Right to Left Shunts and Emboli Detection

Ultrasound can detect, quantify, and localize embolization in real time, and this methodology has been utilized in cerebral vessels with TCD (48-51). Detection of emboli with TCD is based on the definition of microembolic signals (MES) provided by the International Cerebral Hemodynamics Society Consensus Statement (52). MES (Fig. 3) have the following characteristics on spectral Doppler analysis: (i) random occurrence during the cardiac cycle, (ii) brief duration (usually < 0.1 second), (iii) high intensity (> 3 dB over background), (iv) primarily unidirectional signals (if fast Fourier transformation is used), and (v) audible component (chirp, pop).

The power motion-mode Doppler [power-M-mode (PMD)] adds extra dimensions to the process of emboli detection. It shows tracks of emboli in time and space and provides simultaneous, real-time assessment of emboli passing through different vessels, thereby increasing the yield of emboli detection with a single transducer (53 ).

Practically all MES that are detected by TCD are asymptomatic, because the size of the particles that produce them is usually comparable to the diameter of brain capillaries or even smaller (54). MES have been associated with velocity changes in the MCA affected by a significant stenosis (47), which suggests that some artery-to-artery emboli may be comparable in size to the residual lumen or that, when they detach from a stenosis, the residual lumen is increasing, as the velocity decreases. Frequent emboli on TCD in acute stroke correlate with crescendo TIAs (55) and reflect the process of thrombus dissolution (56 ).

Air microbubbles produce strong ultrasound echoes and appear as MES during testing for right-to-left cardiac shunts. A bedside TCD test with agitated saline can detect these shunts,

Figure 2 High-grade (>80%) distal M1 MCA stenosis. Abbreviations. MCA, middle cerebral artery; MFV, mean flow velocity. Source. From pnaim.ll ml mr a mp. i.', .m..\ dmüi \m/|ir..\ m2 mca mi a' tm.\ Ref. 40.

Figure 3 Microembolic signals on TCD. (Upper left panel). Single-gate Doppler spectral presentation of microembolic signals (see International Consensus definition). (Upper middle panel). Air microbubble appearance on echocardiography B-mode image. (Upper right pane). Acute stroke in a 16-year-old girl with patent foramen ovale (PFO) and history of smoking and birth control pill use. (Lowerpanel). Power M-mode (PMD)-TCD "curtain" appearance of air microbubbles with functional PFO. Source: courtesy of Dr. Zsolt Garami is at the University of Texas-Houston Medical School.

Figure 3 Microembolic signals on TCD. (Upper left panel). Single-gate Doppler spectral presentation of microembolic signals (see International Consensus definition). (Upper middle panel). Air microbubble appearance on echocardiography B-mode image. (Upper right pane). Acute stroke in a 16-year-old girl with patent foramen ovale (PFO) and history of smoking and birth control pill use. (Lowerpanel). Power M-mode (PMD)-TCD "curtain" appearance of air microbubbles with functional PFO. Source: courtesy of Dr. Zsolt Garami is at the University of Texas-Houston Medical School.

and it is essential in patients with acute stroke or TIA thought to be caused by paradoxical embolization (57,58). Although TCD cannot localize the shunt [i.e., patent foramen ovale (PFO) or atrial septal defect], it provides complimentary information to transesophageal echocardiography. For instance, "bubble" testing with TCD can be done in a matter of minutes at bedside with minimal or no discomfort to the patient. TCD can offer results of shunt detection with accuracy equal or superior to that of echocardiography for the detection of functional PFO (59) and may detect shunts, even when transesophageal echocardiography is negative, i.e., in the case of pulmonary arteriovenous malformation or inability of the patient to perform Valsalva maneuver during echocardiography (57,58 ).

To optimize TCD performance for right-to-left shunt, the following protocol should be followed (57-59):

  1. Patient is in supine position; 18-gauge needle is inserted into the cubital vein.
  2. Three-way stopcock connector with 2 10-mL syringes is connected to intravenous (IV) access.
  3. 9 mL isotonic saline is forcefully mixed with 1 cc of air.
  4. Less than 1 mL of patient blood can be suctioned into syringe for better bubble formation with agitation.
  5. At least 1 MCA is monitored with TCD.
  6. The first bolus injection of agitated saline is made with the patient breathing normally.
  7. The second bolus injection of similarly prepared, agitated saline is made with a 10-second Valsalva maneuver initiated 5 seconds after the beginning of saline injection.
  8. If negative, TCD monitoring is extended up to 1 minute in order to detect potentially late-arriving bubbles, suggesting a pulmonary shunt.

A 4 -level categorization is proposed by the criteria of the International Cerebral Hemo-dynamics Society (58 ).

  1. No MES were detected (negative "bubble"-test).
  2. 1-10 MES detected (positive "bubble"-test).
  3. >10 MES detected with no curtain.
  4. Curtain (test indicates the presence of a large and functional shunt).

Spencer introduced a new shunt grading system on PMD-TCD, which that takes into account exponential increases in the function of the shunt, and correlated this system with the "platinum" standard of intracardiac, catheter-based ultrasound diagnosis of PFO (59 ). Prospective validation of these diagnostic criteria at other centers is necessary. Nevertheless, the report should comment on whether MES or bubbles were detected at rest or were provoked by the Valsalva maneuver. If few single bubbles were detected at rest and a curtain appeared with Valsalva, this also should be reflected in the report.

Clinical interest in shunt testing with TCD is increasing, as several stroke prevention strategies are being tested, including catheter-based closure procedures. We routinely perform bedside screening with the "bubble" test TCD in patients who have suffered acute stroke or TIA and have a suspected paradoxical embolism, i.e., young patients or those who developed symptoms after prolonged immobility, etc.

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