired, the sonographer can use a subcostal four-chamber view as an alternate.
- Sonographer Direction and Supervision: It is the sonographer’s responsibility to take charge of the shunt study procedure, verifying that the saline has been agitated correctly and directing the nurse when to perform the injection. The sonographer is responsible for the image quality and may need to ask for repeated injections as necessary, in order to complete the examination.
- Assure Proper Injection Techniques: For the acquisition of a good shunt study, the micro-bubbles should be quite dense. Inadequate agitation of the saline and/or injecting too slowly are the primary reasons for a sparse appearance of micro-bubbles within the right heart chambers. More dense bubble population can be achieved if the staff member performing the injection raises the patient’s arm immediately after the injection.
- Shunt vs Pulmonary AVM’s: Pulmonary arteriovenous malformations (PAVM’s) are rare vascular anomalies in the lungs, in which abnormal dilated vessels provide a right-to-left shunt between the pulmonary and systemic circulation. While conducting a saline shunt study, the sonographer observes the number of beats it takes for bubbles to appear on the left side of the heart. Generally, if they appear within the first three to four cardiac cycles, the shunt can be attributed to an ASD or PFO. If however, there is a delay in the appearance in micro-bubbles (usually five to ten beats or more), the prescence of pulmonary AVM’s is likely. Some shunt studies are specifically conducted in order to rule-out pulmonary AVM’s and so, a very long video clip may be required for the examination.
- Follow Workplace Protocols: Every workplace has unique protocols for conducting a saline shunt study and the sonographer should study and follow it in order to assure patient safety.
- Valsalva Maneuver: If the first injection results in a positive result, no further images are necessary. If the initial injections are negative, the patient is instructed to perform a Valsalva maneuver and the saline is injected just before the patient releases it. This will cause a sudden increase in the volume of blood moving in to the right atrium and can cause a patent foramen ovale to briefly open.
- TR Gradient: A supplemental saline injection is made in addition to the shunt study injections if the TR jet was not well-defined on the routine echocardiography exam. See the previous topic for tips on enhancing the TR jet.