A bubble study is an ultrasound test that checks for abnormal openings in your heart by tracking tiny air bubbles injected into your bloodstream. It’s most commonly used to detect a patent foramen ovale (PFO), a small hole between the upper chambers of the heart that should have closed shortly after birth. The test takes about 45 to 60 minutes and requires no fasting or changes to your medications.
How the Test Works
A technician places a small IV line in your arm and injects a sterile salt solution that has been vigorously shaken to create microbubbles. These bubbles are tiny, averaging 16 to 38 micrometers across, and they show up clearly on an echocardiogram (heart ultrasound) as bright, swirling dots moving through your heart chambers.
The key principle is simple: the bubbles are too large to pass through your lungs. Pulmonary capillaries are only about 9 micrometers wide, so the bubbles get trapped and dissolve harmlessly before reaching the left side of your heart. If the ultrasound shows bubbles appearing in the left atrium, it means they found a shortcut, either through a hole in the heart wall or through an abnormal connection in the lungs. That shortcut is called a right-to-left shunt.
Why Doctors Order a Bubble Study
The most common reason is to look for a PFO after an unexplained stroke. When a younger patient has a stroke and doctors can’t find an obvious cause like a blood clot from an irregular heartbeat, they call it a “cryptogenic stroke.” A PFO can allow small clots that form in the veins to bypass the lungs and travel directly to the brain. If a PFO is found and you’ve had a stroke or have low blood oxygen levels, your doctor may recommend a procedure to close the hole.
Bubble studies are also used when patients have unexplained low oxygen levels. In intensive care settings, the test can help determine whether blood is bypassing the lungs through an abnormal pathway, which changes the treatment approach entirely.
What Happens During the Procedure
You’ll lie on an exam table, typically on your left side, while a sonographer places an ultrasound probe on your chest. A small IV catheter goes into a vein in your arm or hand. Staying well hydrated and warm beforehand makes vein access easier.
The saline solution is rapidly pushed back and forth between two connected syringes to create the microbubbles, then injected through the IV. You’ll watch the screen fill with bright specks flowing into the right side of your heart. The sonographer records whether any of those specks cross over to the left side.
During the test, you’ll be asked to perform a Valsalva maneuver: bearing down as if you’re straining, then releasing. This briefly increases pressure on the right side of the heart, which can force open a PFO that might stay closed under normal conditions. The injection is repeated with this maneuver to maximize the chance of detecting a shunt. Some labs ask you to cough forcefully instead.
How Results Are Graded
Results aren’t simply positive or negative. Doctors count the number of bubbles that cross to the left side and assign a grade that reflects the size of the shunt:
- Grade 0 (negative): No microbubbles appear on the left side.
- Grade I (small shunt): 1 to 10 microbubbles cross over.
- Grade II (moderate shunt): 11 to 30 microbubbles cross over.
- Grade III (large shunt): More than 30 microbubbles, with the left atrium nearly filled or appearing opaque on the screen.
Timing also matters. If bubbles appear on the left side within three to five heartbeats of reaching the right side, the opening is likely inside the heart itself. If they appear later, the shunt may be in the lungs rather than the heart wall.
Accuracy and Limitations
A bubble study done through the chest (transthoracic echocardiography) is extremely reliable when it finds something. A meta-analysis of 13 prospective studies covering over 1,400 patients found that a positive result on a chest-based bubble study has a 99% specificity, meaning false positives are almost nonexistent. In patients with cryptogenic stroke, a positive result carries a 95% probability of being a true positive.
The weakness is sensitivity: the same analysis found that only about 46% of shunts are detected this way. That means a negative result doesn’t rule out a PFO. If your doctor still suspects a hole after a negative chest-based study, the next step is typically a transesophageal echocardiogram, where the ultrasound probe is guided into the esophagus to get a closer view of the heart. This approach is more sensitive because the probe sits just millimeters from the structures in question. A properly performed Valsalva maneuver also significantly improves detection rates, so the quality of your effort during the test genuinely affects the result.
Safety and Side Effects
Bubble studies are considered low-risk. The microbubbles dissolve within seconds and don’t circulate through your body the way contrast dye does in a CT scan. There’s no radiation involved, no need to fast, and you can take all your usual medications on the day of the test.
The primary theoretical concern is air microembolism, where a small amount of air enters the bloodstream and causes a temporary blockage. In practice, the volume of air used is small and complications are rare. Researchers have recommended standardizing the amount of air used in preparation to further minimize this risk. You may feel a brief cool sensation when the saline is injected, but most people tolerate the test without any discomfort beyond the IV insertion itself.
What Happens After the Test
If your bubble study is negative, the IV is removed and you can go about your day immediately. If a shunt is detected, your doctor will consider the grade, your symptoms, and your medical history to decide on next steps. For someone with a PFO and a history of unexplained stroke, that often means referral to a neurologist and discussion about whether closing the hole with a catheter-based procedure makes sense.
If you’ve already had a PFO closure, a follow-up bubble study is typically performed around six months after the procedure to confirm the opening has sealed and no residual shunt remains.