doi: 10.1161/01.cir.0000141262.91992.f1
(Circulation. 2004;110:e55-e56.)
© 2004 American Heart Association, Inc.
Correspondence |
Carbon Dioxide Insufflation on the Number and Behavior of Air Microemboli in Open-Heart Surgery
Guy’s Hospital, London, England
To the Editor:
Although useful neuropsychological (NP) benefits may result from the recent report by Svenarud et al1 of a reduction in air microemboli in a randomized controlled trial of CO2 insufflation during single-valve surgery, there are a number of issues relating to cerebral microemboli, valve versus coronary bypass surgery, and NP outcome that are not adequately addressed in this paper.
A number of studies have linked cerebral microemboli to NP outcome, but the relationship between the two remains unclear. Other factors such as inflammatory response and altered cerebral blood flow must also be considered. Neville et al2 compared 193 patients having coronary artery bypass surgery with 73 patients having valve replacement surgery and confirmed previous findings that there were more microemboli detected during valve surgery but there was no difference found in NP outcome. Their result suggests that the number of microemboli may not always be the most significant determinant of NP outcome. Additionally, Svenarud et al1 only measured microemboli within the heart and ascending aorta, and these may not correlate with cerebral microemboli.
I would be interested to know why Svenarud and colleagues1 did not assess cerebral microemboli when they have done so in the past.3 They quote two papers from the same group4,5 as evidence that air microemboli are harmful, but both papers are flawed. Taylor et al5 assume that the microemboli they detect are air, but this is unproven, and the technology to distinguish gaseous from particulate microemboli has not yet been used clinically. Borger et al4 found that the group with fewer than 10 interventions had less NP impairment after surgery; however, this group also had significantly shorter cross-clamp and cardiopulmonary bypass times. A further flaw was the failure to perform actual microemboli counts such that Borger et al4 were not able to make a direct correlation between the microemboli count and NP outcome.
How did Svenarud et al1 count microemboli on a 2-dimensional image over time? Counting cerebral microemboli as they appear sequentially on a Doppler recording is relatively straightforward. However, when microemboli are "whirling around in the left ventricle and left atrium,"1 it must be very difficult to keep track of which microembolus is which for a whole minute and not doublecount any. It is not adequately explained how the microemboli were counted so that their methods could be repeated by others.
References
- Svenarud P, Persson M, van der LJ. Effect of CO2 insufflation on the number and behavior of air microemboli in open-heart surgery: a randomized clinical trial. Circulation. 2004; 109: 1127–1132.
[Abstract/Free Full Text]
- Neville MJ, Butterworth J, James RL, Hammon JW, Stump DA. Similar neurobehavioural outcome after valve or coronary operations despite differing carotid embolic counts. J Thorac Cardiovasc Surg. 2001; 121: 125–136. 2001.
- Van der Linden J, Casimir-Ahn H. When do cerebral emboli appear during open heart operations? A transcranial Doppler study. Ann Thorac Surg. 1991; 51: 237–241.[Abstract]
- Borger MA, Peniston CM, Weisel RD, Vasiliou M, Green RE, Feindel CM. Neuropsychologic impairment after coronary bypass surgery: effect of gaseous microemboli during perfusionist interventions. J Thorac Cardiovasc Surg. 2001; 121: 743–749.
[Abstract/Free Full Text]
- Taylor RL, Borger MA, Wiesel RD, Fedorko L, Feindel CM. Cerebral microemboli during cardiopulmonary bypass: increased emboli during perfusionist interventions. Ann Thorac Surg. 2001; 68: 89–93.[Medline]
[Order article via Infotrieve]
Response
Karolinska Institute, Karolinska University Hospital, Stockholm, Sweden
We thank Dr Whitaker for his interest in our study.1
First, as stated in the title, we investigated the "effect of CO2 insufflation on the number and behavior of air microemboli in open-heart surgery" (p 1127)1 and not the effect of the gas on neuropsychological outcome. We confirmed that most microemboli that appear in the heart after release of the cross-clamp stay there until the beating heart is being filled, which is in accordance with our earlier transcranial Doppler study.2 Moreover, we showed that the number of microemboli ejected into the peripheral circulation during and after weaning from cardiopulmonary bypass can be substantially reduced with CO2 insufflation into the cardiothoracic wound. Thus, it seems reasonable to assume that also the number of microemboli ending up in the brain will become less. It also seems reasonable to assume that the microbubbles that do end up there are less harmful to the brain if they consist of CO2 instead of air. Of course, these findings do not exclude that other factors, be it cerebral blood flow or inflammatory response, may influence postoperative neuropsychological outcome.
Second, regarding the question whether air microemboli are harmful to the brain, there is an abundance of studies showing that this is indeed the case. We refer to the review recently published in the New England Journal of Medicine3 and to the convincing study of Martens et al,4 who compared the effect of the injection of air and of CO2 into the carotid artery. "All animals of the ‘air’ group showed marked circulatory reactions leading to death in 2 animals. In the whole group, diffusion-weighted magnetic resonance imaging revealed irreversible hyperintense signals in both hemispheres" (p 51). Martens et al4 concluded, "In contrast to the dramatic effect of air emboli, identical quantities of carbon dioxide injected into cerebral arteries of the pigs were not associated with major clinical symptoms. The early reversibility of ischemic reactions visualized in diffusion-weighted magnetic resonance imaging encourages the use of carbon dioxide insufflation as a protective method in cardiac surgery" (p 51).
Third, it is clearly stated in Methods how the emboli were counted, so that others can repeat the methods. "The TEE probe was positioned in such a manner that a mid-esophageal long axis view could be kept. That view included 3 areas of interest, ie, the left atrium, the left ventricle, and the proximal part of the ascending aorta.5 Video recordings of this view were started from the release of the aortic cross clamp until 20 minutes after [cardiopulmonary bypass] was discontinued. After the study was finished, an examiner, who was unaware of the treatment given, analyzed the videotapes of all the patients. The maximal number of microemboli in the left atrium, the left ventricle, and the proximal part of the ascending aorta that appeared on one frame, was determined for each minute by scrolling the tape back and forth in slow motion. Thus, microemboli could be differentiated from moving heart tissue including the valve structures" (p 1128).
Finally, if the method had been unreliable, it would not have been able to discriminate between the 2 treatment groups.
References
- Svenarud P, Persson M, van der Linden J. Effect of CO2 insufflation on the number and behavior of air microemboli in open-heart surgery: a randomized clinical trial. Circulation. 2004; 109: 1127–1132.
[Abstract/Free Full Text]
- van der Linden J, Casimir-Ahn H. When do cerebral emboli appear during open heart operations? A transcranial Doppler study. Ann Thorac Surg. 1991; 51: 237–241.[Abstract]
- Muth CM, Shank ES. Gas embolism. N Engl J Med. 2000; 342: 476–482.
[Free Full Text]
- Martens S, Theisen A, Balzer JO, Dietrich M, Graubitz K, Scherer M, Schmitz C, Doss M, Moritz A. Improved cerebral protection through replacement of residual intracavital air by carbon dioxide: a porcine model using diffusion-weighted magnetic resonance imaging. J Thorac Cardiovasc Surg. 2004; 127: 51–56.
[Abstract/Free Full Text]
- Shanewise JS, Cheung AT, Aronson S, Stewart WJ, Weiss RL, Mark JB, Savage RM, Sears-Rogan P, Mathew JP, Quinones MA, Cahalan MK, Savino JS. ASE/SCA guidelines for performing a comprehensive intraoperative multiplane transesophageal echocardiography examination: recommendations of the American Society of Echocardiography Council for Intraoperative Echocardiography and the Society of Cardiovascular Anesthesiologists Task Force for Certification in Perioperative Transesophageal Echocardiography. Anesth Analg. 1999; 89: 870–884.
[Free Full Text]
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