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Eur J Cardiothorac Surg 2003;23:334-340
© 2003 Elsevier Science NL
Heart Center, Department of Rehabilitation Medicine and Department of Health and Society, Faculty of Health Sciences, Linköping University Hospital, 581 85 Linköping, Sweden
Received 9 April 2002; received in revised form 16 September 2002; accepted 2 December 2002.
* Corresponding author. Tel.: +46-13-224824; fax: +46-13-100246
e-mail: ewa.ahlgren{at}lio.se
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Abstract |
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 Top Abstract 1. Introduction2. Materials and methods3. Results4. DiscussionReferences |
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Key Words: Cardiac surgery • Cerebral complications • Cognitive decline • Driving performance
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1. Introduction |
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TopAbstract1. Introduction2. Materials and methods3. Results4. DiscussionReferences |
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The mechanisms responsible for cerebral complications are multifactorial and include cerebral embolization originating from the surgical field, the ascending aorta and the cardiopulmonary bypass (CPB) circuit. Hypotension and a low blood flow state during the heart operation may also play a role in the mechanisms of cerebral injury in patients with a reduced cerebrovascular collateral reserve (for review see Ref. [6]). Anesthesia and nonspecific effects of surgery are also discussed as causes of postoperative cognitive impairment in the elderly. Previous findings suggest that postoperative cognitive dysfunction may persist in about 10% of patients three months after non-cardiac surgery [7]. Despite the substantial number of studies reporting cognitive impairment after cardiac surgery, the short- and long-term effects of this impairment on everyday behavior have not received much attention. An activity of daily life not previously studied, in this particular patient population, is the ability to drive safely after heart surgery. Driving is a complex skill that largely relies on automated motor performance and flexibility learned through continual practice, but also involves attention, rapid cognitive processing and fast judgment [8,9]. Persons above 60 years of age are the fastest growing segment of the driver population and results from a recent study indicate that patients with coronary artery disease are to a large extent active car drivers and continue to drive despite old age [10]. It is not known whether decline in cognitive function after cardiac surgery influences driving performance. Considering the fact that about 800 000 patients worldwide undergo CABG every year [11], this is a topic of great importance.
The aim of this prospective study was to evaluate cognitive function and driving performance, both in real traffic and in an advanced driving simulator before and 4–6 weeks after elective CABG.
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2. Materials and methods |
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TopAbstract1. Introduction2. Materials and methods3. Results4. DiscussionReferences |
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The study period was from April 1999 to September 2000. The study was approved by the Ethics Committee of the University Hospital, Linköping, Sweden.
2.1. Interventions
In the study group standard non-pulsative cardiopulmonary bypass (CPB) technique with membrane oxygenators, roller pumps, moderate hypothermia (32–35 °C) and normocapnia was used. General anesthesia was administered using fentanyl, thiopental or midazolam and isoflurane [1].
In the control patients’ coronary intervention was performed using standard angiography and PCI procedures [12]. The procedure was carried out under local anesthesia without sedation.
2.2. Neuropsychological examination (Table 1)
The neuropsychological test battery consisted of tests covering cognitive domains that have shown to be affected after cardiac surgery such as verbal and visual memory, psychomotor speed, attention and concentration [13]. In addition a set of computerized tests considered important when assessing the ability to drive was included [14]. Testing and scoring were performed by two research nurses, blinded to the results of the driving tests, with special training in cognitive function measurements and supervised by an experienced neuropsychologist (A.L.). The test conditions were standardized and external distraction avoided. Any patient showing a drop of more than one (pre-intervention) standard deviation after intervention was considered to have deterioration for that test variable. A cognitive decline after intervention was defined as deterioration in at least two test variables in the neuropsychological test battery [15]. The evaluation of neuropsychological test results was done in collaboration with the neuropsychologist (A.L.).
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The driving inspector rated the patients performance in each area using a rating scale ranging from 1 to 5, with steps of 0.5. A rating of 3 or above is generally considered a pass. Immediately after the driving task, the patients also rated themselves, for each of these areas using the same scale.
2.4. Simulator driving
The Swedish Road and Transport Research Institute (VTI) advanced driving simulator, previously used in studies concerning driving behavior and cognitive demands, consists of a moving-base system, a wide-angle (120°) visual screen, a vibration-generating system, a sound-regulating system, and a temperature-regulating system [18,19]. These five systems can be controlled to operate in a way that gives the driver a realistic dynamic impression of traffic scenes. The interior of the driving simulator was a standard Volvo 850. The test route was 80 km but before the actual test the participants had a 20-km training session. The participants were instructed to drive normally. The test route, mimicking a two-lane tarmac road, consisted of long sequences of uncomplicated driving conditions combined with complex situations such as junctions, road works and unpredictable situations like a moose suddenly running out in front of the car. During the ‘drive’ a mobile telephone rang and the participants answered by pushing a button on the instrument panel. The two test routes, before and after the coronary intervention, had the same degree of difficulty but the sequences of complex situations and distracting tasks were different. The moose situations were not exactly alike. The dependent measurements from the simulator driving were: Speed (km/h), lateral position (meter), reaction time (s) and time to collision (TTC) (s). TTC is the time before a following vehicle collides with a leading vehicle if the speed of the vehicles remain constant. TTC reflects safety margin in time.
2.5. Statistical analysis
All statistical analyses were performed using SPSS for Windows version 10.1. Results are given as mean values with one standard deviation (±1SD). To evaluate differences between groups and within groups Mann–Whitney U and Wilcoxon signed rank tests were used, since a number of measurements were made on the ordinal scale, and few of the continuous variables met normality assumptions. Differences in absolute frequencies between groups of patients were analyzed using Pearson's chi-square test. P-values less than 0.05 were considered statistically significant.
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3. Results |
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TopAbstract1. Introduction2. Materials and methods3. Results4. DiscussionReferences |
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As seen in Table 2, the study and control groups were comparable as regards to baseline characteristics, except that three-vessel coronary artery disease was more common in CABG patients than in PCI patients (P<0.05).
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3.2. On-road driving before and 4–6 weeks after coronary artery intervention (Table 4)
Before intervention there were no statistically significant differences in driving scores between patients in the study and control group. After intervention CABG patients tended to deteriorate in traffic behavior (P=0.01) and in attention (P=0.04), while PCI patients tended to deteriorated in maneuvering (P=0.04). There were no differences in driving scores related to education level. Patients in both groups rated themselves about one point higher (range: 0.7–1.2) than did the driving inspector in all five areas, both before and after the intervention (P<0.001).
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4. Discussion |
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TopAbstract1. Introduction2. Materials and methods3. Results4. DiscussionReferences |
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Newman and coworkers reported in a recent study that there appears to be a biphasic course in cognitive change after cardiac surgery. After an immediate postoperative decline in cognitive function in 53% of the patients at discharge, the patients improved and after 6 weeks and 6 months 36 and 24%, respectively, still had cognitive impairment. However, after 5 years 42% of the patients performed below their baseline levels [3]. In the present study, neuropsychological test scores at group level showed unchanged or improved test results after surgery. This is a common finding and is explained by the pronounced effect of practice in testing which caused the overall group performance to improve after the operation [20]. Accordingly, when the postoperative mean is compared with the preoperative mean the decline in cognitive function of some individuals is overshadowed by the improvements in others. In contrast, PCI patients improved more than CABG patients in Complex reaction time and in Simultaneous capacity after intervention and this finding may indicate a cognitive deterioration in reaction time and divided attention after CABG.
Since the ability to drive a car requires a set of cognitive complex skills and the fact that even subtle neuropsychological deficits are of importance for driving performance, several studies have investigated driving performance in patients with medical conditions affecting cognitive function [18,19]. To our knowledge driving performance after cardiac surgery has not previously been studied. In a review Blumenthal et al. emphasized the importance of further research in this particular field [21]. In the present study patients in both groups tended to deteriorate in all areas of the on-road driving test after intervention. The major changes in the CABG group were in traffic behavior and attention, and in the PCI group in maneuvering. We have no obvious explanation of these results but it is noteworthy that CABG patients deteriorated in more complex and more cognitive demanding areas of the on-road driving test. Also, the deterioration in driving performance appeared to be mostly confined to the patients older than 65 although due to the small numbers of patients this finding must be regarded as tentative. Previous studies have shown that elderly drivers have difficulties at intersections and in roundabouts and are also more often than younger drivers involved in traffic accidents in these complex traffic situations [22]. Different models of driving behavior have been used to describe the driving process and one of the most cited is Michon's three-level hierarchic model [23]. Decisions on the strategic level concerns planning safe driving, taking into account weather, time and the personal condition and is generally made before actual driving. In real traffic, cognitive processing and decisions are made at the tactical level such as judgment of traffic situations and adaptation of speed and distance. Attention combined with speed of information processing makes it possible to select from several possible solutions.
The operational level consists of elementary driving tasks based on automated, over-learned perceptual-motor functions such as steering, accelerating, shifting and braking. Nine of 11 CABG patients with a significant postoperative cognitive decline deteriorated in some areas of the on-road driving test such as speed adjustment to current traffic situations, positioning of the car in the road lane and in traffic behavior including looking ahead and adjusting to traffic rules and traffic conditions, i.e. tactical level. This may indicate that cognitive functions important for safe driving may be influenced after cardiac surgery, at a time when most of the patients probably resume driving after their heart operation.
The finding that deterioration in maneuvering was seen in the control group was unexpected. Focal neurological deficits after PCI are rare and angioplasty has not been associated with short-term cognitive impairment but data are sparse [24].
The driving scores before intervention in both groups were unexpectedly low when compared with scores for a healthy control group of the same age in another study using the same test route and evaluation criteria [19]. This may indicate that widespread atherosclerotic disease itself has an impact on cognitive functions and driving ability.
There was a great discrepancy between the self-rating of the patients in both groups and how the driving inspector evaluated their driving ability in the on-road driving test both before and after the intervention. Other studies have reported similar data and most people tend to overestimate their own driving skill and safety [25]. This may not be so important if driving performance is at a high level but in this case it seems that the patients were not fully aware of the deficits in their driving performance which may increase the risk for unsafe driving and motivation to take compensatory action.
Driving performance in the simulator in both groups appeared largely unaffected after the intervention. The absence of postoperative deterioration in the driving simulator test may be explained by compensatory behavior with very careful driving in this new, insecure driving situation [8,18]. The design of the simulator test route was a country road with low-density traffic including complex sections as well as unpredictable situations in order to reveal changes in driving performance at the tactical level. The drawback of creating more complex situations in the simulator are the problem of nausea, which increases. Patients would also be expected to drive even more carefully to compensate for the increased complexity tending to make simulator driving less realistic [8].
The on-road driving test, which involves both urban and rural areas and contains more complex driving situations, seems sensitive in detecting changes in driving performance [18,19]. There may be a risk for judgment bias by the inspector but the driving route and evaluation criteria were standardized and used in previous studies of driving performance in other patient groups [17–19]. Anxiety and tiredness of the patient and variation in traffic density, weather and road conditions may also have affected performance but the conditions were similar for both groups.
One limitation of the study is the small number of patients included. As the test procedures in this study were both extensive and costly, study size had to be limited.
Treatments (CABG or PCI) were not randomly assigned and although care was taken to match the two groups with respect to age, gender, education and driving experience, a systematic difference between the two groups cannot be ruled out. More patients in the CABG group had three-vessel coronary artery disease but the two groups were similar with respect to risk factors for cerebrovascular disease such as hypertension, diabetes, peripheral vascular disease as well as baseline neurocognitive test score and driving behavior.
In conclusion, data from this study indicate that patients with coronary artery disease to a larger extent suffer a decline in cognitive function after surgery with cardiopulmonary bypass than after percutaneous coronary intervention in local anesthesia. Patients with a cognitive decline also tend to be the same patients who show deterioration in driving performance after intervention. Further research on the impact of cardiac surgery on neurocognitive function and driving performance seems warranted.
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Acknowledgments |
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References |
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TopAbstract1. Introduction2. Materials and methods3. Results4. DiscussionReferences |
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