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Ann Thorac Surg 2010;89:1091-1097. doi:10.1016/j.athoracsur.2009.11.056
© 2010 The Society of Thoracic Surgeons
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Original Articles: Adult Cardiac

Postoperative Cognitive Dysfunction and Aortic Atheroma

Lisbeth A. Evered, BS, MBiostata, Brendan S. Silbert, FANZCAa,b,*, David A. Scott, PhD, FANZCAa,b

a Centre for Anaesthesia and Cognitive Function, Department of Anaesthesia, St. Vincent's Hospital, University of Melbourne, Melbourne, Australia
b Department of Surgery, St. Vincent's Hospital, University of Melbourne, Melbourne, Australia

Accepted for publication November 19, 2009.

* Address correspondence to Dr Silbert, Department of Anaesthesia, St. Vincent's Health, PO Box 2900, Fitzroy, Vic, Australia (Email: brendan.silbert{at}svhm.org.au).


ADULT CARDIAC SURGERY: The Annals of Thoracic Surgery CME Program is located online at http://cme.ctsnetjournals.org. To take the CME activity related to this article, you must have either an STS member or an individual non-member subscription to the journal.

 

   Abstract
Top
 Abstract
Introduction
 Material and Methods
 Results
 Comment
 References
 
Background: The relationship of aortic atheroma to postoperative cognitive dysfunction (POCD), a common complication of coronary artery bypass graft surgery, has not been resolved. We undertook assessment of aortic atheroma using intraoperative ultrasonography and related the degree of aortic atheroma to POCD.

Methods: Aortic atheroma was assessed using intraoperative transesophageal and epiaortic echocardiography in 311 patients who underwent coronary artery bypass graft surgery. Atheroma was graded from 0 (normal or minimal) to 3 (mobile or rough) in each of four quadrants of the proximal ascending to proximal descending thoracic aorta. Atheroma burden was defined as the atheroma score divided by the total possible score for that patient. Patients also completed a neuropsychological battery consisting of eight tests taken the week before surgery and at 1 week and 3 and 12 months afterward. Decreased cognitive function for each test was defined as an individual decrease of at least 1 standard deviation of the group baseline mean for that test, and POCD was defined as a decrease in two or more tests.

Results: The atheroma burden (%) was greater in the patients with POCD. The difference was greatest at 1 week (10.4 ± 14.7 versus 4.4 ± 9.0, p = 0.0002) and diminished progressively at 3 months (8.9 ± 14.1 versus 5.4 ± 10.1, p = 0.06) and 12 months (6.6 ± 12.0 versus 5.6 ± 10.2, p = 0.56). Multivariable analysis showed that atheroma burden strongly predicted POCD at 1 week.

Conclusions: Aortic atheroma burden predicts POCD at 1 week but has less impact on POCD as time progresses. Atheroma burden is highly correlated with age and may be a good predictor of early POCD.

Atheroma of the ascending aorta is an independent predictor of perioperative stroke in patients undergoing coronary artery bypass graft (CABG) surgery because atheroma is believed to be a rich source of cerebral emboli [1–4]. The prevalence of aortic atheroma is common among patients undergoing CABG surgery because of the diffuse nature of atherosclerotic disease, which affects many vascular beds in addition to the coronary circulation. Consequently, postoperative stroke has been reported as 1% among patients undergoing CABG surgery [5] and as high as 15.3% among patients with severe arch plaque [6]. Indeed, the recognition of the part played by aortic atheroma in stroke after CABG surgery has been an important impetus for initiating surgical techniques that minimize the manipulation and possible disruption of aortic atheroma. Thus, the surgical techniques of off-pump surgery [7] and Y-grafts [8] aim to re-establish coronary circulation without cross-clamping the aorta and thus avoid dislodgement of atheroma leading to cerebral emboli.

Postoperative cognitive dysfunction (POCD) does not manifest as overtly as stroke, but the incidence is very much greater, and it remains the most common neurologic complication after CABG surgery. The incidence of POCD depends on the time of testing after surgery, but has been reported as 53% at discharge, 24% at 6 months, and 42% at 5 years [9]. Despite intensive research into the etiology of POCD, the exact cause remains unclear. Atheroma of the aortic arch is a prominent source of cerebral emboli [10, 11], and it has been suggested that emboli that are not sufficient to cause overt stroke may manifest as POCD [12, 13]. Despite the recognition that cerebral emboli may derive from aortic arch atheroma, the link between these emboli and POCD remains uncertain [14]. Initial reports using diffusion-weighted magnetic resonance imaging (MRI) linked cerebral emboli with POCD [15], but that has not been supported by subsequent studies [16, 17].

To clarify the relationship between aortic atheroma and POCD, we assessed the extent of aortic atheroma in patients undergoing elective CABG surgery and related this to POCD.


   Material and Methods
Top
 Abstract
 Introduction
 Material and Methods
Results
 Comment
 References
 
Study Participants
The study group consisted of a subgroup of 311 patients from the 349 patients who participated in the AustraliaN Trial Investigating PostOperative Cognitive Dysfunction, Early extubation and Survival (ANTIPODES), a prospective randomized controlled trial [18]. The primary outcome was the incidence of POCD after CABG surgery in patients who had received either high or low doses of fentanyl. These patients underwent detailed transesophageal or epiaortic ultrasonography scanning (if available), or both, as part of the protocol. The results of the ultrasonographic scanning have not been analyzed previously. Institutional Ethics Committee approval was granted, and written informed consent was obtained from all patients. Eligible patients were aged 55 years or older, were scheduled to undergo elective first-time CABG surgery (on pump), and were randomly allocated to receive either high- or low-dose opioid anesthesia. Blood from the cardiotomy suction was returned to the main venous reservoir for reinfusion.

Anesthetic and Surgical Management
Premedication consisted of oral temazepam (10 to 20 mg) and ranitidine (150 mg). Intravenous midazolam (as much as 0.1 mg/kg) was used as sedation for insertion of monitoring lines. All patients received fentanyl (either 10 µg/kg or 50 µg/kg), propofol if indicated (as much as 1 mg/kg), and muscle relaxants. Maintenance of anesthesia was with propofol infusion, with the option of supplemental boluses of fentanyl. Dose adjustments were at the discretion of the anesthesiologist. The lungs were ventilated with oxygen and air, and volatile agents were not used at any time.

After manual palpation the aortic inflow cannula was inserted into a suitable part of the distal ascending aorta. Proximal and distal anastomoses were performed under single aortic cross-clamping, and no side clamp was used. For cardiopulmonary bypass, the circuit was primed with 2 L heparinized crystalloid solution. Standard hemodynamic management involved maintaining mean arterial pressures of 60 to 80 mm Hg with moderate body hypothermia and alpha-stat pH management. Tepid blood cardioplegia (approximately 25°C) was given antegrade to induce asystole with subsequent doses administered retrograde. Perfusion flow rates were 2.0 to 2.4 L · min–1 · m–2 using nonpulsatile flow. All circuits used a Cobe Optima membrane oxygenator (Cobe Cardiovascular, Arvada, CO) and a 40-µm arterial line filter.

Neuropsychological Testing
All patients completed a battery of eight neuropsychological tests administered by a trained interviewer. This was done on four occasions: (1) baseline tests, during the week before surgery; (2) early postoperative tests, 1 week (postoperative day 6) or before discharge if earlier than that; (3) intermediate postoperative tests, 3 months after surgery; and (4) late postoperative tests, 12 months after surgery.

The tests were selected because they had been used commonly and recommended in an expert consensus statement [19]. The test battery consisted of the Consortium to Establish a Registry for Alzheimer's Disease Auditory Verbal Learning Test, Digit Symbol Substitution Test, Trail-Making Test parts A and B, Controlled Oral Word Association Test, Semantic Fluency Test, and Grooved Pegboard Test (dominant and nondominant hands). All of these tests have been described elsewhere [20, 21]. Parallel forms were administered for the Consortium to Establish a Registry for Alzheimer's Disease Auditory Verbal Word Learning Test, and all the tests were administered in the same order. The National Adult Reading Test was used to estimate intelligence quotient [22] and was administered at the baseline assessment. Absolute test scores were reversed for timed tasks so that a decrease implied cognitive decline for every test.

Visual analog scales were used to assess anxiety and depression at each time of testing. Patients were asked to mark an ungraded line (10 cm in length), anchored by 0 and 100 at each end.

Postoperatively, decreased cognitive function for each test was defined as a decrease of an individual patient's score of at least 1 SD [23, 24] of the baseline mean for all patients for the relevant test (after adjusting for systematic factors such as learning and practice effects). This correction was done by calculating the change in population score at each testing time from baseline (systemic population change) and subtracting this value from each individual patient's change score [25, 26].

POCD in each patient was defined as two or more abnormal test results [27]. Tests not attempted were treated as omissions and not as failures, and if fewer than two tests were completed at one timepoint, assessment for deficit at that time point was omitted. The process of testing and the analysis of the results has been described in detail elsewhere [24].

Previous analysis has shown that the incidence of POCD was less at 1 week among patients receiving lower dose opioids (23.6% versus 13.7%; p = 0.03), but there was no difference in the incidence of POCD between high- and low-dose opioid groups at 3 and 12 months after surgery [18].

Echocardiographic Analysis
After induction of anesthesia, transesophageal echocardiography (TEE) was performed using a Vivid Five ultrasound system (GE Vingmed, Horten, Norway). An adult multiplane TEE probe (6Tv) was inserted, and a systematic examination of the heart and aorta performed by an anesthesiologist experienced in echocardiography. After exposure of the aorta, ultrasonic epiaortic scanning was performed using a sterile linear probe (10T). The combination of the TEE and the epiaortic probe allowed for the most comprehensive scanning of the aortic arch. This imaging was attempted at six sites in four quadrants (giving a maximum of 24 sites) in all patients (Table 1).


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  		Table 1 Ultrasound Probe Used to Scan Aortic Segment
 
At each site, atheroma was assessed according to the following classification based on Royse and colleagues [28]: 0 = normal aorta/minimal atheroma less than 3 mm; 1 = atheroma 3 to 5 mm; 2 = atheroma more than 5 mm; 3 = atheroma that is mobile or rough. Atheroma burden was defined as the total atheroma score divided by the total possible maximum score from all the scanned sites in each patient and converted to a percentage. As it is not always possible to produce good images at all sites, only patients who were scanned at a minimum of eight sites were included in the analysis.

Statistical Analyses
Group comparisons were made using unpaired t tests for continuous variables, the Spearman ranked correlation coefficient for ranked data, and {chi}2 or Fisher's exact test for dichotomous variables. Incidence analysis was used to examine cognition after patients were classified for POCD status [29, 30]. Atheroma burden was considered as a continuous variable and approximated a normal distribution appropriate for parametric analysis.

Associations were determined using univariable analysis and multivariable logistic regression, with a probability value of less than 0.2 set for entry into the multivariable regression models. Univariable and multivariable analysis for POCD at each time point was performed for atheroma burden, age, sex, intelligence quotient, anxiety, depression, clinical risk factors for cardiovascular disease, perioperative indicators of severity of cardiovascular disease (number of distal grafts, a history of myocardial infarction, and left ventricular function), and high- or low-dose opioid anesthesia. High- or low-dose opioid was included because it was the variable of interest in the initial study and was significantly related to POCD at 1 week postoperatively.

Tests were performed using STATA (Version 10.0; StataCorp, College Station, TX). A probability value of less than 0.05 was taken to indicate statistical significance.


   Results
Top
 Abstract
 Introduction
 Material and Methods
 Results
Comment
 References
 
Demographic and preoperative data are shown in Table 2. Three hundred and eleven patients underwent echocardiographic examination. Of these, 298 were scanned at a minimum of eight sites. Intraoperative details are shown in Table 3.


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  		Table 2 Patient Demographics, Cardiovascular Risk Factors, and Medications
 

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  		Table 3 Intraoperative Data
 
Two patients had postoperative strokes, one presenting as hemianopia and the other as mild hemiparesis. Both these patients were still able to complete all cognitive assessments. Seven patients died during the 12-month study period: 2 patients had acute perioperative cardiac complications and did not complete any postoperative tests and were thus excluded from analysis; 3 of the remaining 5 patients completed cognitive assessments at 3 months after surgery. Therefore, a total of 296 patients were included for analysis.

Successful imaging was obtained of the proximal and mid ascending aorta in 183 patients, the distal ascending and proximal arch in 136 patients (epiaortic scan), and the distal arch and proximal descending aorta in 296 patients. The atheroma burden at each site of the aorta is shown in Table 4. From the proximal ascending aorta to the proximal arch the atheroma burden did not exceed 1.8% ± 6.0%. The atheroma burden in the distal arch (8.2% ± 14.0%) and proximal descending aorta (8.9% ± 15.8%) was greater.


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  		Table 4 Atheroma Burden at Each Site of the Aorta
 
Of the 296 patients analyzed, 34 did not complete neuropsychological assessment at 1 week, 20 did not complete it at 3 months, and 22 did not complete it at 12 months. This lack of assessment was most commonly due to patients being unwell (especially at 1 week), unable to be contacted, or in a few cases, withdrawal from the trial. Consequently, the number of patients analyzed was 262 at 1 week, 276 at 3 months, and 274 at 12 months. The atheroma burden and the incidence of POCD at each time point are shown in Table 5. POCD was identified in 52 patients (19.8%), 38 patients (13.8%), and 41 patients (15.0%) at each of these time intervals. The atheroma burden in patients with POCD was greater than in patients without POCD at each testing time. This finding was statistically significant at the 1-week testing time (10.4% ± 14.7% versus 4.4% ± 9.0%, p = 0.0002).


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  		Table 5 Atheroma Burden and Incidence of POCD at Each Time Point
 
When the sum of the atheroma burden from the proximal ascending to distal arch was analyzed separately from the proximal descending arch, the same relationships held (Table 6). There was good correlation between the atheroma burden seen in the proximal descending aorta and the burden in more proximal aortic regions (r = 0.71; p = 0.00), as shown in Figure 1.


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  		Table 6 Results of Univariable and Multivariable Analyses
 

Figure 1
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  		Fig 1. Relationship of percent atheroma burden in proximal descending aorta and proximal ascending to distal aortic arch.

 
The results of the univariable and multivariable analyses are shown in Table 6. At 1 week, POCD was associated with atheroma burden and age on univariable analysis, and this association was retained only for atheroma burden on multivariable analysis. The odds ratio for predicting POCD at 1 week was 1.04 (95% confidence interval: 1.01 to 1.07), indicating an increased risk of POCD of 4% for every 1% increase in atheroma burden (p = 0.007).

Atheroma burden and age were highly correlated (r = 0.342, p = 0.00; Fig 2). Low-dose opioid anesthesia maintained an association with POCD at 1 week on both univariable and multivariable analysis. On multivariable analysis, hypertension was associated with POCD at 3 months, and number of distal grafts was associated with POCD at 12 months (Table 7).


Figure 2
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  		Fig 2. Relationship between age and atheroma burden.

 

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  		Table 7 Predictors of Postoperative Cognitive Dysfunction
 

   Comment
Top
 Abstract
 Introduction
 Material and Methods
 Results
 Comment
References
 
We have shown that patients who experience POCD at 1 week have significantly greater atheroma in the aorta than do patients who do not experience POCD. Atheroma burden was also greater at 3 months among patients with POCD but fell short of statistical significance (p = 0.06). At 1 week postoperatively, multivariable analysis showed that age, a consistent predictor of POCD [9, 31], no longer retains an association, but atheroma burden remains highly associated with POCD.

Age was a univariable predictor of POCD at 1 week in this study, but lost its significant association when covaried with atheroma burden. This relationship is explained by the strong correlation of age and atheroma burden, which is high considering the small age range of the subjects (r = 0.342, p = 0.00). Age, which has been repeatedly shown to be a predictor of POCD, may acquire this association by virtue of its link with atheroma. Indeed, age itself is purely a descriptor, not a causative factor, and must be a proxy for some etiologic mechanism. In this investigation, the close correlation of age and atheroma suggests that age may be a proxy for atheroma in the development of POCD.

The association of atheroma with early POCD does not necessarily imply that POCD is a consequence of cerebral emboli. Direct proof of a part played by emboli would require measurement of microembolic events (eg, by intraoperative transcranial Doppler) or the use of postoperative diffusion-weighted MRI. Neither of these techniques has provided unequivocal evidence that cerebral emboli are associated with POCD. A recent systematic review of the emboli detected by transcranial Doppler and cognitive function was unable to show any definitive relationship between these [32]. Similarly, one postoperative diffusion-weighted MRI study implicated cerebral emboli [33], whereas others have failed to confirm this finding [16, 17].

An alternative explanation is that the presence of atheroma in the thoracic aorta represents a measure of the severity of systemic cardiovascular disease. Aortic wall thickness has previously been shown to indicate the severity of coronary artery disease [34]. Population studies have shown that clinical risk factors for cardiovascular disease such as hypertension, diabetes mellitus, and hyperlipidemia are also risk factors for cognitive impairment in the general population [35]. It may be that atheroma of the aorta may be a good predictor of POCD, a concept underscored by previous studies that have found age to predict POCD.

Our results agree with those of Goto and colleagues [36], who showed that atheroma burden (assessed in the ascending aorta only—using an epiaortic ultrasound probe) was associated with neuropsychological dysfunction 7 days after CABG surgery. Although they utilized different neuropsychological tests, and a different definition of POCD (they defined POCD as a decrease by 2 SD from baseline score), they found that atheroma was strongly associated with POCD at day 7 (total atherosclerotic score odds ratio = 5.063 (95% confidence interval: 2.023 to 10.578); p < 0.001).

Bar-Yosef and coworkers [37] were unable to show an association of aortic atheroma burden with POCD at 6 weeks after CABG surgery. Their study differed from the present study in a number of key aspects. Firstly, they used a continuous cognitive change score (derived from primary component analysis of the neuropsychological tests) as a continuous outcome variable in addition to using a dichotomous definition as secondary endpoint. In contrast, we used a dichotomous outcome for POCD as recommended by the Consensus Statement [38]. Secondly, they did not use epiaortic scanning to visualize that part of the aortic arch that cannot be seen by TEE, thus limiting the complete assessment of atheroma. Thirdly, their definition of atheroma burden relied on complex analysis of the cross-sectional area of the aortic atheroma rather than simple grading. Finally, their study was limited to 162 patients, which may have been insufficient to detect a relationship between atheroma burden and POCD at 6 weeks.

Our study both reconciles and explains the results of Goto and associates [36], who did cognitive testing at 7 days, and Bar-Yosef and colleagues [37], who did cognitive testing at 6 weeks, because we tested patients both at 1 week and 3 months after CABG surgery. We found a strong association at 1 week in common with Goto and coworkers [36] but failed to find a significant association at 3 months in common with Bar-Yosef and coworkers [37]. However, in common with Bar-Yosef and coworkers, we were able to show a weak association of atheroma burden and POCD at later testing times that may have become significant if more patients had been studied. Post-hoc power calculations show that 388 patients would need to be analyzed to show a significant difference in atheroma burden between those with and without POCD at 3 months ({alpha} = 0.05, 1-β = 0.8). The relationship between atheroma burden and POCD was weaker at 12 months, when 3,600 patients would be required to show a significant difference.

These observations suggest that factors influencing early POCD (1 week) have a lesser effect on POCD as time progresses (3 months and 12 months). We have previously shown that low fentanyl dose is associated with POCD at 1 week but does not affect POCD at 3 and 12 months [18]. This relationship still holds independently of atheroma burden, which also predicts POCD at 1 week. Early POCD may be more vulnerable to factors that have less of an influence on POCD at 3 and 12 months. For example, residual drug effects both from anesthesia and postoperative analgesia and sedation, stress response, inflammatory response, or even oligomerization of central nervous system amyloid beta [39]. Early POCD is important because it has been shown to delay hospital discharge [18] and also to be associated with cognitive changes 5 years later [9].

In conclusion, aortic atheroma is strongly associated with POCD at 1 week after surgery, and this association weakens with time. This finding does not imply that aortic emboli originating from the atheroma lead directly to POCD, but it clearly identifies the presence of aortic atheroma as a risk factor. Atheroma burden is highly correlated with age and may be a good predictor of early POCD.


   References
Top
 Abstract
 Introduction
 Material and Methods
 Results
 Comment
 References
 

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