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LETTER TO THE EDITOR |
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1 London Health Sciences Centre and University of Western Ontario Ontario, Canada 2 Children's Hospital and Harvard Medical School Dept. of Cardiac Surgery Children's Hospital 300 Longwood Avenue Boston, MA 02115, USA |
The authors are to be commended for addressing an important issue with a carefully designed prospective randomized study. Since the mortality for routine open-heart procedures is now so low, it is essential for us all to focus on the quality of life of these patients after surgery. One of the most important sources of morbidity is central nervous system injury. Stroke continues to be an important cause of morbidity following bypass in adults. It is most likely related to macroembolic events, usually dislodged atheromatous material occluding a vessel of significant size in the cerebral circulation. In addition to a defined stroke, many groups have documented a decline in neuropsychological test results following cardiopulmonary bypass which could be the result of watershed hypoperfusion or the microemoblic load to the brain. Moody and colleagues2 have used histopathologic techniques to demonstrate multiple emboli throughout the brains of adults who have died following recent cardiopulmonary bypass. The histologic markers of embolism they identified most likely represent fat emboli as well as perhaps microatheromatous emboli that could potentially be reduced in number by appropriate arterial line filtration. Blauth and colleagues3,4 have previously demonstrated that arterial line filtration reduces microemboli as documented by fluoroscein retinal angiography. They have also shown that more emboli are seen with bubble oxygenators than with membrane oxygenators and that there is a correlation between embolic load and neuropsychometric score results. Furthermore, in a prospective trial of 100 patients undergoing cardiac surgical procedures with cardiopulmonary bypass and randomized to use or non-use of 40 µm arterial filters, Pugsley and colleagues5 showed both a significant reduction in transcranial Doppler-detected cerebral emboli and a corresponding reduction in postoperative neuropsychological deficits in patients in whom filters were used. Thus, previous literature strongly supports the routine use of arterial line filters, particularly when a bubble oxygenator is used. How then does one explain the result that Shahani and colleagues describe in their recent report in the Asian Annals?
The fundamental problem that we see in the paper by Shahani's group is that they have compared the mean score of the group randomized to arterial line filtration with the mean score of the group that did not have an arterial line filter. The limitation of mean score results has been addressed at a recent consensus conference.6 The fundamental limitation of mean scores is that patients who are not impaired by the surgical procedure will perform better on the same test when they undertake that test for the second time, that is following the surgery, because of the practice effect. The authors have indeed documented that within each group there was an improvement in the mean score for trail making test part A. When the majority of a group improves its performance because these patients are unimpaired and perform normally and others in the group show no change or an actual decrement, if one looks at overall group mean scores the net result may be no change or slight improvement. In other words, the patients who have experienced some form of insult are masked because their decline in test score is offset by those who demonstrate the expected improvement in performance due to the practice effect. How then should one define "significant impairment"? This remains controversial but most investigators currently take a one standard deviation decrease from baseline test score, or a specific determinant, for example, 20%. They then compare the incidence of dysfunction thus defined that occurs in each group. Only by identifying the percentage of patients who suffer a specific decline in performance, either with or without the treatment under test, can one be confident that there is indeed no difference with an intervention such as arterial line filtration.
One of the other critical limitations of the study by Shahani and colleagues relates to the power of the study to detect differences. The power is very much influenced by the amount of variability in postoperative performance, which itself is strongly influenced by factors that include the educational level, age, testing site, and number of days postoperatively. Variations in these factors in the study population can result in a large standard deviation in postoperative performance, thus requiring a much larger number of patients in order to detect any change. Our sense from previous studies is that a minimum group size of 50 to 75 patients is required to show a meaningful change occurring in approximately 40% of patients. It is quite probable that because of within-group heterogeneity the group sizes of 21 and 23 patients in the study by Shahani and colleagues did not allow sufficient power to detect differences between groups.
In conclusion, once again, the authors are to be congratulated on addressing a critically important topic. It is only through efforts such as this that central nervous system injury both overt and subtle will be reduced following procedures employing cardiopulmonary bypass.
REFERENCES
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