TCD Detection of Right-to-Left Shunt (Bubble Test) in PFO: Spencer Scale and International Consensus
Physiological Basis of the Method
In the presence of a right-to-left shunt, bubbles bypass the pulmonary filter and appear in cerebral arteries, where they are recorded as microembolic signals (MES). This is the essence of the bubble test (c-TCD). The method does not anatomically localize the shunt and does not differentiate PFO from other cardiac right-to-left shunts; the scales do not predict the size of the PFO but assess conductance — the ability of the opening to transmit material from the venous system to the brain, which depends on the size of the opening and the pressure gradient.
Methodology
Contrast: ~9 ml of saline is agitated between two syringes through a three-way stopcock (a drop of blood/air can be added for bubble stability), galactose-based contrasts are also used. It is injected into the antecubital vein. According to the international consensus, bilateral monitoring of both MCAs (or MCA and basilar artery) with phased-array 2 MHz probes is preferred for increased sensitivity. The protocol includes testing at rest and during the Valsalva maneuver, with an observation window >30 s (according to different protocols ~40 s), with an assessment of the time of signal appearance.
Grading Systems by Number of MES
| System | Grade |
|---|---|
| International Consensus | Categories 1–4: 0 / 1–10 / 11–25 / >25 or "curtain" |
| Spencer Logarithmic Scale | 0–V |
The report specifies the system used and the highest grade achieved.
Features of Bilateral Monitoring
In bilateral monitoring, the threshold between small and medium shunts is shifted (approximately from 10 to ~20 MES), as signals are counted from both sides — this is noted in the report.
Permanent and Latent Shunt
A permanent shunt — MES appear both at rest and during Valsalva; a latent shunt — only during Valsalva.
Structure of the Conclusion
The conclusion of the bubble test includes: presence of a shunt; grading (system + highest value); type (permanent/latent); approximate level; recommendation for TEE. A positive test leads to echocardiography, not to a final diagnosis of the defect type, as c-TCD does not anatomically localize the shunt.
Frequently asked questions
What are the two grading systems for right-to-left shunt used in c-TCD?
International consensus (categories 1–4: 0 / 1–10 / 11–25 / >25 or "curtain") and Spencer logarithmic scale (0–V). The report specifies the system used and the highest grade achieved.
How does a permanent shunt differ from a latent one?
A permanent shunt — MES appear both at rest and during Valsalva; a latent shunt — only during the Valsalva maneuver.
How does the threshold change in bilateral monitoring?
The threshold between small and medium shunts is shifted approximately from 10 to ~20 MES, as signals are counted from both sides; this is noted in the report.
Does c-TCD allow determination of PFO size?
No. The scales do not predict the size of the PFO but assess the conductance of the opening, which depends on the size and pressure gradient. The method also does not anatomically localize the shunt.
What follows a positive bubble test?
Transesophageal echocardiography (TEE) is recommended: a positive test leads to echocardiography, not to a final diagnosis of the defect type.