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Eur J Cardiothorac Surg 2001;19:611-618
© 2001 Elsevier Science NL
a Department of Cardiothoracic Surgery, Linköping Heart Center, University Hospital, Linköping, Sweden
b Department of Cardiothoracic Anesthesia, Linköping Heart Center, University Hospital, Linköping, Sweden
Received 13 October 2000; received in revised form 7 February 2001; accepted 8 March 2001.
Corresponding author. Tel.: +46-13-22-20-00; fax: +46-13-10-02-46
e-mail: rolf.svedjeholm{at}lio.se
| Abstract |
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Key Words: Neurological injury Atherosclerosis Cerebrovascular disease Postoperative complications Glutamate Glucose Cardiac surgery
| 1. Introduction |
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| 2. Materials and methods |
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CPB was conducted with a membrane oxygenator and a roller pump generating non-pulsatile flow. Ringer's acetate and mannitol were used for priming the extracorporeal circuit. During the time frame of the study moderate hemodilution (hematocrit 2025%) and moderate hypothermia (3235°C) were employed. At normothermia the target CPB flow was 2.4 l/m2 body surface area and the target pressure was 60 mmHg. Antegrade or combined ante- and retrograde delivery of St. Thomas' cold crystalloid cardioplegic solution was used for myocardial protection. Weaning off CPB was started at a rectal temperature of 3536°C. Heparin was neutralized with protamine chloride. Ringer's acetate was used for volume substitution postoperatively. Shed mediastinal blood was routinely retransfused in the intensive care unit. Postoperative rewarming was facilitated by radiant heat provided by a thermal ceiling. Postoperative anticoagulation routines during the study period are given in Appendix A.
The patients were operated by two surgeons. Perioperative management differed from traditional care by its increasing reliance on metabolic measures for treatment of postoperative heart failure. During the early part of the study period metabolic interventions were reserved for the most severe cases of postoperative heart failure [13]. Due to encouraging experience inotropes for weaning from bypass were gradually replaced by prolongation of CPB and metabolic interventions (Fig. 1 and Appendix A). During the latter part of the study period the following measures were undertaken in order, depending on the severity of heart failure: prolongation of CPB, intravenous glutamate, high-dose GIK, inotropic support and mechanical circulatory support. SvO2 and diuresis served as the main guidelines for assessment of the adequacy of the hemodynamic state, rather than cardiac output measurements [14]. The basic concepts and details regarding the metabolic interventions with intravenous glutamate and high-dose GIK have been reported previously [13,14].
2.3. Definitions
Complications presented refer to in-hospital events occurring at our institution. Neurological injury in this study included the following cerebral complications: (1) stroke; (2) depression of consciousness or confusion if associated with signs of cerebral injury on CT scan or focal neurological deficit; and (3) transient ischemic attacks with focal neurological deficit. The majority of patients with suspected neurological injury were examined by a neurologist and by CT scan. Cognitive dysfunction was not assessed. Perioperative myocardial infarction was diagnosed according to screening routines at our institution [14]. Patients were classified as having had postoperative heart failure if the hemodynamic state had warranted active measures directed at improving myocardial function such as prolonged CPB, metabolic, pharmacological or mechanical circulatory support. In general, this implied difficulty in weaning from CPB or deteriorating hemodynamics (usually decreasing SvO2 below 5055% associated with either oliguria, decreasing arterial blood pressure or increasing atrial filling pressures) after weaning from CPB. A history of cerebrovascular disease was defined as previous stroke or transient ischemic attack, or prior surgical repair of or documentation of obstructive carotid disease (
50%) on angiography or duplex ultrasound examination.
2.4. Statistical analysis
Data are presented as mean±standard deviation (SD). Non-parametric tests, the MannWhitney U-test and Fisher's exact test, were used for comparison of patients with and without postoperative cerebral complications. For evaluation of independent risk factors for postoperative cerebral complication, univariate logistic regression was first employed. Variables undergoing univariate analysis are given in Appendix A. Variables were tested in a stepwise forward multivariate logistic regression model if the univariate P value was less than 0.25. Due to the limited number of events cross-validation of the final model was undertaken. Statistical significance was defined as P<0.05. Statistical analyses were performed with a computerized statistical package (Statistica 5.5, StatSoft, Inc., Tulsa, OK).
| 3. Results |
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3.2. Outcome in patients with neurological injury (Tables 1 and 2)
Two patients died during primary hospitalization. One further patient died during rehabilitation. One-year mortality was 20.0% compared with 2.0% in patients without cerebral complications. The median duration for hospitalization including recovery stay at the referring hospital was 21 days. Complete or near complete recovery was reported in half of the surviving patients. In the remaining patients various residual symptoms were described in the records as presented in Table 2.
3.3. Risk factors for neurological injury
The patients with cerebral complications were on average 4 years older (69±11 years) compared with those without postoperative cerebral complications. A previous history of cerebrovascular disease was found in one-third and signs of advanced peripheral vascular disease or calcification of the ascending aorta on palpation were found in another third of the patients with cerebral complications (Table 2). The patients with cerebral complications also had more extensive coronary artery disease as indicated by a higher number of bypasses performed. Furthermore, the patients with cerebral complications had longer aortic cross-clamp time and CPB time, and postoperative heart failure and atrial fibrillation were more frequently encountered.
The incidence of cerebral complications was 3.5% in diabetics, 3.5% in patients with postoperative atrial fibrillation, 3.9% in patients aged 70 or older, 5.4% in patients undergoing combined valve and CABG procedures, 6.0% in patients with COPD and 8.2% in patients with a previous history of cerebrovascular disease.
According to forward stepwise multiple logistic regression analysis, a history of cerebrovascular disease was the most important risk factor for postoperative neurological injury. Advanced age, aortic cross-clamp time, the number of bypasses and history of COPD also emerged as independent risk factors (Table 3). Of these variables COPD and the number of bypasses did not resist cross-validation.
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To address the potential role of postoperative heart failure as a risk factor for neurological injury, a model recalculation was undertaken by entering this variable to the final multivariate model. This yielded a risk model with an odds ratio (OR) of 2.7 (95% confidence interval (CI) 0.98.6, P=0.08) for postoperative heart failure. Repeated recalculation of this model (including postoperative heart failure) by separately entering each of the measures to treat or prevent postoperative heart failure resulted in the following ORs with respect to neurological injury: use of high-dose GIK, OR 0.3 (95% CI 0.11.7, P=0.16); use of intravenous glutamate, OR 0.3 (95% CI 0.12.0, P=0.23); inotropes for weaning from CPB, OR 1.8 (95% CI 0.311.6, P=0.51); and use of mechanical circulatory support, OR 2.1 (95% CI 0.225.3, P=0.56).
| 4. Discussion |
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History of cerebrovascular disease emerged as the most important independent risk factor for postoperative cerebral complications [6,8,15,17,18]. Furthermore, advanced age, prolonged aortic cross-clamp time, COPD and number of bypasses emerged as independent risk factors in the multivariable analysis. COPD and number of bypasses did not resist cross-validation of the final multivariate model and, hence, their role should be interpreted cautiously. However, these findings deserve attention as other studies also suggest that these factors predict neurological injury after cardiac surgery [6,18,19]. Possible mechanisms behind the relationship between COPD and neurological complications include increased hemoglobin levels and carbon dioxide retention with associated changes in cerebral vasoreactivity [18,20]. The extent of coronary artery disease has been identified as a strong predictor for postoperative neurological injury [19]. This could reflect the degree of associated systemic atherosclerotic disease but also a relationship between the extent of coronary disease and postoperative heart failure (the latter being associated with a three-fold increase in the incidence of stroke) has been described [19].
Although it did not enter the final risk model our results also suggest that postoperative heart failure is a potential risk factor for neurological complications. Further support has been provided by more recent data showing the need for inotropes to be a risk factor for neurological injury [6]. Moreover, other investigators have found the combination of atrial fibrillation and low output syndrome to predict delayed strokes [8]. In our study, patients treated with inotropes for weaning or mechanical circulatory support had a higher incidence of neurological complications, which considering the employed strategy suggests that the severity of postoperative heart failure could play a role in the risk of developing neurological injury. No evidence or tendencies suggesting an increased risk for neurological complications associated with the use of intravenous glutamate or high-dose GIK were found.
The concerns regarding the potential neurotoxicity of glutamate have been based on animal experimental data mainly obtained in rodents [11,12]. However, in primates glutamate does not pass the blood brain barrier and exogenous glutamate has therefore not been shown to cause cerebral injury [12]. In cardiac surgery, the possible influence of CPB on the blood brain barrier has to be considered. As the cerebral concentration of glutamate is reported to be 50100 times higher than in blood it is essential that results are interpreted in relation to the dosages applied [12,13]. The dosages of glutamate employed here were moderate and have been reported to cause two- to three-fold elevations of whole blood levels [12]. The present study supports the safety of such dosages but obviously further surveillance is warranted. The situation regarding GIK is related to the quality of blood glucose management. Both hypoglycemia and hyperglycemia are potentially hazardous to the brain. Our database did not include results of blood glucose measurements and, hence, further studies with respect to blood glucose management are warranted. Furthermore, assessment of the extent of neurological injury would have been desirable as the major concern with hyperglycemia is aggravation of existing injury [9,10]. Currently it seems that GIK can be safely used in cardiac surgery provided that blood glucose management is adequate. In fact, properly managed it can be used to improve blood glucose control, which potentially could have a beneficial effect on the outcome of neurological injury [21].
One further observation related to the treatment of postoperative heart failure is the discrepancy regarding CPB time and aortic cross-clamp time as risk factors for neurological injury. Previous studies have identified CPB time as a risk factor [6,8,17,22]. Potential hazards associated with CPB include hypoperfusion, microembolism and inflammatory response [23]. Furthermore, prolonged CPB can be a marker of a complicated procedure or a more severe state of postoperative heart failure [6]. In our multivariable analysis, aortic cross-clamp time was found to be an independent risk factor for neurological injury. However, CPB time did not emerge as an independent risk factor although CPB time was longer in the group of patients with neurological injury. The diverging results could be explained by the employed strategy, which was adopted due to concerns that premature termination of CPB with the aid of inotropes in postoperative heart failure can delay myocardial recovery and lead to more severe states of heart failure. Consequently, prolonged reperfusion of the heart on CPB (together with metabolic support) was used as the first line of treatment in postoperative heart failure and therefore usually did not reflect a severely compromised hemodynamic state or a complicated procedure. Given the circumstances, an extended aortic cross-clamp time more likely reflected a complicated procedure or advanced atherosclerotic disease. An extended aortic cross-clamp time obviously also prolonged the CPB time and with regard to the potential hazards associated with CPB it can be speculated that extra-corporeal perfusion during cardiac arrest may be less adequate for the brain. In fact, it has recently been suggested that the quality of pulsatile flow may be important for brain perfusion during CPB [24].
In individual patients the cause for neurological injury may occasionally be identified with reasonable certainty. One of the patients with postoperative heart failure and neurological injury had been resuscitated immediately prior to surgery, which illustrates that preoperative hemodynamic state also deserves consideration. One patient with recurrent atrial fibrillation developed a stroke in close association with conversion from atrial fibrillation to sinus rhythm. However, atrial fibrillation did not emerge as an independent risk factor although it was more common in patients with neurological complications. This association can be explained by the age distribution but it is noteworthy that other investigators have identified atrial fibrillation as a risk factor for stroke [7,8]. This issue and the observation that more than one-fourth of the neurological complications presented a few days after surgery suggests that coagulative state could play a role in the development of this complication. Interestingly, two of the surviving 12 patients developed pulmonary emboli (Table 2). It remains to be clarified if improved anticoagulation protocols could help to reduce the risk for late neurological events.
Retrospective studies generally yield a lower incidence of neurological complications as compared to prospective studies [2,4]. Furthermore, the reported complications refer to events during hospitalization at our hospital and events after discharge may have been missed [4,8]. Thus, although the study included 775 procedures it was small considering the incidence of neurological complications. The limited number of events renders multivariate analysis susceptible to misclassification and as false positive diagnosis can be expected to have a more pronounced influence only patients with certain neurological injury were included. These circumstances also explain the wide CIs obtained and, hence, ORs should be interpreted cautiously. Cross-validation of the model supported the validity of the main risk factors and the overall incidence of cerebral complications did not differ markedly from the previous studies. Also, other variables traditionally associated with neurological complications such as combined CABG+valve procedures and diabetes, that did not emerge as independent risk factors, exhibited an incidence of neurological injuries in agreement with recent large-scale studies [4,6]. It is conceivable that history of cerebrovascular disease, advanced age, aortic cross-clamp time and extent of coronary disease all to some extent constitute markers for advanced atherosclerotic disease.
Several measures that have been introduced in recent years to prevent neurological injury appropriately address atheromatous disease of the ascending aorta [25]. Epiaortic scanning of the ascending aorta by ultrasound can identify patients with atherosclerotic aortas and provide guidance in the choice of strategy [8]. Manipulation of the aorta can be reduced by a single-clamp technique or avoided by off-pump surgery on the beating heart [25,26]. Use of intra-aortic filtration to capture particulate emboli in association with CPB is another measure that deserves evaluation [27]. Epiaortic scanning of the ascending aorta was not available at our institution during the study period. Since its introduction measures to reduce or avoid manipulation of the aorta are frequently undertaken. Further studies are warranted to address the impact of these strategies on neurological outcome.
To conclude, our results support previous findings suggesting history of cerebrovascular disease to be the most important risk factor for postoperative cerebral complications and in general identify markers for advanced atherosclerotic disease to be of importance. Use of intravenous glutamate and high-dose GIK did not increase the risk for neurological injury.
| Acknowledgments |
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| Appendix A |
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A.2. Postoperative anticoagulation routines during the study period
CABG patients were given 500 mg acetylsalicylic acid (ASA) rectally 8 h after surgery and thereafter 160 mg ASA orally every day. Heparin (5000 IU) was given subcutaneously three times daily until the patients were fully mobilized. After valve replacements warfarin treatment was started on the first postoperative day. Heparin (7500 IU) was given subcutaneously three times daily (starting 12 h postoperatively) until the prothrombin level had reached the therapeutic level.
A.3. Metabolic interventions during the study period
The changing measures for treatment and prevention of postoperative heart failure are given in Fig. 1. Overall inotropes for weaning from CPB were used in 24 patients (3.1%) and metabolic support was given to 148 patients (19.1%). Glutamate was used in 117 patients (15.1%), in 65 patients (8.4%) as treatment for perioperative heart failure and in the remaining cases prophylactically because of severe myocardial ischemia or for study purposes. High-dose GIK was given to 82 patients (10.6%) mainly because of cardiac failure on weaning from CPB. Seventy of these patients also received glutamate infusion.
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kanson E., Vanhanen I. Glutamate and high-dose glucose-insulin-potassium (GIK) in the treatment of severe cardiac failure after cardiac operations. Ann Thorac Surg 1995;59:S23-S30.
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