A bubble study is often performed to help diagnose a patent foramen ovale (PFO)—a hole between the upper chambers of the heart. This test takes about 30 minutes to perform.
An echo with a bubble study is often called contrast echocardiography or a transcranial Doppler study (TCD).
How a Bubble Study Is Done
A bubble study takes advantage of the fact that when sound waves encounter different types of physical media — in this case, gas vs. liquid — they bounce around more, and create more “echo waves.” These echo waves show up on an echocardiogram as increased density.
In a typical bubble study, a saline solution is shaken vigorously to produce tiny bubbles and is then injected into a vein. As the bubbles travel through the vein and into the right side of the heart, the increased density they produce on the echocardiography image allows the physician to actually watch the bubbles move through the cardiac chambers.
If the heart is functioning normally, the bubbles will be seen entering the right atrium, then the right ventricle, then out the pulmonary artery and into the lungs, where they are filtered out of the circulation.
However, if the bubbles are seen to enter the left side of the heart, this indicates that there is an abnormal opening between the two sides of the heart—a so-called intracardiac shunt.
Currently, commercial forms of “bubbles” are available to be used during bubble studies. These new agents usually are comprised of tiny protein or phospholipid casings which enclose a gas. These new agents appear to be safe in most cases and may provide better echo imaging. However, they are substantially more expensive than a shaken saline solution.
Bubble Study for Patent Foramen Ovale
The most common reason for performing a bubble study is to look for a PFO. In these studies, while the bubbles are being injected into the vein, the patient is asked to perform a Valsalva maneuver (that is, bearing down as if having a bowel movement).
The Valsalva maneuver transiently raises the pressures in the right side of the heart, so that if a PFO is present, the bubbles can often be seen entering the left atrium. Bubbles appearing in the left atrium during the test confirm the presence of a PFO.
Fortunately, while PFOs are quite common (occurring in up to 25% of adults), they lead to stroke only rarely. So, while a positive bubble study might confirm the presence of a PFO, it does not tell the healthcare provider very much about the likelihood of stroke.
Most experts think that a better way to assess whether a PFO is likely to produce a stroke is to perform a transcranial Doppler study (TCD) in conjunction with a bubble study.
In a TCD study, echo techniques are used to visualize bubbles traveling through the blood vessels of the brain. The TCD study can detect whether the bubbles injected into a vein are actually entering the brain circulation. If so, the PFO appears to be more likely to increase the risk of stroke, and the doctor will be more likely to recommend anticoagulation therapy, or if a stroke has already occurred, possibly surgical closure of the PFO.
