Mild to Moderate Atheromatous Disease of the Thoracic Aorta and New Ischemic Brain Lesions After Conventional Coronary Artery Bypass Graft Surgery

The presence of new brain infarcts detected by MRI has been reported in almost one third of patients after coronary artery bypass graft (CABG) surgery.^1–4 The purpose of this study was to determine the role of proximal thoracic aortic atheroma (ascending aorta and aortic arch) in predicting cerebral embolic events and new ischemic brain infarcts in patients undergoing CABG surgery.

Methods

Study Population and Management Strategies

After institutional research board approval, informed consent was obtained from 110 patients >60 years of age scheduled for CABG surgery. Patients with history of stroke, claustrophobia, atrial fibrillation, and symptomatic carotid artery disease were excluded. Anesthetic and cardiopulmonary bypass (CPB) management was standardized.

Thoracic Aorta Assessment

Intraoperative transesophageal echocardiography and epiaortic scanning was used to evaluate proximal thoracic aorta. Normal aorta was defined as intimal thickness ≤2 mm (grade 0). Atheroma was classed as >2 to 4 mm (grade 1), >4 mm (grade 2), and mobile atheroma (grade 3). Patients with grade 1 to 3 atheroma were allocated to high-risk group and patients with grade 0 atheroma to low-risk group.

Cerebral Emboli Detection

Using transcranial Doppler, the middle cerebral artery was monitored continuously from 2 minutes before cannulation of the aorta to 2 minutes after aortic decannulation. The technique of detection and analysis of embolic hits was used as described previously.⁵ The embolic load (sum of embolic hits) was calculated for 2 time periods: first, 1 minute before and 2 minutes after surgical interventions (aortic cannulation and decannulation, cross-clamp application and removal, CPB start and end, and start of cardiac ejection), and second, embolic load during entire CPB.

MRI Scanning

MRI was performed 3 to 7 days after surgery. New ischemic lesions were identified as regions of signal hyperintensity on diffusion-weighted imaging (3 orthogonal gradient direction; b=1000 s/mm²) and signal hypointensity on the apparent diffusion coefficient maps.

Consciousness Assessment and Monitoring

The NEECHAM Confusion Scale⁶ was used for assessment and monitoring of patient consciousness level preoperatively (baseline) and postoperatively daily (days 1 to 5).

Statistical Analysis

The emboli count was analyzed with Mann-Whitney U test, and the association between atheroma and MRI brain infarcts was assessed using a 2×2 contingency table. One-way ANOVA was used to compare the NEECHAM scores. A P value <0.05 was considered significant.

Results

Demographic Data

Patients in the high-risk group were considerably older (71±6 [n=38]) versus 67±6 [n=72] years; P=0.004) and were more likely to have impaired left ventricular (LV) function (26 versus 11%; P=0.041).

Aortic Atheroma Characteristics

The mean thickness of atheroma/intima was 4.8±1.9 and 1.4±0.6 mm in the high- and low-risk groups, respectively (P<0.00001). Eleven patients presented with grade 1/grade 2 atheroma in the ascending aorta. Twenty-three patients had grade 1/grade 2 atheroma restricted to the aortic arch with normal ascending aorta. Grade 3 atheroma was identified in 4 patients (1 ascending aorta and 3 aortic arch). Surgical management was modified in these 4 patients. Distal arch cannulation with conventional CPB was performed in 2 patients, and beating heart surgery without CPB was achieved in the other 2 patients.

MRI Findings

A total of 50 (45%) patients underwent MRI investigation. In patients who had MRI, the overall prevalence of new ischemic brain lesions was 16%; however, new lesions were found only in the high-risk group (Table). Half of these patients had a single 3- to 12-mm lesion, and the other half presented with multiple 3- to 15-mm lesions. Four patients with new brain lesions had the atheroma restricted to aortic arch. Pre-existing lacunar infarcts were present in 3 and 4 patients in the high- and low-risk groups, respectively (P=0.63). One patient with pre-existing infarct had a new brain lesion.

Transcranial Doppler Findings

Adequate transcranial Doppler signal was acquired in 24 and 52 patients from the high- and low-risk groups, respectively (P=0.328). The high-risk group had a 3-fold increase in embolic events during CPB (Figure 1).

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Assessment of Consciousness

Confusion was identified in 6 (16%) and 5 (7%) patients in the high- and low-risk groups, respectively (Figure 2). None of the patients had any clinically significant metabolic changes that would justify the altered level of consciousness. One patient in the high-risk group had a clinical stroke with a lesion in the motor cortex of the left hemisphere. There was a significant correlation between the NEECHAM scores and embolic count in the high-risk group only (r=0.63; P<0.001). Patients with mobile atheroma had good clinical outcomes and no neurological sequele. Postoperative morbidity and hospital length of stay were comparable between the 2 groups.

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Discussion

The new brain infarcts were found in 8 patients (n=6 grade 1 and n=2 grade 2 atheroma). Four of these patients had the atheroma restricted to aortic arch. Proximal thoracic atheroma was associated with increased intraoperative embolic load, a change in postoperative mental status, and development of acute ischemic brain infarcts. The prevalence of these brain infarcts in patients with atheroma exceeded 60%. These findings suggest that mild to moderate atheroma of the proximal thoracic aorta was a major contributor to ischemic brain injury after cardiac surgery. The surgical procedure was altered in 4 patients with mobile atheroma. All of these patients had good clinical outcomes and no neurological sequele.

Patients in the high-risk group presented for surgery on average 4 years later, most likely accounting for the observed difference in the degree of atherosclerosis in the thoracic aorta between the 2 groups. Furthermore, these patients were more likely to have impaired LV function preoperatively. Advanced age and impaired cardiac function have been recognized previously as independent predictors of neurological complications after cardiac surgery.^7,8 Our results suggest that these predictors may simply represent a surrogate measure of the degree of atheromatous changes of the thoracic aorta, which has not been assessed consistently in the previous studies.

In conclusion, this investigation demonstrates that a patient population at increased risk of suffering new ischemic brain lesions as a result of heart surgery can be identified reliably before surgery by means of detailed comprehensive echocardiography. At present, it is unclear, whether the choice of alternative surgical, endovascular, or medical therapy would result in better neurological outcomes in patients with mild and moderate aortic atherosclerosis. However, the absence of new MRI-identified infarcts in the low-risk group suggests that for patients with no significant atheromatous disease, routine CABG with CPB is a safe and effective management strategy. Patient stratification based on the aortic atheromatic burden should be addressed in future trials designed to tailor treatment strategies to improve long-term outcomes of coronary heart disease and reduce the risks of perioperative neurological injury.

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