Paradoxical Emboli Study - Transcranial Doppler (PES)

What is Paradoxical Emboli Study (PES)?

PES is a specialised Transcranial Doppler (TCD) ultrasound test. It is used to check whether injected tiny bubbles can pass from the body’s venous circulation to the arterial circulation and travel toward the brain.  This test investigates whether small clots could travel in the same way. 

A positive test suggests there could be a connection, or “shunt,” between the body’s venous circulation and arterial circulation. This connection can occur in the heart or, less commonly, in the lungs. The most common cause is a patent foramen ovale (PFO), a small opening between the upper chambers of the heart that did not fully close after birth.

A PFO occurs in about 20–25%¹ of the general population, and most people are not aware they have it.

¹Koutroulou I, et al. Epidemiology of Patent Foramen Ovale in General Population and Stroke Patients. Front Neurol. 2020.

Your doctor may recommend this test to:

• Investigate the possible cause of a stroke or transient ischemic attack (TIA)

How should I prepare for PES?

• Keep well hydrated before the test
• If you have a fear of needles or do not wish to have an intravenous (IV) line inserted, please inform your doctor prior to the test date.
• Not required to fast
• Continue with your regular diet and medications
• If you feel unwell, please inform the lab prior to the test

How is PES performed?

The test is done by a doctor and a sonographer and takes approximately 45 minutes. The sonographer will check if the test can be performed (refer to below section “Are there any limitations to the test?”). 

• The test is performed while you are lying on a bed.
• A clear, water-soluble gel is applied to two small ultrasound probes to help transmit sound waves.
• The probes are placed at your temples and secured with a soft, padded headframe.
• You may hear a continuous “whooshing” sound, which represents blood flow in your brain arteries.
• An intravenous (IV) line is inserted into a vein (near your elbow) in your right arm.
• Saline solution containing tiny bubbles (9ml Sodium Chloride with 1ml air) is injected through the IV line. You may feel slight discomfort during the injection, but it should not be painful. Please inform the doctor or sonographer if you experience any pain.
• The injection is repeated while you perform Valsalva breathing exercises. This helps simulate everyday situations that temporarily increase pressure in the chest, such as lifting heavy objects or exercising at the gym.
• To ensure accurate results, please avoid talking or moving during the test unless instructed.
• The probes detect any tiny bubbles that reaches your brain arteries.
• Once the test is complete, the headframe and IV line are removed, and the gel is cleaned off.
• A report will be sent to your doctor and he/she will discuss the results with you at your next appointment.

Frequently Asked Questions about PES

Is the test safe?

• The amount of air used in the injection is very small (1 mL), adhering to the international consensus protocol.^2,3
• Some patients may experience pain at the area where the IV is inserted, similar to a blood draw. 

²Jauss M, Zanette E. Detection of right-to-left shunt with ultrasound contrast agent and transcranial Doppler sonography. Cerebrovasc Dis. 2000;10:490–496.

³Tsivgoulis, Georgios, et al. Safety of Transcranial Doppler ‘Bubble Study’ for Identification of Right-to-Left Shunts: An International Multicentre Study. Journal of Neurology, Neurosurgery & Psychiatry, vol. 82, no. 11, 2011, pp. 1206–1208. https://doi.org/10.1136/jnnp.2010.219733

If you have any questions or concerns, please contact your designated neurodiagnostic laboratory or healthcare team.

Are there any limitations to the test?

In some patients, the ultrasound signal cannot pass effectively through the skull at the temple area due to thicker bone (this is known as a limited or absent temporal window). When this occurs, it may not be possible to monitor the brain arteries, and the test cannot be done. 

This is more common in older adults and may occur more frequently in Asian women due to natural variations in skull thickness. If this occurs, your doctor will advise on the next appropriate steps. In such cases, there will be no charge for the Paradoxical Emboli Study test.

Can I use Medisave to pay for this test?

This test may be claimable under:

• Flexi-MediSave for Seniors; or
• Outpatient scans for diagnosis or treatment of a medical condition. 

Please refer to the Ministry of Health's Outpatient Care page for the latest claim limits and eligibility criteria.

The information is correct as of July 2026 and subject to revisions without notice.
Content adapted from NNI Neurodiagnostic Laboratory (NDL), NNI@TTSH.