(Stroke. 1999;30:863-866.)
© 1999 American Heart Association, Inc.
Original Contributions |
From the Department of Neurology, Kanazawa University (E.F., M.T.), Kanazawa; Department of Cardiovascular Surgery, Niigata University (K.H., H.O.), Niigata; and the Department of Neurology, National Saigata Hospital (T.N., N.F.), Oogata, Japan.
Correspondence and reprints requests to Eisuke Furui, Department of Neurology, Kanazawa University, 13-1, Takaramachi, Kanazawa, 920-8641 Japan. E-mail efurui{at}med.kanazawa-u.ac.jp
| Abstract |
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MethodsIn an animal study, adult pigs with venoarterial extracorporeal membrane oxygenation and minimal anticoagulation therapy were used as a model for cerebral embolism. After performing TCD monitoring with a multigated approach, we did an offline analysis to investigate several parameters concerning MES and TS. We also examined TS in patients in a clinical study.
ResultsFrom a total of 362 MES investigated in the animal study, 72.9% were followed by TS. We could not find any TS associated with artifacts. The time delay between TS and MES was negatively correlated with the velocity of MES. MES almost always appeared first in the proximal channel, whereas TS conversely appeared first in the distal channel. In the clinical study, we were also able to observe TS associated with MES.
ConclusionsTS may represent emboli passing down a branch vessel or twisted downstream vessel. TS are specific for MES and can be used as another criterion for MES identification.
Key Words: cerebral embolism diagnostic imaging ultrasonography, Doppler
| Introduction |
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change associated with MES in both animal and clinical studies.
| Subjects and Methods |
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In an animal study, 2 adult pigs weighing 24 to 26 kg, treated with
venoarterial extracorporeal membrane
oxygenation (VA ECMO) and minimal anticoagulation
therapy, were used as a model for cerebral embolism. During the
experiment, the animals were anesthetized with 2.0% to 3.0%
halothane and treated following Niigata University Animal Care
Guidelines. Following a left thoracotomy, VA ECMO was performed by
aortic and pulmonary cannulation with use of a roller pump and
membrane oxygenator. Heparin was administered once before the
cannulation. After the bypass was started, numerous MES were
recorded, and anticoagulation therapy was added only when the
frequency of MES increased excessively. A major branch of the carotid
artery, which displayed flow directed toward the probe, was insonated
through the left transorbital route at a depth of
50 mm. Two
sample volumes of 5 mm were set at a distance of 5 mm. The
probe was fixed with an external device. During TCD monitoring, the
automatic embolic detection system in this Doppler machine was able
to detect and save any relevant segments, which we reviewed later. The
criteria for MES identification were as follows: (1) short duration
(<100 ms), (2) unidirectional signal, (3) intensity increase at
least 7 dB above the background, (4) random appearance in the cardiac
cycle, and (5) sufficient time delay between the 2 channels. We
performed an offline analysis of the recorded MES that met
the criteria to investigate the prevalence of the TS, the time delay
between TS and MES, the relative dB level, and the velocity of MES
in the proximal channel. TS were counted only when unequivocal reversal
signals were recognized after MES on the high time resolution spectral
display in both channels. To calculate these parameters,
MES and TS were defined as the point of Doppler frequency shift at
the maximum amplitude. In addition, 100 episodes of artifact were
produced by lightly tapping skin around the probe before the
cannulation. We could not determine the beginnings of MES and TS on the
raw Doppler time displays unless both of them were clear. The time
delays of both MES and TS between 2 channels were calculated on the raw
Doppler time displays when possible.
In a clinical study, we analyzed patients with a mechanical prosthetic cardiac valve or ischemic stroke and performed TCD monitoring from 1 middle cerebral artery for more than 30 minutes using the universal technique. Informed consent was obtained from all patients before the study. The sample volumes setting and the MES identification criteria were same as in the animal study. We performed an offline analysis of the recorded MES to investigate the prevalence of the TS.
For statistical analysis, the nonparametric Mann-Whitney U test was used to compare different groups. Significance was declared at the P<0.05 level.
| Results |
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In the clinical study, we were also able to observe TS associated with
MES (Figure 1A
). Among a total of 160 MES investigated, 16 (10.0%)
were followed by TS; 6 of the patients (42.9%) presented with
TS (Table
). TS in the clinical study also
had the same characteristics as in the animal experiment: (1) TS
appeared first in the distal channel, then in the proximal one and (2)
the intensity of TS was lower than that of the corresponding
MES.
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To confirm that TS were not artifacts specific to this Doppler
machine, we examined TS in the animal and clinical studies with another
Doppler machine (TC 2020, Nicolet/EME). We were also able to
record TS associated with MES using that machine (Figure 1C
).
| Discussion |
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What accounts for TS? TS tended to follow larger and faster MES,
so the turbulence flow after the embolus may be one explanation for TS.
If an embolus becomes larger and flows faster, the turbulence flow
after the embolus may become larger and easier to detect. However, our
results with the multigate approach do not support this hypothesis. If
this were true, TS might have appeared sequentially in both channels,
first in the proximal then in the distal one, just as MES did. The main
points of our results are summarized as follows: (1) the intensity of
TS was lower than that of the corresponding MES, (2) TS appeared as a
reversal signal first in the distal channel and then in the proximal
one, and (3) the time delay between MES and TS negatively correlated
with the velocity of MES. According to these points, TS seem to have
been recorded farther away from the core of the sample volume than
MES were. TS may be estimated to represent emboli passing down
a branch vessel or twisted downstream vessel, which has a direction of
flow away from the probe (Figure 3
). Some
pairs of MES and TS, recorded only in the proximal channel (Figure 4
), may represent emboli that
pass down branch vessels before they are detected in the distal
channel. The presence of these pairs supports the hypothesis that TS
represent emboli passing down a branch vessel or twisted
downstream vessel. The time delay between MES and TS depended on the
velocity of MES. The faster the MES flowed, the earlier the TS appeared
with a shorter time delay. The detection of TS is highly dependent on
the spatial arrangement of the vessels and the sample volumes. The
smaller size of a pig's brain may explain the higher prevalence of TS
in the animal study than in the clinical study. Not the prevalence but
the presence of TS has important implications for clinical
practice.
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We could find only one report8 dealing with postembolic signals. We thought that this sign was not unique to our study, because we found postembolic signals resembling TS in the TCD manufacturer's brochure or a literature concerning emboli detection.9 Ries and colleges8 reported that specific changes in Doppler spectral patterns could be identified after all embolic signals caused by solid particles in a phantom model by use of a high-resolution analysis of Doppler raw data. These postembolic spectral patters were always characterized by a Doppler frequency shift decreasing in time and resembling the Greek letter lambda. According to them, in 60% of all signs the velocity of the signal finally passed zero to a reverse flow direction, like TS. They assume that lambda signs can be explained by Doppler reflection phenomena caused by postembolic flow disturbances or by technical factors concerning beam geometry. Lambda signs and TS have some similarities on the spectral display. However, we suppose that the origin of lambda signs is basically different from that of TS for the following reasons: (1) lambda signs were recorded in a phantom model using a straight polyethylene tube without branch or twisted segments, contrasting with our study, and (2) lambda signs could not be found in cases of embolism by small air bubbles, whereas TS were recorded associated with MES from a patient with a patent foramen ovale during the contrast medium injection.
We conclude that the TS are characteristic of the changes in Doppler spectra associated with MES and that the presence of TS can be a useful criterion for MES identification.
| Acknowledgments |
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Received August 31, 1998; revision received December 3, 1998; accepted December 3, 1998.
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Ries F, Teimann K, Pohl C, Bauer C, Mundo M, Becher H.
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9. Ackerstaff RGA, Jansen C, Moll FL. Carotid endarterectomy and intraoperative emboli detection: correlation of clinical, transcranial Doppler, and magnetic resonance findings. Echocardiography. 1996;13:543550.[Medline] [Order article via Infotrieve]
Guest Editors Non-Invasive Laboratory Neurological Institute ColumbiaPresbyterian Medical Center, New York, NY
| Introduction |
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Whether artifact or not, the attempt here provides another target for validation by future investigators, possibly leading us closer to a reliable definition of particulate matter embolism.
Ultrasonographers continue to be unclear on what constitutes the material(s) that make up the MES found in a variety of settings, which span the gamut from harmless microcavitations generated by artificial valves at one end to particulate matter emboli from the heart and great vessels at the other. That some MES are associated with high-grade stenoses of brainbrain vessels is well established, but that the MES are harbingers of potentially serious brain-bound emboli is not. Experiences vary widely as to whether any of a number of treatments, from platelet antiaggregants to anticoagulation, alter the frequencies of MES, although most centers fail to document the MES after successful endarterectomy (or, in some cases, angioplasty and stenting) of brain-bound vessels. But what, exactly, is the range of materials is still not clear. Animal models seem a good source for such studies.
Received August 31, 1998; revision received December 3, 1998; accepted December 3, 1998.
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