A bubble echocardiogram is ordered when your doctor needs to find out whether blood is crossing between the left and right sides of your heart through a hole that shouldn’t be there. The most common reason is to check for a patent foramen ovale (PFO), a small flap-like opening between the upper chambers of your heart that was supposed to close shortly after birth but never fully sealed. About one in four people have one without knowing it, and it usually causes no problems, but in certain situations it can allow blood clots to bypass the lungs and travel to the brain.
What the Test Is Looking For
A standard echocardiogram uses ultrasound to show your heart’s structure and how well it pumps, but it can’t reliably detect a small hole between the upper chambers. A bubble study adds a simple twist: a small amount of salt water is rapidly mixed with a tiny bit of air between two syringes, creating a solution full of microbubbles. These bubbles are injected into a vein in your arm and show up brightly on the ultrasound as they flow into the right side of your heart.
Here’s the key principle: the microbubbles are roughly 16 to 38 micrometers wide, while the tiny blood vessels in your lungs are only about 9 micrometers across. Normal lungs act as a filter, trapping and dissolving every bubble before blood returns to the left side of the heart. If bubbles do appear in the left atrium, it means they found a shortcut, either through a hole in the heart wall or through abnormal blood vessels in the lungs. That shortcut is called a right-to-left shunt.
Common Reasons Your Doctor Ordered It
The single most frequent reason is a stroke that has no obvious cause. When standard testing rules out blocked arteries, irregular heart rhythms, and blood-clotting disorders, the stroke is classified as “cryptogenic,” meaning its origin is unknown. One leading theory for these unexplained strokes is paradoxical embolism: a blood clot forms in a vein (often in the legs), travels to the right side of the heart, slips through a PFO or similar defect, and reaches the brain. A bubble study can reveal whether that pathway exists. The American Stroke Association specifically recommends considering a bubble study with a Valsalva maneuver as part of the workup for cryptogenic stroke.
Other reasons your doctor may order the test include:
- Unexplained low oxygen levels. If your blood oxygen is persistently low and lung disease doesn’t fully explain it, a shunt could be allowing oxygen-poor blood to mix into your arterial circulation.
- Migraine with aura. There is an observed association between PFO and chronic migraines, particularly migraines with visual disturbances. While closing a PFO isn’t currently recommended as a first-line migraine treatment, some doctors investigate whether a shunt is present when migraines are frequent and resistant to standard therapy.
- Before certain procedures. If you’re being evaluated for a catheter-based PFO closure or another cardiac intervention, a bubble study helps confirm the defect and measure its severity.
What Happens During the Test
No special preparation is needed. You can eat, drink, and take your regular medications beforehand. The appointment typically takes 45 to 60 minutes.
You’ll lie on an exam table, and a technician will place an IV line in your arm and apply the standard ultrasound probe to your chest. First, a baseline echocardiogram records your heart at rest. Then the agitated saline solution is injected through the IV. On the monitor, you’ll see the right atrium light up with a dense cloud of bright dots. The sonographer watches closely to see whether any of those dots cross over to the left side.
You’ll then be asked to bear down as if you’re straining, a technique called the Valsalva maneuver, or to cough forcefully. Both actions temporarily raise pressure on the right side of the heart, which can push the flap of a PFO open and force bubbles through. Research shows that coughing is actually more sensitive than the Valsalva maneuver for revealing a PFO. In one study, coughing detected a shunt in 44% of patients who tested negative during quiet breathing, compared to 16% with the Valsalva technique alone. The injection is typically repeated a few times with these different maneuvers to maximize accuracy.
How Results Are Graded
If no microbubbles appear on the left side, the test is negative, meaning no shunt was detected. When bubbles do cross over, doctors grade the severity based on how many appear in the left atrium:
- Grade I: 1 to 10 microbubbles
- Grade II: 11 to 30 microbubbles
- Grade III: More than 30 microbubbles, with the left atrium nearly filled or fully opacified
A higher grade means more blood is crossing through the defect. This grading helps your doctor decide whether the shunt is large enough to be clinically significant and whether closure might be warranted, particularly after a stroke.
Timing also matters. If bubbles appear in the left atrium within three to five heartbeats of filling the right atrium, the shunt is likely inside the heart itself (a PFO or atrial septal defect). If they show up later, after five or more beats, the pathway is more likely through abnormal blood vessels in the lungs.
Accuracy and Follow-Up Testing
A bubble study done through the chest wall (transthoracic) detects about 76% to 77% of PFOs. A transesophageal echocardiogram, where a small ultrasound probe is guided down the throat to sit right behind the heart, detects roughly 79%. Neither method reaches higher than 80% sensitivity on its own. For this reason, cardiologists often treat the two approaches as complementary. If your transthoracic bubble study is negative but suspicion remains high, your doctor may recommend the transesophageal version for a closer look.
Safety and Side Effects
The bubble study is considered very safe. The microbubbles dissolve within seconds, and the saline solution is the same sterile salt water used in standard IV fluids. A survey by the American Society of Echocardiography reported transient side effects in about 0.06% of cases, which works out to roughly 6 in 10,000 tests.
In rare instances, particularly in patients who turn out to have a shunt, a small number of bubbles can reach the brain’s blood vessels and cause brief neurological symptoms: dizziness, temporary vision changes, tingling in the face or hand, or short-lived weakness on one side of the body. These events resemble a mini-stroke and typically resolve within minutes. They are uncommon enough that the true incidence hasn’t been precisely established, but the risk is low enough that the diagnostic value of the test is generally considered to far outweigh it, especially in someone being evaluated after a stroke.