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J Thorac Cardiovasc Surg 2003;126:1638-1639
© 2003 The American Association for Thoracic Surgery
Brief communications |
a Department of Neurology, Martin-Luther University of Halle-Wittenberg, Halle/Saale, Germany,
b Department of Neurology, University of Zürich, Zurich, Switzerland,
c Department of Cardio-Thoracic Surgery, University of Basel, Basel, Switzerland.
Received for publication November 7, 2002; revisions received February 10, 2003; revisions received April 15, 2003; accepted for publication April 21, 2003.
* Address for reprints: Dimitrios Georgiadis, MD, Department of Neurology, University of Zürich, Frauenklinikstrasse 26, CH-8091 Zürich, Switzerland
Dimitrios.Georgiadis{at}usz.ch
Several studies have demonstrated that the counts of Doppler microembolic signals (MESs) detected during cardiac surgery are related to (1) outcome and length of hospital stay,1 (2) degree of postoperative neuropsychologic deficit,2 and (3) incidence of new ischemic lesions on magnetic resonance imaging of the brain.3 Extracorporeal oxygenation causes varying amounts of air to be diluted in the blood; this air is only partially removed by the arterial filters that are routinely used. We undertook this study to evaluate (1) the percentage of MES reduction caused by the arterial filter and (2) the proportion of MESs actually reaching the brain, by comparing the MES counts detected before the arterial filter, after the arterial filer and in both middle cerebral arteries (MCAs).
Patients and methods
Eleven patients, 7 men and 4 women aged 59 ± 18 years, were monitored during elective cardiac surgery (coronary artery bypass grafting n = 9, valve replacement n = 2). Anesthesia was induced by fentanyl and midazolam and maintained with fentanyl, midazolam, and isoflurane. Pancuronium was used for neuromuscular blockade. The pH was maintained by the alpha-stat strategy. Surgery was performed under normothermic bypass (35°C-37°C). Blood aspirated from the surgical field was subsequently reinfused to the patient. A nonpulsatile blood flow pump (Stöckert S 3; Stöckert Instrumente GmbH, Munich, Germany) and two membrane oxygenators (COBE CML Duo; COBE Cardiovascular, Inc, Arvada, Colo; DIDECO D 703 Compactflo; Dideco SpA, Mirandola, Italy) with tubing systems that included an arterial line filter (40 µm; COBE) were used in all patients.
Both MCAs were continuously insonated at a depth of 52 to 58 mm for the complete duration of cardiopulmonary bypass (from skin incision until the end of the reperfusion period) in the presence of an experienced observer with two fixed 2-MHz transducers of a pulsed-wave transcranial Doppler machine (Multi-Dop X-4; DWL, Singen, Germany). Additionally, the arterial line of the oxygenator was continuously monitored with two further 2-MHz transducers with a sample volume of 10 mm. These were inserted in a tubing device, assuring their stable placement and a constant distance between the transducer and the arterial line (5 cm). The space between the transducer and the tubing was filled with ultrasonic gel. Exact details of this monitoring procedure have been published elsewhere.4 All ultrasonographic data were recorded on digital audio tapes with an 8-channel digital audio recorder (TASCAM DA 88; TEAC Deutschland GmbH, Wiesbaden, Germany) for later reevaluation.
MESs were detected according to the following criteria: intensity increase at least 6 dB above background, characteristic acoustic qualities, and unidirectionality of signal. Evaluation of MESs was only performed during extracorporeal circulation. It was initiated after cannulation and clamping of the ascending aorta and terminated shortly before the aortic clamp was removed.
Nonnormally distributed data are expressed as medians with 95% confidence intervals and compared with the Mann-Whitney U test.
Results
The exact MES counts detected in each patient are listed in Table 1. The arterial filter resulted in a 58.9% reduction of MESs (59132 of 100394). Only 4.4% (2624 of 59132) of the MESs detected after the arterial filter were actually detected in the MCAs. No significant differences in MES counts were observed when comparing the results of the right MCA with those of the left MCA (P = .95, Mann-Whitney test; Table 1).
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Our study produced two major findings: (1) the arterial filter resulted in a 60% reduction of MESs and (2) only approximately 4% of MESs that crossed the arterial filter finally reached the MCAs. The proportion of MESs detected in the MCAs corresponds to the total cerebral perfusion under cardiopulmonary bypass, as this is estimated as 5% to 10% of the total perfusion volume. Furthermore, the mean number of MES detected in the MCAs in this study (163, 95% confidence interval 52-452) is comparable with the data reported by Barbut and colleagues5 (mean MES counts of 133 ± 28) during unilateral TCD monitoring over the MCA of 20 patients undergoing elective cardiac surgery.
Obviously, a main limitation of this study is our inability to distinguish between formed and gaseous embolic material. Unfortunately, such distinction was not feasible as our study commenced. A further study limitation was the low number of examined patients. It must be pointed out though, that a total of 162,150 MESs were analyzed. Because the purpose of this study was the evaluation of MESs associated with cardiopulmonary bypass, we refrained from further evaluating MESs detected during other operation stages. Our study thus can provide no information on the relation between MES counts and various stages of cardiac surgery or surgical manipulations, because the MESs detected in this study were not caused by surgical interventions but solely by the oxygenator.
References
This article has been cited by other articles:
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  R. A. Rodriguez, F. Rubens, D. Belway, and H. J. Nathan Residual air in the venous cannula increases cerebral embolization at the onset of cardiopulmonary bypass Eur. J. Cardiothorac. Surg., February 1, 2006; 29(2): 175 - 180. [Abstract] [Full Text] [PDF]
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 R. A Rodriguez, K. A Williams, A. Babaev, F. Rubens, and H. J Nathan Effect of perfusionist technique on cerebral embolization during cardiopulmonary bypass Perfusion, January 1, 2005; 20(1): 3 - 10. [Abstract] [PDF]
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 D. D. Doblar Intraoperative Transcranial Ultrasonic Monitoring for Cardiac and Vascular Surgery Seminars in Cardiothoracic and Vascular Anesthesia, June 1, 2004; 8(2): 127 - 145. [Abstract] [PDF]
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