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Correspondence

Postoperative Ischemia and Cognitive Impairment in Cardiac Surgery Patients

^a Department of Neurology, Auckland City Hospital, Park Rd, Grafton, Auckland, 1001 New Zealand
^b Departments of Psychology and Anesthesiology, University of Auckland, Private Bag 92019, Auckland, New Zealand
^c Department of Medicine, University of Otago, PO Box 4345, Christchurch, 8001 New Zealand
^d Department of Cardiothoracic Surgery, Auckland City Hospital, Park Rd, Grafton, Auckland, 1001 New Zealand

(Email: abarber{at}adhb.govt.nz).

To the Editor:

We read with interest the article by Knipp and colleagues [1] that addresses several important questions. Among other findings, this group reports that after coronary artery bypass grafting there was no relationship between the presence of ischemic lesions on magnetic resonance diffusion-weighted imaging (DWI) and either short-term or long-term cognitive outcome. This contrasts with findings in our very recent work in which cognitive impairment at 6 weeks post-surgery was found in all patients who received at least one valve to the left heart and who had ischemic change on postoperative DWI [2].

We find it difficult to understand the strength of Knipp and colleagues' [1] conclusions, and we would appreciate clarification of the following points:

(1) In the results section, they report no association between early or late cognitive changes and the number or volume of ischemic lesions on postoperative DWI. Yet there is no specific information given regarding the analyses for the number of lesions in Table 6.

(2) The numbers in the "Results" section are also difficult to follow. The authors report that 20 patients had new postoperative DWI lesions, but the breakdown (ie, "Ten patients had 1, 5 patients had 3 to 6, and 1 patient had 7 new DWI lesions") does not seem to add up.

(3) The distribution of either the volume or number of DWI lesions is clearly skewed, and this does not seem to have been taken into account in the statistical analysis. Would it have been better to treat this outcome as a binary DWI lesion or no lesion?

(4) In Table 6, the odds ratio of 1.000 is reported with very narrow (ie, 0.997 to 1.002) 95% confidence limits. We presume this is because it applies to a 1 mm³ change in the DWI lesion volume and, if correct, we wonder if an analysis based on such a small unit change is meaningful. If a larger volume were used as the unit, the confidence limits would be correspondingly larger, making it clearer that the finding is at best tentative.

A number of other factors may have contributed to the discrepant results. In contrast to Knipp and colleagues [1]:

(1) We used a battery of cognitive tests that included all tests recommended by the International Cardiac Study Group in "A Statement of Consensus on Assessment of Neurobehavioral Outcomes After Cardiac Surgery."

(2) We defined cognitive impairment using the Reliable Change Index, which calculates the distribution of expected change around baseline performance and takes into account factors such as test-retest performance.

(3) We used less stringent criteria to define cognitive impairment with a drop off in function in any one of the tests used as opposed to at least three tests.

We accept that Knipp and colleagues [1] are not alone in their findings. However, we doubt that 32 patients is a sufficient number from which to draw a reliable negative conclusion regarding the relationship between postoperative ischemic lesions on DWI and cognitive impairment. We are continuing our study, and we hope to report a further analysis based on 150 patients in due course.

	References

Top References

 

1. Knipp SC, Matatko N, Wilhelm H, et al. Cognitive outcomes three years after coronary artery bypass surgery: relation to diffusion-weighted magnetic resonance imaging Ann Thorac Surg 2008;85:872-879.[Abstract/Free Full Text]
2. Barber PA, Hach S, Tippett LJ, Ross L, Merry AF, Milsom P. Cerebral ischemic lesions on diffusion-weighted imaging are associated with neurocognitive decline after cardiac surgery Stroke 2008;39:1427-1433.[Abstract/Free Full Text]

This article has been cited by other articles:

				S. C. Knipp, H. Jakob, C. Losch, N. Matatko, H. Wilhelm, H. C. Diener, and M. Schlamann Reply. Ann. Thorac. Surg., February 1, 2009; 87(2): 673 - 674. [Full Text] [PDF]

This Article Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Add to Personal Folders Download to citation manager Permission Requests Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Barber, P. A. Articles by Milsom, P. Search for Related Content PubMed PubMed Citation Articles by Barber, P. A. Articles by Milsom, P. Related Collections Cerebral protection Related Article