What Is a Bubble Test for the Heart?

A bubble test is an ultrasound-based heart exam in which a small amount of agitated saline, shaken until it forms tiny microbubbles, is injected into a vein and tracked on a live echocardiogram as it travels through the heart. Its main purpose is to reveal abnormal connections that allow blood to cross from the right side of the heart to the left, bypassing the lungs. The test is simple, takes only a few minutes on top of a routine echocardiogram, and involves no radiation or contrast dye in the traditional sense. But the information it delivers can reshape how a stroke or unexplained low oxygen level is managed.

How the Test Works

The setup is straightforward. A technician or nurse places an intravenous line, usually in a vein on your arm. Two syringes are connected by a small three-way valve called a stopcock. One syringe holds at least eight milliliters of preservative-free saline, and the other holds a tiny amount of room air, roughly half to one milliliter. The two syringes are pushed back and forth rapidly, forcing the saline and air to mix into a frothy suspension of microbubbles.¹ That cloudy solution is then injected into the IV while an ultrasound probe captures images of your heart in real time.

On the screen, the microbubbles show up as a bright, swirling cloud. Under normal circumstances, they flood the right atrium and right ventricle, travel into the pulmonary arteries, and get trapped in the tiny capillaries of the lungs before they ever reach the left side of the heart. The lungs act as an extremely effective filter. Animal studies have shown that, under normal conditions, the pulmonary capillary bed catches bubbles smaller than about 22 micrometers in diameter.² Since agitated saline bubbles are larger than typical red blood cells, healthy lungs stop virtually all of them. If any bubbles do appear on the left side of the heart, something is letting them through.

Which arm the IV goes into can matter. In most cases either arm works, but certain anatomical variants, like a persistent left superior vena cava, drain differently, and using a left-arm injection may be important for catching those.³ Your sonographer will typically choose the arm based on what the cardiologist suspects.

What the Bubbles Can Reveal

The key finding the test looks for is a right-to-left shunt, meaning blood is crossing from the right heart chambers to the left through an opening that should not be there, or through abnormal blood vessels in the lungs. The most common culprit is a patent foramen ovale, or PFO. Before birth, every heart has a small flap-like opening between the right and left atria that allows oxygenated blood from the placenta to bypass the lungs. In most people, this flap seals shut after birth. In roughly one in four adults, it never fully closes. Most of those people never know and never have problems, but the opening can become clinically relevant in certain situations.

Timing is what separates one type of shunt from another. If microbubbles appear in the left atrium within about three heartbeats after the right atrium fills with the bright cloud, the shunt is almost certainly inside the heart itself, such as a PFO or an atrial septal defect.⁴ If bubbles show up later, after roughly three to six beats, the shunt is more likely happening through abnormal blood vessels in the lungs, known as pulmonary arteriovenous malformations.⁵ That distinction matters because the two conditions are treated very differently.

The Valsalva Maneuver and Why You’ll Be Asked to Bear Down

During the test, you will almost certainly be asked to perform a Valsalva maneuver. This involves bearing down as though you are straining during a bowel movement, holding that pressure for several seconds, and then releasing it just as the bubbles are injected. The maneuver briefly raises the pressure on the right side of the heart relative to the left, which can push open a PFO flap that would otherwise stay closed at rest.

The difference it makes is substantial. In one study of 65 patients with confirmed PFOs, the bubble test was positive in only about 59% of patients at rest. When the Valsalva maneuver was added, the detection rate jumped to roughly 85%.⁶ The maneuver was especially effective at revealing smaller shunts that would have been completely missed otherwise. This is why cardiologists consider a bubble study incomplete if it was performed only at rest. If you are doing the test and the technician does not ask you to bear down, it is worth asking whether that step was intentionally skipped or simply overlooked.

Grading the Results

A bubble test result is not just “positive” or “negative.” The number of microbubbles that cross over is counted and assigned a grade, which gives the cardiologist a rough sense of the shunt’s size. A widely used scale works like this:

Grade 0: No microbubbles seen on the left side.
Grade I: 1 to 5 microbubbles.
Grade II: 6 to 20 microbubbles.
Grade III: 21 to 50 microbubbles.
Grade IV: More than 50 microbubbles.

Grades I and II are generally considered small shunts, while grades III and IV are classified as large.⁷ The grade can influence treatment decisions. A grade I shunt found incidentally in someone with no symptoms or stroke history may warrant nothing more than monitoring, whereas a large shunt in a younger stroke patient may tip the scale toward closure.

Why Doctors Order a Bubble Study

The most common reason for ordering a bubble test is an unexplained, or “cryptogenic,” stroke, particularly in someone younger than about 60 with no obvious cause like atrial fibrillation or clogged carotid arteries. The theory is that a blood clot forming in the veins, typically in the legs, can travel through a PFO into the arterial circulation and lodge in the brain. This is called paradoxical embolism. Without an opening like a PFO, that clot would have been caught in the lungs.

One large retrospective study that looked at over 700 stroke and transient ischemic attack patients referred for bubble studies found that a PFO was detected in about a third of them. But the study also highlighted an important nuance: only about 31% of those patients with a detected PFO ended up having their clinical management changed as a result.⁸ Finding a PFO does not automatically mean the PFO caused the stroke. Many of those patients had other stroke risk factors, such as atrial fibrillation or severe artery narrowing, that better explained the event. Sorting out whether the PFO is a bystander or the culprit is one of the harder clinical judgments in stroke medicine.

Beyond stroke, bubble studies are also ordered when doctors suspect pulmonary arteriovenous malformations. These abnormal connections between arteries and veins in the lungs can cause low oxygen levels and, in some cases, allow clots or bacteria to reach the brain. Patients with hereditary hemorrhagic telangiectasia, a genetic condition that causes these malformations, are routinely screened with bubble tests. The test is also used during workups for unexplained low oxygen levels in patients with liver disease, where a condition called hepatopulmonary syndrome can create shunting through the lung vasculature.

How Accurate Is the Standard Bubble Study

The bubble test performed through a standard transthoracic echocardiogram, the version where the ultrasound probe sits on your chest, is excellent at confirming a shunt when it finds one but less reliable at ruling one out. Meta-analyses have consistently shown very high specificity but more modest sensitivity. One meta-analysis of prospective studies found a sensitivity of about 46% and a specificity of 99%, meaning the test rarely gives a false positive but misses more than half of shunts that are actually there.⁹ A more recent meta-analysis was somewhat more optimistic, reporting sensitivity around 73% with specificity of 94%.¹⁰ The gap between those estimates likely reflects differences in how the Valsalva maneuver was performed, image quality, and patient body habitus.

This means a positive bubble study on a regular echocardiogram is very reliable. If the sonographer sees bubbles crossing, the shunt is almost certainly real. But a negative result on the chest-wall version does not guarantee there is no shunt. When clinical suspicion remains high, the next step is usually a transesophageal echocardiogram, or TEE, in which a specialized ultrasound probe is passed down the esophagus to sit directly behind the heart. The TEE offers much better image resolution of the atrial septum and is considered the standard reference for confirming PFOs and determining their anatomy before any closure procedure.

Transcranial Doppler as a Screening Alternative

There is another way to do a bubble test that does not involve imaging the heart at all. In a transcranial Doppler bubble study, the agitated saline is injected the same way, but instead of watching the heart, a Doppler probe placed on the temple listens for bubbles arriving in the brain’s blood vessels. If bubbles cross through a shunt and reach the cerebral arteries, they produce characteristic “hits” or “microembolic signals” that the machine counts.

Transcranial Doppler is highly sensitive. A meta-analysis of prospective studies reported a sensitivity of 97% and specificity of 93% for detecting right-to-left shunts, which compares favorably to chest-wall echocardiography.¹¹ One smaller head-to-head comparison found that transcranial Doppler with the bubble test matched transesophageal echo perfectly, with both sensitivity and specificity at 100% in a group of young and middle-aged stroke patients.¹² It is also noninvasive and inexpensive, requiring no sedation or special preparation.¹³

The tradeoff is that transcranial Doppler tells you a shunt exists but not where it is. It cannot distinguish a PFO from a pulmonary shunt, and it gives no information about the anatomy of the defect. For that reason, it works best as a screening step: if the transcranial Doppler bubble test is negative, the chance of a significant shunt is very low and you can often avoid the more invasive TEE. If it is positive, a TEE is still typically needed to characterize what is going on structurally before any intervention.

What Happens After a Positive Bubble Study

A positive bubble study opens a conversation, not an automatic pathway to surgery. The clinical decision depends on why the test was ordered, the size of the shunt, the patient’s age, stroke risk factors, and whether the PFO is judged to be the likely cause of symptoms rather than an innocent bystander.

For younger patients with cryptogenic stroke and a large PFO, transcatheter closure has become an established option. This is a catheter-based procedure in which a small device is threaded through a vein and deployed across the PFO to seal it shut. After closure, patients typically undergo a follow-up bubble study months later to check whether the shunt has resolved. One study of over 550 patients who underwent PFO closure found that about 20% still had some degree of right-to-left shunting at around four months, but that dropped to about 8% by a year out.¹⁴ In most of those persistent cases, the residual shunting was mild and did not require additional intervention. A small number had identifiable sources such as pulmonary arteriovenous malformations or leaks around the device itself, and those were treated separately.

For patients in whom the PFO is likely incidental, such as an older adult with several traditional stroke risk factors, the usual approach is medical management with antiplatelet or anticoagulant medication rather than device closure. The evidence for closure is strongest in younger patients with few competing risk factors and anatomical features that favor paradoxical embolism, such as a large shunt or an associated atrial septal aneurysm.

Bubble Studies in Children and Congenital Heart Disease

Bubble tests are not limited to adults investigating strokes. In children with congenital heart disease, particularly those who have undergone certain surgical repairs, bubble echocardiography plays a distinct role. Children who have had a cavopulmonary anastomosis, a type of surgery for single-ventricle heart defects that reroutes blood flow so that venous blood passes directly to the lungs, can develop pulmonary arteriovenous malformations over time. These shunts let deoxygenated blood slip past the lungs and lower the child’s oxygen levels.

Bubble echocardiography is more sensitive at catching these malformations than conventional imaging. In one study of children after cavopulmonary surgery, bubble contrast echocardiography detected pulmonary shunting in about 71% of patients, while pulmonary angiography, the traditional imaging method, caught it in only about 21%.¹⁵ That is a striking difference and has made the bubble study a standard surveillance tool in this population.

For the more common question of whether a child has a PFO or a small atrial septal defect, particularly after a pediatric stroke, the standard chest-wall echocardiogram with agitated saline is generally effective enough to assess the integrity of the wall between the atria in the majority of children, avoiding the need for sedation and the more invasive transesophageal approach.¹⁶ Children tend to have better ultrasound windows than adults because they have thinner chest walls, which helps image quality considerably.

Is the Test Safe

The agitated saline itself is one of the simplest “contrast agents” in medicine. It is just sterile salt water and a tiny amount of air. The microbubbles dissolve within seconds once they reach the lung capillaries, and the air volume involved is far too small to cause an air embolism under normal circumstances. Patients often feel a brief cool sensation or mild flush when the saline is injected, but serious reactions are extremely rare. There is no radiation exposure, no iodine-based dye, and no need for kidney function tests beforehand.

The main discomfort comes from the IV placement. If the test progresses to a transesophageal echo, the swallowing of the probe is more involved and typically requires light sedation and a numbing spray in the throat, but that is the TEE portion, not the bubble study itself. The chest-wall version with agitated saline is about as low-risk as cardiac testing gets.

One misconception worth clearing up: some patients worry that injecting bubbles into the bloodstream sounds dangerous. The volume of air is extremely small, and under normal circumstances the lungs catch every bubble before it can reach the arterial side. Even in patients who do have a shunt, the number of microbubbles that cross is tiny relative to what would be needed to cause harm. The diagnostic value far outweighs the negligible risk, which is why the test has remained a cornerstone of cardiac imaging for decades.