5.3 Agitated-Saline Studies and Intracardiac Shunts

Agitated saline begins in the right heart

An agitated-saline study creates echogenic microbubbles by vigorously mixing a small, protocol-defined amount of air with sterile saline. These bubbles are generally too large to cross normal pulmonary capillaries, so a good peripheral venous injection densely opacifies the right atrium and ventricle. Appearance in the left heart suggests a right-to-left pathway, either through the heart or pulmonary circulation. This physiology is the opposite of commercial UEA left-ventricular opacification, whose stabilized microspheres are designed to traverse the lungs.

Common adult indications include suspected patent foramen ovale or other interatrial communication, suspected intrapulmonary shunt, and evaluation of anomalous systemic venous drainage such as a persistent left superior vena cava. Verify the order, question, patient, venous access, protocol, and administration authority. ASE's comprehensive TTE guidance lists pregnancy and a known significant right-to-left shunt among contraindications for agitated saline; apply current medical direction and local policy rather than assuming a suspected small PFO and a known large shunt are the same circumstance.

Prepare and acquire without shortcuts

The 2019 ASE comprehensive adult TTE protocol describes two 10-mL lockable syringes, a three-way stopcock, 8 to 10 mL saline, and no more than 0.5 mL air. Laboratories may have a specifically approved mixture, sometimes including a small amount of the patient's blood, so follow that protocol rather than improvising. Agitate briskly until the bubbles are fine and uniform; never inject a visible macroscopic air pocket. Locking equipment prevents separation during agitation. Use only a confirmed venous line, never an arterial catheter.

Use an optimized A4C to show the interatrial septum; use subcostal A4C when apical imaging is poor. Use harmonic imaging when helpful. Begin the clip with a bubble-free right atrium, capture injection and full right-heart opacification, and record long enough to detect delayed passage. ASE's comprehensive protocol recommends at least 20 consecutive beats. Annotate injection site, rest or maneuver, and injection number.

At rest, a small PFO may show no right-to-left passage because LA pressure exceeds RA pressure. Rehearse Valsalva before injection: the patient strains, bubbles fill the right atrium, and release transiently raises RA pressure relative to LA. A leftward shift of the interatrial septum supports an effective maneuver. Cough or protocol-approved abdominal compression can help when Valsalva is not feasible. A negative study with weak RA opacification or ineffective provocation is nondiagnostic, not proof that no shunt exists.

Count timing, then confirm the pathway

Count cardiac cycles from dense right-atrial opacification, not from the syringe push. Left-heart appearance within three cycles favors an intracardiac pathway, appearance after more than five cycles favors transpulmonary passage, and cycles three through five form an overlap zone rather than a rigid boundary. Timing is a clue, not an absolute law. Direct visualization of bubbles crossing the interatrial septum strongly supports intracardiac passage, while bubbles entering from a pulmonary vein support a transpulmonary route and outweigh cycle counting alone.

Physiology can blur the boundary. High LA pressure or a poor Valsalva can delay passage through a PFO. A large pulmonary arteriovenous connection can produce relatively early bubbles. Eustachian-valve flow can direct an upper-extremity injection away from the septum, and injection site, cardiac output, respiration, and shunt size affect arrival. Pseudocontrast from stagnant blood and excessive gain can mimic a few bubbles. The sonographer records timing, route, maneuver quality, and limitations for physician interpretation rather than diagnosing PFO or pulmonary AVM from a beat count alone.

Safety and documentation complete the study

Explain the procedure and maneuver, protect the IV, use aseptic technique, and watch the patient throughout. Stop for new neurologic symptoms, chest pain, severe dyspnea, syncope, significant rhythm change, or another concerning response; assess, call for help, and follow emergency policy. Preserve relevant images and document the event without independently attributing cause.

The worksheet should record indication, venous site, mixture per protocol, number of injections, imaging view, baseline and provocation conditions, RA opacification quality, cycle timing, observed route, shunt-grade method if the laboratory uses one, limitations, symptoms, and communications. In a stroke evaluation, a negative resting injection followed by early septal crossing only after an effective Valsalva release is not contradictory; it demonstrates pressure-dependent passage. Conversely, delayed bubbles without a visible route require cautious correlation. Technique, timing, anatomy, and physiology together make the result interpretable.

Count from right-atrial opacification

The timing clock starts when the right atrium is densely opacified, not when the syringe is pushed. Record the route whenever possible because direct septal crossing or pulmonary-vein entry is more informative than timing alone.