(Stroke. 1996;27:87-90.)
© 1996 American Heart Association, Inc.
Articles |
Comparison of Transcranial Doppler Ultrasonography and Transesophageal Echocardiography to Monitor Emboli During Coronary Artery Bypass Surgery
From the Departments of Neurology (D.B., P.K., R.T., D.H.), Cardiothoracic Anesthesia (F.Y.), and Cardiothoracic Surgery (J.G.), Cornell University Medical College, New York, NY.
Correspondence to Denise Barbut, MD, MRCP, Starr-607, 520 E 70th St, New York, NY 10021.
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Abstract |
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 Top Abstract IntroductionSubjects and Methods Results Discussion References |
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Background and Purpose Transcranial Doppler ultrasonography (TCD) is the standard technique for monitoring emboli in the cerebral circulation. Embolic signals have been detected with the use of this technique in most patients undergoing coronary artery bypass surgery. We previously reported that the majority of emboli are detected after release of aortic cross-clamps and partial occlusion clamps. In this study we compare the intraoperative use of TCD with transesophageal echocardiography (TEE) to monitor cerebral emboli.
Methods We simultaneously monitored 20 patients undergoing coronary bypass surgery with TCD and TEE. All patients also underwent routine TEE examination of the aorta.
Results Embolic signals were detected in all patients by both techniques. Mean total number of emboli was 535±109 by TEE compared with 133±28 by TCD. We found correlation between numbers of emboli detected by the two techniques at clamp placement and release (r=.65, P=.002). Clamp placement and release accounted for 84% of all emboli by TEE and 83% by TCD. By TEE, large, highly echogenic particles were detected after clamp release compared with small, barely echo-dense particles at the onset of bypass. No such distinction was apparent by TCD. We found correlation between severity of aortic atheroma and both TEE- (P=.003) and TCD-detected (P=.009) emboli.
Conclusions TEE and TCD can both be used to continuously monitor emboli during coronary artery bypass surgery. However, TEE is invasive and justified only if it is being performed for intraoperative assessment of aortic atheromatosis or cardiac function.
Key Words: aorta • bypass surgery • echocardiography • embolism
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Introduction |
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TopAbstractIntroductionSubjects and Methods Results Discussion References |
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Coronary artery bypass surgery is an effective treatment for patients with multivessel and left main coronary artery disease. However, it is complicated by stroke in 4.7% to 5.2% of cases1 2 3 4 and by cognitive impairment in up to 30% of cases.5 6 Cerebral emboli have been detected intraoperatively with the use of transcranial Doppler ultrasonography (TCD),7 8 9 10 and a correlation between numbers of emboli and neuropsychological outcome has been reported.10 11 We previously showed that a majority of emboli occur after release of aortic clamps.12 Embolic signals (ES) have also been detected intraoperatively in the descending aorta with the use of a TCD probe.13
In this study we monitored 20 patients intraoperatively using TCD and transesophageal echocardiography (TEE) with the goal of comparing the two techniques for detection of emboli.
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Subjects and Methods |
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TopAbstractIntroductionSubjects and Methods Results Discussion References |
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Patient Selection
We monitored 20 patients undergoing elective isolated coronary artery bypass using TCD and TEE. The protocol was approved by our institutional review board, and participating patients gave signed informed consent. Standard TEE was performed in all patients.
Anesthesia
Morphine and lorazepam or midazolam served as
premedication, and
thiopental, fentanyl, and pancuronium were used for induction.
Anesthesia before and after bypass was maintained with
additional boluses of fentanyl and midazolam.
Cardiopulmonary Bypass
We used membrane oxygenators in
conjunction with nonpulsatile
centrifugal pumps. A 40-µm Pall filter was incorporated into the
arterial line. Bypass was initiated at flows of 2.4 L/min
per square meter at a body temperature of 37.5°C and reduced to 1.6
L/min per square meter at 28°C. We regulated systemic blood pressure
pharmacologically to maintain mean pressures between 50 and 80 mm Hg.
Surgical staff members were blinded to TCD and TEE findings during the
operative procedure.
Standard TEE
We performed biplanar or monoplanar TEE on all
patients after
induction of general anesthesia and endotracheal intubation
and again at the end of the surgical procedure. A 5-MHz TEE probe
(Acuson) and an Acuson 128 XP system were used. All studies were
recorded on standard VHS videotape and subsequently interpreted. We
assessed the severity of ascending, arch, and descending aortic
atheroma individually and graded each segment as follows:
grade I, normal to mild intimal thickening; grade II, severe intimal
thickening; grade III, atheroma protruding less than 5 mm
into the lumen; grade IV, atheroma protruding 5 mm or more
into the lumen; and grade V, atheroma with a mobile
component.14
Intraoperative TCD Monitoring
We monitored the middle
cerebral artery of patients from aortic
cannulation to bypass discontinuation using a 2-MHz pulsed-wave TCD
probe (Medasonics-CDS) placed on the patient's temple at a depth of
4.5 to 6.0 cm. ES were defined as high-amplitude, unidirectional,
transient signals less than 0.1 second in duration and associated with
a characteristic chirping sound. We recorded the timing of all
major events and numbers of ES occurring within 4 minutes of the
following events: aortic cannulation, inception and termination of
bypass, aortic cross clamping, and aortic cross-clamp release.
Intraoperative TEE Monitoring
After completion of the aortic
examination, we positioned the
probe in transverse plane at the level of the aortic arch, just before
the origins of the left subclavian artery, for continuous
recording from aortic cannulation until 5 to 10 minutes after
termination of bypass. Monitoring was started after aortic cannulation
in 11 patients. Other specific events were occasionally missed because
of "freezing" of the TEE equipment (Table
). We
recorded data on standard VHS videotape for subsequent
analysis by two examiners. ES were defined as echogenic
intraluminal signals not present in the same position on
consecutive frames.
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The recorded images were displayed with the use of the Data Translation DT 2867 frame grabber. Since each particle appears on several consecutive frames, only three frames with large numbers of emboli were selected of 30 frames for each second and numbers of particles in each frame counted. In this way we avoided recounting the same particle more than once. The means of these three frames were obtained for each of the first 5 seconds after the event and were summed. Only the first of each 5-second interval thereafter to a maximum of 240 seconds or cessation of emboli was similarly counted. This mean for the first of each 5-second period was then multiplied by the number of seconds in the interval to arrive at the maximum number for the 240 seconds. Emboli occurring outside these 4-minute time frames were classified as interim signals.
Statistical Analysis
Statistical analyses were performed on a
Gateway P5-90
computer with the use of S-PLUS for Windows. Statistical
comparison between groups and significance of correlations were
performed with the use of ANOVA.
We performed multivariate linear modeling for the clamp-related numbers of emboli by TEE or TCD using the grade of severity of atheroma (as defined above) in the ascending aorta, aortic arch, and descending aorta as predictors. The "best" model was selected on the basis of minimization of Aikaike information criteria and was used to compute the "expected" number of emboli for a given patient and to test correlation with observed values.
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Results |
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TopAbstractIntroductionSubjects and Methods Results Discussion References |
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We detected ES intraoperatively by TCD and TEE in all 20 patients monitored (15 men, 5 women; mean age, 69 years [range, 50 to 78 years]). The mean number of emboli for the entire procedure was 535±109 by TEE compared with only 133±28 for TCD, although counting methods are not directly comparable. The largest number of ES occurred in the 4-minute period after aortic cross-clamp release (903 for TEE, 279 for TCD). Both techniques detected the majority of emboli after seven specific operative events: aortic cross clamp and partial occlusion clamping and release, bypass onset and discontinuation, and aortic cannulation. ES were distributed unevenly across these specific time points (Fig 1
) (two-way ANOVA: TEE,
P=.00014; TCD, P=.000009). Aortic cross clamp
release accounted for 42% and 41% of the total number of emboli for
TEE and TCD, respectively. Partial occlusion clamp release accounted
for another 24% by TEE and 21% by TCD. Combined aortic
cross-clamp and partial occlusion clamp placement and release
accounted for 84% of all ES by TEE and 83% by TCD. Numbers of ES
detected by the two techniques correlated for aortic cross-clamp
placement (r=.66, P=.003) and release
(r=.70, P=.0005), partial occlusion clamp
placement (r=.77, P=.0008) and release
(r=.88,
P=1.86x10-6), and bypass
discontinuation (r=.98,
P=8.4x10-14) (Fig 1).
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The largest numbers of ES per frame were detected during the first 10 seconds after clamp release (mean, 13) and tapered down to 0 to 5 ES per frame by 20 seconds (mean, 1.57). A second smaller flurry was apparent between 150 and 200 seconds.
The difference between signals generated by the onset of bypass and
clamp release was easily and consistently noticeable by TEE.
Intensely echogenic, large particles followed clamp release (Fig
2), whereas those detected at the onset of bypass
were poorly echogenic, less distinct, and smaller. TCD-detected
ES generated by these two events were not distinguishable on the basis
of amplitude or duration.
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Mobile atheromatous plaque (grade V) was present in
four patients, mostly in the descending aorta. Severity of TEE-detected
aortic atheroma correlated with numbers of TEE- and
TCD-detected emboli (Fig 3). Atheroma
severity in all three aortic segments combined significantly predicted
numbers of ES by TCD (r=.63, P=.009) and TEE
(r=.82, P=.003). Severity of atheroma
in the aortic arch alone also had predictive value, particularly for
TCD (r=.57, P=.009).
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Discussion |
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TopAbstractIntroductionSubjects and Methods Results Discussion References |
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TCD is the standard technique for monitoring emboli during coronary artery bypass surgery.7 8 9 10 11 With this technique, emboli have been detected in most patients.7 11 12 We have previously shown that the bulk of embolization occurs after aortic cross clamp and partial occlusion clamp release, which suggests an association with the severity of aortic atheroma.12 The distribution of emboli in relation to specific operative events is very similar with TEE monitoring. In accordance with previous results,12 aortic cross clamp and partial occlusion clamp release accounted for 66% of TEE-detected and 62% of TCD-detected emboli. The numbers of emboli detected by TEE were substantially higher than with TCD, reflecting the fact that only a fraction of the aortic emboli enter the cerebral circulation. Rates of embolization on a second-by-second basis after clamp release are better delineated with TEE monitoring. Analysis of sequential frames reveals that the vast majority of emboli occur within the first 20 seconds after clamp release.
The difference between the large, intense emboli after clamp release and the small, indistinct particles seen at onset of bypass is quite striking with TEE. The significance of this difference in terms of pathological material is unclear. In vitro, the size of injected particles has been shown to correlate with TCD-detected signal amplitude and duration.15 Neither TCD nor TEE, however, can differentiate between particles of differing constitution in vivo. The flurry of whirling intra-aortic echo densities seen on TEE at the onset of bypass has no consistent counterpart at the level of the middle cerebral artery by TCD. It is our impression that they consist of gaseous microbubbles formed by turbulent flow, most of which are too small to detect by the time they reach the middle cerebral artery, or they are merely of variable echogenicity caused by mixing crystalloid priming solution and blood. The highly echo-dense particles detected by TEE after clamp release are accompanied by substantial numbers of high-amplitude ES on TCD. Preliminary results suggest that numbers of ES correlate with severity of aortic atheroma.16 17 This has led us to believe that these signals mostly represent atheromatous debris rather than gaseous microbubbles trapped behind the clamps. In this study we found a strong association between numbers of emboli and severity of aortic disease by both TEE and TCD. Taken individually, only the aortic arch significantly predicted numbers of emboli. The biologically most plausible association would be correlation with ascending segment disease and proximal aortic arch, since emboli originating from the distal arch or descending segment cannot enter the cerebral circulation. Unfortunately, the ascending segment and proximal arch are the least well visualized parts of the aorta by TEE, even with a biplanar probe. Atheromatous plaque in this segment is frequently missed unless the epiaortic probe is used.18 Marschall et al19 has shown a compelling association between aortic arch atheroma and stroke after bypass surgery; emboli monitoring was not performed. Much larger patient numbers are needed to establish the exact relationship between aortic atheroma, numbers of emboli, and more importantly, neurological and cognitive outcome.
TEE is increasingly being used during coronary artery bypass surgery to assess the severity of aortic atheroma. Positioning the probe at the level of the aortic arch to monitor intraluminal emboli after completion of the aortic examination is technically quite simple. The technique enables more detailed analysis of embolization after specific operative events and may yield more information regarding size and constitution of emboli. Unlike TCD, TEE is an invasive procedure, and its use cannot be justified unless examinations of the heart and aorta are being performed. The TEE equipment is considerably more expensive and bulkier than TCD, and the review of tapes is labor intensive. A fully automated emboli counting system would have to be developed to make it a useful adjunct to standard intraoperative TEE.
Received June 16, 1995; revision received September 21, 1995; accepted October 4, 1995.
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References |
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C. F. Bladin, L. Bingham, L. Grigg, A. G. Yapanis, R. Gerraty, and S. M. Davis Transcranial Doppler Detection of Microemboli During Percutaneous Transluminal Coronary Angioplasty Stroke, November 1, 1998; 29(11): 2367 - 2370. [Abstract] [Full Text] [PDF]
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H. Miyano, M. Inagaki, N. Hashimoto, T. Shishido, T. Kawada, Y. Miyake, and K. Sunagawa Regional cerebral blood flow during rewarming of cardiopulmonary bypass correlates with posthypothermic regional glucose use J. Thorac. Cardiovasc. Surg., September 1, 1998; 116(3): 503 - 507. [Abstract] [Full Text]
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D. Barbut, D. Grassineau, E. Lis, L. Heier, G. S. Hartman, and O. W. Isom Posterior Distribution of Infarcts in Strokes Related to Cardiac Operations Ann. Thorac. Surg., June 1, 1998; 65(6): 1656 - 1659. [Abstract] [Full Text] [PDF]
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R. Vanninen, M. Aikia, M. Kononen, K. Partanen, H. Tulla, P. Hartikainen, J. Partanen, H. Manninen, P. Enberg, and M. Hippelainen Subclinical Cerebral Complications After Coronary Artery Bypass Grafting: Prospective Analysis With Magnetic Resonance Imaging, Quantitative Electroencephalography, and Neuropsychological Assessment Arch Neurol, May 1, 1998; 55(5): 618 - 627. [Abstract] [Full Text] [PDF]
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A. Jacobs, M. Neveling, M. Horst, M. Ghaemi, J. Kessler, H. Eichstaedt, J. Rudolf, P. Model, H. Bonner, E. R. de Vivie, et al. Alterations of Neuropsychological Function and Cerebral Glucose Metabolism After Cardiac Surgery Are Not Related Only to Intraoperative Microembolic Events Stroke, March 1, 1998; 29(3): 660 - 667. [Abstract] [Full Text] [PDF]
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G. S. Aldea, P. O'Gara, O. M. Shapira, P. Treanor, A. Osman, E. Patalis, C. Arkin, R. Diamond, V. Babikian, H. L. Lazar, et al. Effect of Anticoagulation Protocol on Outcome in Patients Undergoing CABG With Heparin-Bonded Cardiopulmonary Bypass Circuits Ann. Thorac. Surg., February 1, 1998; 65(2): 425 - 433. [Abstract] [Full Text] [PDF]
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 M A DE BELDER Ultrasonic imaging of aortic atheroma Heart, February 1, 1998; 79(2): 111 - 112. [Full Text]
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E. J. Heyer, D. C. Adams, E. Delphin, D. J. McMahon, S. D. Steneck, M. C. Oz, R. E. Michler, and E. A. Rose CEREBRAL DYSFUNCTION AFTER CORONARY ARTERY BYPASS GRAFTING DONE WITH MILD OR MODERATE HYPOTHERMIA J. Thorac. Cardiovasc. Surg., August 1, 1997; 114(2): 270 - 277. [Abstract] [Full Text]
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D. Barbut, F.-S. F. Yao, Y.-W. Lo, R. Silverman, D. N. Hager, R. R. Trifiletti, and J. P. Gold Determination of Size of Aortic Emboli and Embolic Load During Coronary Artery Bypass Grafting Ann. Thorac. Surg., May 1, 1997; 63(5): 1262 - 1267. [Abstract] [Full Text]
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