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J Thorac Cardiovasc Surg 2000;119:132-137
© 2000 Mosby, Inc.
CARDIOPULMONARY SUPPORT AND PHYSIOLOGY |
From Oxford Heart Centre, John Radcliffe Hospital,a Oxford, United Kingdom, Department of Biomedical Engineering, University of Groningen, The Netherlands,b and University Department of Experimental Psychology, Oxford, United Kingdom.c
Financial support was received from Cobe Cardiovascular, Inc, Arvada, Colo. Haemoprobe, Groningen, The Netherlands, performed the S-100ß analysis.
Address for reprints: S. Westaby, BSc, FRCS, MS, Oxford Heart Centre, John Radcliffe Hospital, Oxford OX3 9DU, England.
| Abstract |
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| Introduction |
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In recent years S-100ß has been used, in a research context, to identify cerebral injury after cardiac surgery in preference to the more labor-intensive neuropsychologic evaluations.
7,8 Several studies suggest that transient elevations in levels of serum S-100ß protein reflect subclinical cerebral damage in the absence of frank neurologic signs.
9,10 To date, however, no study has demonstrated that S-100ß levels reflect neuropsychologic performance. The aim of the present study was to determine whether S-100ß could predict early or late neuropsychologic and functional impairment after coronary bypass surgery with CPB. We attempted to avoid noncerebral sources of S-100ß by discarding cardiotomy suction blood.
11
| Patients and methods |
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Conduct of the operation
All operations were performed with Cobe flat sheet membrane oxygenators and bypass circuits (COBE Cardiovascular Inc, Denver, Colo) and a pump flow of 2.4 L · m2 · min1 at between 32°C and 36°C. Vent suction was not used. The mean arterial pressure was maintained between 50 and 80 mm Hg and alpha-stat arterial carbon dioxide management was used. Myocardial protection was achieved with antegrade crystalloid cardioplegia. Shed mediastinal blood was minimized by careful surgery and was discarded to avoid noncerebral sources of S-100ß. Anticoagulation during CPB was monitored with the activated clotting time (Hemochron 800; International Technidyne Corp, Edison, NJ). Additional heparin was administered if the activated clotting time was shorter than 400 seconds. Heparin was neutralized by means of protamine chloride infusion (3 mg/kg) after the completion of CPB.
S-100ß analysis
Serum was analyzed for S-100ß protein concentration 24 hours before the operation (T1), 5 minutes before beginning CPB but after heparin injection (T2), immediately after termination of CPB but before protamine injection (T3), and 8 hours after the operation (T4). The serum or heparinized plasma samples were stored at 80°C until analyzed. S-100ß protein concentration was determined with the use of a new, more sensitive monoclonal immunoluminometric method (sensitivity 0.02 µ/L) (Sangtec LIA 100, Sangtec Medical AB, Bromma, Sweden). This assay method uses 3 monoclonal antibodies, SMST 12, SMSK 25, and SMSK 28, to detect the beta chains in the betabeta and the alphabeta dimers of S-100. Since S-100ß is a calcium-binding protein, plasma treated with ethylenediaminetetraacetic acid cannot be used, but heparin does not affect the results, as we showed in a separate unpublished validation study. The test is based on sandwich formation of immobilized ant-S-100ß antibody, S-100ß from the test sample and luminescence-labeled S-100ß antibody. The assay was performed according to the manufacturers instructions and label was measured by means of flash luminescence (Lumat LB 9507, EG & G, Berthold, Germany).
Neuropsychologic and functional assessment
Each patient underwent a comprehensive neuropsychologic examination administered by a research psychologist (S. White). The National Adult Reading Test (NART) was used to estimate pre-morbid intelligence. Tests were carried out preoperatively (1 day before the operation), before hospital discharge (5 days after the operation), and again at 3 months after the operation. All tests have been described in detail elsewhere.
12-16Table I
lists the neuropsychologic tests and provides a brief description of the cognitive domains evaluated.
In addition to neuropsychologic performance, functional and emotional measures were also measured before the operation and at 3 months follow-up. These tests were as follows:
Statistical analysis
Clinical data were expressed as mean and standard deviation. Blood test values were described as mean and standard error of the mean. Neuropsychologic performance was assessed by calculating the change scores (preoperative minus predischarge and preoperative minus follow-up) for each patient on every test. Likewise, change scores (preoperative minus follow-up) were also calculated for each of the 4 functional measures. Change scores were then correlated with the S-100ß measures taken at each of the 4 different time points. Computerized statistical analysis was undertaken with the use of the SPSS for Windows (version 7) statistical program (SPSS, Inc, Chicago, Ill). Comparisons between groups were done with the Mann-Whitney test for unpaired data. Correlations between each of the mean S-100ß time points and age, CPB time, crossclamp time, blood loss, blood transfusions, ventilatory times, and neuropsychologic performance were calculated by means of the Pearson correlation coefficient and, unless specified, adopted the .01 level as significant given the large number of comparisons. Because the distribution of S-100ß values was generally skewed, these data are presented as median, 25th, and 75th percentiles.
| Results |
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Neuropsychologic performance
A total of 95 patients completed the preoperative assessment. The remaining 5 were eliminated from follow-up. The mean estimated IQ for the group was 108. Before hospital discharge (5 days after the operation), 79 patients could be assessed. Two patients were excluded because of stroke and multiple organ failure. The remaining 14 patients declined for reasons of ill health. However, 3 months after the operation, 89 patients had completed all tests. Pearson correlation coefficients for all serial time points for S-100ß versus neuropsychologic and functional change scores are presented inTables IV and V.
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| Discussion |
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The serum levels of S-100ß after uncomplicated cardiac operations follow a different pattern from those after acute ischemic cerebral events, wherein peak values occur after 3 days.
20,21 Studies on S-100ß release during and after CPB have shown peak S-100ß levels soon after perfusion, presumably resulting from transient increase in permeability of the blood-brain barrier as part of the inflammatory response.
8-10,18,19 Joensson and associates
9 showed that S-100ß levels immediately after CPB did not correlate with cerebral outcome although grossly elevated levels of S-100ß 48 hours after the operation were associated with clinically obvious stroke.
9 The early release of S-100ß after cardiac surgery may thus be a transient serum elevation without any relation to permanent neuronal damage. However, the early S-100ß studies were interpreted without the awareness that S-100ß from noncerebral sources might confuse the issue by elevating overall levels.
11 In the present study, extracerebral S-100ß was reduced by discarding cardiotomy blood and not retransfusing shed mediastinal blood, which has been shown to contain high levels of S-100ß.
11 Consequently, our S-100ß levels were lower and also peaked later than in earlier reports.
8-10,18,19 Lower levels might also be caused by other factors. We used a new, highly sensitive immunoluminometric assay test capable of detecting much lower levels of S-100ß, whereas in earlier studies levels of S-100ß below 0.2 µ/L were not detectable. Consequently, more patients in our study had detectable levels of S-100ß. As most of these were at or below the previous less-sensitive level, it is likely that they contributed to the lowering of the mean levels, since previous studies in which less-sensitive assays were used did not always include patients with undetectable S-100ß levels.
Grocott and colleagues
10 described a weak but significant relationship between cerebral microemboli and S-100ß levels. In an experimental study, blood aspirated from the surgical field during CPB and reinfused into dogs generated a greater density of small capillary and arterial dilatations in the brain than CPB without cardiotomy suction.
23 The presence of so-called small capillary and arterial dilatations was first demonstrated by Moody and colleagues
24 in autopsy specimens from patients who had undergone cardiac operations with CPB. There is evidence to suggest that lipid microembolism is the cause of these diffuse cerebral changes.
25 We have also shown that an arterial line filter can significantly reduce levels of S-100ß, although this early investigation was performed with cardiotomy suction and the less-sensitive S-100ß assay.
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A recent study by Backstroem and associates,
22 using a kinetic model employing elimination rates of S-100ß during the first 5 hours after operation, found a positive correlation with early adverse cognitive outcome. The authors proposed early S-100ß elimination rate (as an expression of postbypass release) in preference to absolute levels to correct for contamination from extracerebral sources.
9 However, the formula to calculate elimination rate requires a progressive decrease in postoperative levels, whereas our study without cardiotomy suction shows an increase over this time frame. Consequently, for our study the calculation would have been meaningless for most patients.
Appearance of S-100ß in serum after CPB may indicate cellular damage, increased permeability of the blood-brain barrier, or both. Transient permeability of the blood-brain barrier to S-100ß protein could be caused by the inflammatory response generated by CPB itself. Since the development of inflammation is time related, we might expect a positive correlation between perfusion time and S-100ß release. In fact, correlation between perfusion time and S-100ß levels has been an inconstant finding.
8-10,18,19,27,28 Our present study found no relationship between S-100ß and perfusion time, although in general the duration of CPB was short. However, we did find a significant correlation between the prebypass value and the postbypass levels, and patients without detectable prebypass levels had significantly lower levels of S-100ß afterward, irrespective of perfusion time. These findings suggest that factors other than perfusion time may influence the release of S-100ß. Surgical injury and the anesthesia may well contribute to this release, and differences have been shown in valve, aortic aneurysm, and hypothermic circulatory arrest patients where S-100ß levels are greater and remain significantly higher for between 5 and 48 hours.
18 In open cardiac operations (as opposed to coronary operations), the relationship between S-100ß and neuropsychologic outcome might be different irrespective of the contribution of cardiotomy suction.
The finding of significantly higher S-100ß levels in patients requiring ventilation for more than 5 hours is interesting and consistent with a recent report claiming a relationship between postoperative S-100ß levels and prolonged intubation.
28 Cerebral edema has been demonstrated within an hour of CPB by magnetic resonance scanning in patients undergoing routine coronary bypass.
29 The swelling is variable and may similarly reflect transient change in permeability of the blood-brain barrier without injury to the brain itself. The transient increase in S100ß levels may then represent a reversible increase in the permeability of cellular and vascular membranes as opposed to structural neuronal damage. All patients having coronary operations in this study were subjected to fast track and early extubation protocols (irrespective of age), so that shorter extubation time reflected early awaking and conscious awareness.
30
In summary, we were unable to demonstrate any correlation between serum S-100ß protein levels and early or late neuropsychologic impairment after elective coronary operations with CPB. It is possible that the transient passage of S-100ß from cerebrospinal fluid to serum bears little relationship to the type of neuronal injury necessary to produce impairment of neuropsychologic and functional measures.
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