Neurological dysfunction is a common problem after cardiac surgery with cardiopulmonary bypass (CPB) [1,2]. Several investigations provide strong evidence that extracorporeal circulatory support is one of the major contributors to postoperative brain dysfunction most probably caused by transient or manifest cerebral ischaemia [3-5]. The increased serum concentrations of the neuronal-ischaemic markers neuron-specific enolase (NSE) and S-100β have been related to CPB-induced cerebral cell injury. Based on this assumption an arterial-cerebral venous concentration gradient should be detectable, reflecting cerebral cell injury and the increased release neuronal-ischaemic markers from the brain.
The CPB process itself results in an increase of inflammatory mediators in the blood, subsequently leading to multiple cellular interactions [6,7]. Although cerebral circulation is maintained by CPB, global cerebral hypoperfusion or cerebral microemboli from the extracorporeal circuit are regarded as potential risk factors for neuronal-ischaemic events. However, little is understood as to whether cerebral cell injury and a transient or sustained cytokine release from the brain may in part contribute to the systemically inflammatory response syndrome. Our hypothesis was that cerebral cytokine release caused by cerebral cell injury should be detectable in an arterial-cerebral venous gradient.
This clinical investigation was performed to verify these hypotheses and to evaluate the time course of arterial-cerebral venous concentration gradients for specific markers of cerebral ischaemia (NSE and S-100β) as well as for pro- and anti-inflammatory cytokines (IL-6, IL-8 and IL-10).
The study was approved by the local Ethics Committee and written informed consent was obtained from each patient. Twenty-five male patients scheduled for elective coronary artery bypass grafting (CABG) were included in this investigation. According to clinical and duplex-ultrasonic investigation of intra- and extracranial arteries, no patient showed evidence of pre-existing cerebrovascular disease. Patients with a history or laboratory evidence of hepatic, renal or nervous system disease were excluded from the study. None of the patients had general anaesthesia within a period of 30 days prior to surgery. Individual cardiac medication such as anti-hypertensive drugs, nitroglycerin or β-adrenoceptor blocking agents, or both, was continued until the day of surgery. Flunitrazepam (2 mg orally) premedication was given on the evening before and on the morning of surgery.
Anaesthesia and catheterization procedure
Before induction of anaesthesia, routine haemodynamic monitoring was established including electrocardiography (leads II and V5), and arterial and central venous catheterization. Induction of anaesthesia was performed by intravenous (i.v.) administration of fentanyl 7 μg kg−1 and midazolam 0.2 mg kg−1; endotracheal intubation was facilitated by administration of pancuronium 0.15 mg kg−1. Anaesthesia was maintained with fentanyl 10 μg kg−1 h−1 and midazolam 0.1 mg kg−1h−1. In addition to routine monitoring procedures, jugular bulb catheterization was performed via retrograde cannulation of the right internal jugular vein with a single lumen catheter (14-G Arrow® Int. Inc, Reading, PA, USA). The correct catheter position was verified by fluoroscopy. Mechanical ventilation of the lungs was performed using a volume-controlled anaesthesia ventilator (Cato®, Dräger AG, Lübeck, Germany) with a fresh gas flow rate of 6 L min−1 at an inspired oxygen fraction (FiO2) of 0.5 in air. A tidal volume of 10 mL kg−1 body weight, a respiratory rate of 10 breaths min−1 and a positive end-expiratory pressure of 5 cmH2O were applied in all patients. PaCO2 was maintained at 4.7-5.3 kPa during mechanical ventilation.
The anaesthetic and surgical management did not differ from our routine clinical practice. For CPB a non-pulsatile flow of 1.8-2.4 L m−2min−1 was established with a centrifugal pump (Jostra Rotaflow®, Hirrlingen, Germany) and a membrane oxygenator (Jostra Quadrox®, Hirrlingen, Germany). Priming of CPB consisted of 1 L Ringer's lactate and 500 mL 6% hydroxyethylstarch. Cardiac arrest and myocardial protection were obtained by administration of Bretschneider's cardioplegic solution at 4°C (Custodiol®, Dr Franz Köhler Chemie GmbH, Alsbach-Hähnlein, Germany). A two-stage cannulation technique was performed in all patients. Patients were cooled to 32°C applying α-stat acid-base management. Perfusion pressure was maintained between 50 and 60 mmHg.
After admission to the intensive care unit (ICU) postoperative therapy with mandatory ventilation of the lungs, fluids and pharmacological support was carried out in accordance with standard critical care practice. Analgesia and sedation using pirinitramide (piritramide) and midazolam was provided as clinically deemed necessary.
Study design and analytical procedures
After induction of anaesthesia and catheterization arterial and cerebral venous blood samples were taken simultaneously at the following time points: after induction of anaesthesia for baseline (baseline), during hypothermic CPB at 32°C (CPB), after termination of CPB at sternal closure (after CPB), after 2, 4 and 6 h from admission to the ICU, respectively (ICU 2, 4 and 6 h). Plasma was immediately separated by centrifugation at 3000 rpm at 0°C for 10 min and frozen at −70°C for later analysis. Plasma levels of IL-6, IL-8 and IL-10 were measured in duplicate using enzyme immunometric assays (Milenia Biotec®, Bad Nauheim, Germany). Serum concentrations for NSE and S-100β were measured in duplicate using automated luminometric immunoassays, Liaison Sangtec 100® and Liaison NSE® (Byk-Sangtec Diagnostica GmbH, Dietzenbach, Germany). Average values were used for further calculations.
Almost all sampled data approximately followed a standard normal distribution. All values presented in the figures are given as median, 25th/75th percentiles, and ranges of concentrations. Analysis of variance (ANOVA) was performed between the preoperative measurement period (equal to baseline) and the sequential measurements (CPB, after CPB, ICU 2, 4 and 6 h) using a repeated measures design. Concentration differences between arterial and cerebral venous blood were assessed by the Wilcoxon signed rank sum test. Results were considered statistically significant at P < 0.05. Additional descriptive analysis was performed using the Statistica® software package (Kernel-Version 6.0.591, StatSoft Inc.®, Tulsa, OK, USA).
None of the investigated 25 patients developed any intra- or postoperative complications that would have resulted in additional interventions beyond routine management. During the postoperative period none of the patients showed manifest neurological damage. Five patients required i.v. low dose epinephrine support (1-3 μg h−1) after termination of CPB. Administration could be terminated after arrival on the ICU and prior to the start of measurement point ICU 2 h.
Patients mean age was 62 yr (range 49-77 yr), height was 173 ± 6 cm and body weight was 79 ± 7 kg, respectively. Duration of operation was 205 ± 47 min, aortic cross-clamping time was 58 ± 18 min and CPB time was 94 ± 26 min.
Results are shown as median, 25th/75th percentiles, and ranges of concentrations. During as well as after CPB no significant differences for NSE, S-100β, IL-6, IL-8 and IL-10, were found between arterial and jugular bulb plasma concentrations (Figs 1-5).
S-100β concentration significantly increased during CPB with a peak after termination of CPB (Fig. 2). NSE concentration was significantly increased during CPB but in contrast to S-100β, maximum concentrations were observed 2 h after ICU admission. No significant decline was measured during the on-following time-period (Fig. 1).
Compared to the baseline measurement, increased plasma concentrations were found for pro-inflammatory (IL-6, IL-8), and anti-inflammatory (IL-10) cytokines after termination of CPB (Figs 3-5). The highest concentrations were observed after termination of CPB followed by a gradual decrease over time. No phased response of the anti-inflammatory cytokine IL-10 was detectable compared to the induction of pro-inflammatory IL-6 and IL-8 (Figs 3-5).
During the investigated period of time neither neuronal-ischaemic markers nor cytokines reached baseline concentrations.
In this clinical investigation no arterial-cerebral venous concentration gradients were found for neuronal-ischaemic markers as well as for pro- and anti-inflammatory mediators during and after CPB. However, NSE and S-100β plasma concentrations were increased during and after hypothermic CPB, whereas a significant elevation of cytokines was only observed after termination of CPB. Plasma concentrations of neuronal-ischaemic markers as well as cytokines did not return to baseline levels during the investigated time-period.
Cerebral dysfunction is common after cardiac surgery with CPB [1,2]. The implementation of CPB is thought to play a key role in many pathophysiological mechanisms, which in detail remain unclear. Associated to CPB, global hypoperfusion even of short duration, gaseous and particulate microembolic events may induce ischaemic episodes in the brain [8-10]. Post-ischaemic reperfusion results in the release of biochemical markers of injured tissue from the ischaemic area. In principle, this observation should also apply for the brain, which may be injured by an ischaemic event associated to the CPB procedure. In turn, this event should become apparent by an additional increased release of biochemical markers in the cerebral venous blood. Therefore, simultaneous measurement of arterial and cerebral venous blood concentrations basically may be seen as a suitable approach to assess ischaemic marker release from the brain.
Postoperative neurocognitive impairment in patients undergoing cardiac surgery has been associated with the implementation of extracorporeal circulation [2,5]. Detailed analysis suggests that increased plasma concentrations of biochemical markers NSE and S-100β may be associated with cerebral ischaemia due to CPB [11-14] or acute brain injury . Presently however, little information on the ischaemia-related increased release of inflammatory markers from the brain is available. Acute brain injury has been demonstrated to cause increased IL-6 production, as detected by arterial-cerebral venous gradient measurement, while IL-1β, IL-8 and TNFα remained unchanged . In a different clinical setting, a significant IL-8 release into cerebral venous plasma during CPB was reported, while other pro-inflammatory cytokines were not elevated . In the present investigation an increase for the arterial-cerebral venous gradient for neuronal-ischaemic markers as well as cytokines was not found during and after CPB. This result therefore does not confirm the initial hypothesis that CPB may lead to a cerebral inflammatory response. Even more, it suggests that the elevated blood concentrations of the neuronal-ischaemic markers may have a different source other than the brain [18-21]. Recent findings indicate that retransfusion of mediastinal shed blood, cardiotomy suction or pleural drainage blood may contribute to elevated S-100β systemic blood concentrations. In addition, derived from the results of this investigation, the lack of NSE and S-100β atrioventricular (A-V) gradient elevation must be understood in terms of a non-evident cerebral cell injury and increased systemic blood concentrations of these markers as a strong evidence for extra-cerebral sources. However, this in turn may be understood as a reason for the missing A-V gradient for interleukins. Nevertheless, an increase for the investigated variables may occur in the presence of severe neurological damage, which was not observed in any of the patients in this study.
Our study may have some methodological limitations. First, the accuracy of the analytical methods are not sensitive enough to detect minor concentration differences in arterial and cerebral venous blood, which nevertheless might result in a slow increase in systemic plasma concentration. This however seems unlikely, as our results demonstrate a steep increase after termination of CPB. Second, the selected measurement points (CPB - ICU 6 h) may have been chosen falsely, thereby not detecting a transient release appropriately. This however seems unlikely, as measurements were performed repeatedly during as well as after CPB within short intervals of time.
NSE has been shown to be associated with elevated blood concentrations during and after CPB for cardiac surgery and has been correlated to cerebral injury. In this study the results are in accordance with observations reported by other investigators [22,23]. However, in contrast to S-100β the blood concentrations of NSE showed no gradual decline and remained elevated. Since NSE is contained in platelets and erythrocytes, mechanical destruction by the CPB resulting in haemolysis leads to increased plasma concentrations . Therefore, NSE as a marker for cerebral ischaemia in a setup with CPB is of limited value.
The concentration time course of inflammatory and neurobiochemical markers, during and after CPB, has extensively been investigated [25-27]. In summary, most studies show a peak after termination of CPB and a gradual decrease during the postoperative period. In principle, the results of this investigation are in accordance with these observations. Cytokine concentrations were not elevated during hypothermic CPB. Attenuation of the inflammatory reaction due to hypothermia must be considered as an explanation for this observation [17,27]. Furthermore, the lack of a significant cytokine increase must be understood as an indicator for adequate systemic perfusion by the extracorporeal circuit. However, reperfusion of the heart and lungs after aortic declamping may be responsible for the concentration time course of cytokine concentrations.
The balance of pro- and anti-inflammatory cytokines has been demonstrated in paediatric cardiac surgery with a systemic plasma increase in IL-8 concentration and a phased response in IL-10 elevation . Currently, no data are available on the arterial-cerebral venous blood concentration ratio during and after CPB as to whether pro-inflammatory cytokines represented by IL-6 and IL-8 are counterbalanced by the anti-inflammatory IL-10. With no A-V gradient apparent for the anti-inflammatory marker the results of the present study indicate an almost congruent time pattern for IL-10 compared to IL-6 and IL-8. Therefore, the overall increase must be understood in association with the elevated concentrations of IL-6 and IL-8 counterbalanced by the anti-inflammatory response. This observation supports the maintenance of a pro- and anti-inflammatory cytokine balance during and after cardiac surgery.
In summary this study demonstrates an overall increase of neuronal-ischaemic markers and cytokines during and after CPB. Furthermore, changes of arterial-cerebral venous gradients for NSE and S-100β as well as IL-6, IL-8 and IL-10 as indicators for neuronal cell injury were undetectable. Although none of our patients suffered from major neurological complications, the results of this clinical study question the specificity of increased NSE and S-100β plasma concentrations as a marker of cerebral tissue injury in CABG patients. Therefore, the increases of neuronal- ischaemic markers during CPB, as well as the elevated blood concentrations of pro- and anti-inflammatory cytokines after termination of CPB, are most probably caused by a systemic reaction due to extracorporeal circulation and not by a release from the brain.
Preliminary results of this investigation were presented in part at the annual EACTA meeting, June 2002 in Dublin, Ireland.
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