The bubble test, formally known as an agitated saline contrast echocardiogram, is a non-invasive diagnostic procedure. It is a specialized form of cardiac ultrasound that helps visualize blood flow patterns within the heart. The primary goal is to detect abnormal connections, or shunts, that allow blood to bypass the lungs. Using a temporary contrast agent, the test identifies these structural issues.
The Purpose and Mechanism of the Bubble Test
The test identifies a right-to-left shunt. This is often used to diagnose a Patent Foramen Ovale (PFO) or an Atrial Septal Defect (ASD), openings in the wall separating the upper chambers of the heart. Identifying these shunts is important because they can allow blood clots to enter the systemic circulation, contributing to conditions like stroke or transient ischemic attacks.
The “bubbles” are microscopic air bubbles suspended in a sterile saline solution. This solution is created by rapidly agitating saline and air in a syringe. Once injected into a vein, these microbubbles are highly visible on the ultrasound imaging because they effectively reflect the sound waves.
In a normally functioning heart, the bubbles travel from the vein into the right side of the heart. They are then pumped into the lungs, where the bubbles are filtered out by the pulmonary capillaries, disappearing from the bloodstream. If a shunt is present, the bubbles bypass the lung filter and appear on the left side of the heart, confirming an abnormal connection.
What to Expect During the Procedure
Few special preparations are required; patients are instructed to take their normal medications. The procedure is performed while a standard transthoracic echocardiogram (TTE) is underway, with the patient often lying on their left side to optimize the heart’s image quality.
An intravenous (IV) line is inserted into a vein for the saline solution injection. Electrodes are also placed on the chest to monitor the heart’s electrical activity throughout the imaging process. The technologist or cardiologist performing the test will then begin capturing real-time ultrasound images of the heart chambers.
During the test, the prepared saline solution is injected rapidly through the IV line. The patient will be asked to perform specific maneuvers, such as the Valsalva maneuver or a cough, which briefly increases pressure within the chest and the right side of the heart. This temporary pressure change helps unmask shunts that may only open under strain. The entire procedure is relatively quick, often taking less than 45 minutes, and most patients can resume their regular activities immediately afterward.
Interpreting Positive and Negative Results
A negative result indicates that the heart and major blood vessels are functioning normally. On the echocardiogram screen, the injected microbubbles will be seen filling the right atrium and ventricle. Following the normal circulatory path, these bubbles will then disappear as they enter the lungs, and no bubbles will be visible in the left atrium or left ventricle.
A positive result for an intracardiac shunt is confirmed when microbubbles appear in the left side of the heart. The timing of their appearance is a key diagnostic detail; bubbles that appear in the left heart chambers within three to eight heartbeats after entering the right side strongly suggest an intracardiac connection, like a PFO. The number of bubbles that cross over also helps grade the size of the shunt, ranging from mild (a few bubbles) to severe (a dense cloud of bubbles).
If the bubbles appear in the left side of the heart but with a delay—more than eight heartbeats after their arrival in the right atrium—it suggests a different issue, such as a pulmonary shunt. This late appearance means the bubbles passed through the lungs via abnormal connections, known as pulmonary arteriovenous malformations.
The results of the bubble test are reviewed by a cardiologist. These findings are used to guide decisions about further diagnostic testing or necessary treatment, such as medication or a procedural intervention.