Ann Thorac Surg 2003;75:1361-1362
© 2003 The Society of Thoracic Surgeons
Correspondence
Cerebral perfusion and cognitive function: Reply
Wilko Reents, MDa,
Joerg Babin-Ebell, MDa,
Wolfgang Muellges, MDb
a Department of Thoracic and Cardiovascular Surgery, University Hospital Wuerzburg, Josef-Schneider-Str. 6, 97080 Wuerzburg, Germany
b Department of Neurology, University Hospital Wuerzburg, Josef-Schneider-Str. 11, 97080 Wuerzburg, Germany
e-mail: wilko.reents{at}mail.uni-wuerzburg.de
To the Editor:
We thank Dr Alston for his interest in our study. The study by Alston and colleagues [1] of the relation between jugular venous bulb saturation (SjO2) and cognitive performance in patients undergoing coronary artery bypass grafting led to the same conclusion as ours [2], which used different methods. Yet it should be noted that in the study of Alston and colleagues [1], there was poor agreement between intraoperatively obtained fiberoptic and bench oxymetry SjO2 measurements, so their analysis is restricted to postoperative SjO2 desaturation and cognitive outcome. This precludes a direct comparison with our data.
Cerebral oxygen saturation (ScO2) assessed by near-infrared spectroscopy (NIRS) correlates with jugular venous bulb oxygen saturation (SjO2) in instances of global changes of cerebral perfusion and oxygenation. Thus global cerebral hypoxia/ischemia should be detectable in a representative area as investigated by NIRS. However, we agree that an event causing focal cerebral hypoxia/ischemia may be missed by NIRS. It remains speculative whether different areas of the brain in individual patients are differently susceptible to hypoxia/ischemia. Our study was designed to investigate the influence of ScO2 on postoperative cognitive performance and not to examine the influence of various parameters on ScO2. The effect of PaCO2 on cerebral blood flow is well known [3]. Since ScO2 was the investigated target parameter, correction by the actual PaCO2 seems unnecessary.
There is no perfect method to measure and to statistically evaluate postoperative cognitive function [4]. With respect to statistical analysis, we concur that dichotomization causes loss of individual differences and could thereby lead to a reduction in the statistical power to detect differences between groups [5]. We provide here the correlation between individual test score differences and the time and amount of ScO2 below the stated cut-off values (Table 1).
There was no significant relationship, thus leaving our conclusions unchanged.
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Table 1. Correlation Between Individual Test Score Differences For Each Neuropsychologic Test and Cerebral Oxygen Saturation as Assessed by Near-Infrared Spectroscopy
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References
- Robson M.J., Alston R.P., Deary I.J., Andrews P.J.D., Souter M.J., Yates S. Cognition after coronary artery surgery is not related to postoperative jugular bulb oxyhemoglobin desaturation. Anesth Analg 2000;91:1317-1326.[Abstract/Free Full Text]
- Reents W., Muellges W., Franke D., Babin-Ebell J., Elert O. Cerebral oxygen saturation assessed by near-infrared spectroscopy during coronary artery bypass grafting and early postoperative cognitive function. Ann Thorac Surg 2002;74:109-114.[Abstract/Free Full Text]
- Henriksen L. Brain luxury perfusion during cardiopulmonary bypass in humans: a study of the cerebral blood flow response to changes in CO2, O2 and blood pressure. J Cerebr Blood Flow Metab 1986;6:366-378.[Medline]
- Borowicz L.M., Goldsborough M.A., Selnes O.A., McKhann G.M. Neuropsychologic change after cardiac surgery: a critical review. J Cardiothorac Vasc Anesth 1996;10:105-112.[Medline]
- MacCallum R.C., Zhang S., Preacher K.J., Rucker D.D. On the practice of dichotomization of quantitative variables. Psychol Methods 2002;7:19-40.[Medline]
