What is a Bubble Study (or a PFO Test)?
PFO is a hole between the left and right atriums of the heart, which creates a right to left shunt for blood flow. This hole typically exists before birth, but it most often closes shortly after birth. If the hole fails to close naturally after birth, it is referred to as a Patent Foramen Ovale (PFO).
Embolism through a PFO can be a significant cause of stroke in young adults. Transcranial Doppler is one of the preferred modalities for quick and non-invasive identification and assessment of PFO.
The PFO test with TCD is often called a “bubble test” or “bubble study”, because it involves the intravenous injection of micro-bubbles (air mixed saline). Along with the injection, the Doppler signal is recorded with a TCD system, most often in the Middle Cerebral artery (MCA).
In the event of an existing Patent Foramen Ovale, the air bubbles that are actually microemboli pass from the right heart to the left side through the open shunt. Therefore, the blood which is pumped out of the heart includes a mix of these air microemboli and reaches the cerebral circulation.
How to use TCD for the PFO (Bubbles Test)
During bubble study, the spectral Doppler easily detects air bubbles that pass through the cerebral arterial circulation (such as the MCA). Such air bubbles have the characteristics of High-Intensity Transient Signals (HITS) with distinct increased signal energy.
To help direct the blood towards the cerebral circulation and maximize the effect of the bubble test, the patient is asked to perform the Valsalva maneuver during the test. The total emboli counted during the test is a clear indication of whether PFO exists and, if it exists, how severe it is.
Using Dolphin for PFO Test
The Dolphin TCD Equipment has a dedicated PFO protocol designed to complete the test rapidly per all clinical guidelines. The PFO protocol supports both a unilateral examination or a more complete assessment, a bilateral examination using the Dolphin/XF Robotic Headframe.
For improved HITS identification, the Dolphin supports a Power M Mode display, which helps visualize the high-intensity bubbles that pass through the target vessel. More importantly, the Dolphin allows measuring the total HITS count not only from the main selected depth but also all of the HITS identified at all depths along the ultrasound beam. This unique feature allows measuring the suspected emboli counts in a single measurement, for example, in the MCA and ACA arteries.
Frequently, the examination calls for performing several measurement sessions, such as with/without the Valsalva maneuver, which can take place before or after the bubble injection. The Dolphin PFO protocol allows performing multiple examination sessions in a single continuous measurement. Each session is given its own name, with automatic cursors placed to mark the different stages of the examination. A separate automatic HITS count is performed for each session, as well as a series of additional clinical information that is provided for optimal PFO diagnosis.
The Dolphin HITS analysis has an unmatched temporal resolution, which allows for digging deep and counting HITS during the embolic shower. A sophisticated HITS analysis program allows a detailed investigation of each suspected embolic event, which includes the energy and motion in the artery of each event, a high-resolution embolic m-mode display, and more.
Expected Results of a Bubble Study
There are several grading criteria to characterize the PFO or the right-to-left shunt, based on the count of the Micro-Embolic Signals (MES).
The criteria by the ICC (International Consensus Criteria) is as follows:
|3||> 30, with embolic shower|
Another grading scale is referred to as Spencer’s Logarithmic Scale:
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