The incidence of stroke after CABG increases with age about 6-fold and advanced age is considered as an indicator of raised atherosclerotic burden. The post-CABG stroke risk is higher in older patients with cardiovascular disease than younger patients with no risk factors for cardiovascular disease.13 Mérie et al. reported a moderate increase in the risk of stroke after CABG with an age increase.14 In Yamamoto et al.’s study on patients from the CREDO-Kyoto PCI/CABG Registry Cohort-3, advanced age is reported as an independent risk factor of periprocedural stroke post-CABG and PCI.10 In a study by Aribox et al., congestive heart failure, chronic renal disease, previous cerebrovascular disease, and atrial fibrillation were significantly associated with ischemic stroke in 85 years old or older patients.15

Studies have shown that undergoing CABG in the acute phase after AMI has been recognized as a risk factor for ischemic stroke. Besides, the risk of stroke was increased while the CABG was done later after AMI, however, the occurrence rate was noticeably lower with late CABG in comparison to early CABG.16 In another study by Anwar Shah et al., MI was related to the increased occurrence of stroke after CABG.17 In a study by Åström et al., in the early cohort, the rates of stroke were respectively 15.8% and 10.9% in patients with and without CABG, and in the late cohort, the rates were 4.0% and 2.3%, in patients with and without CABG, respectively.16 The increased risk of stroke after AMI seems to be related to pro-thrombotic factors, including inflammation, platelet, and sympathetic activation.18

Post-operative atrial fibrillation happens in about 30% of CABG patients before they discharge from the hospital and is an important stroke predictor.19 A meta-analysis by Megens et al. reported that the risk of stroke in CABG patients with post-operative atrial fibrillation remained meaningfully higher during a 2 years follow-up in comparison to patients without post-operative atrial fibrillation.20 Lahtinen et al. study reported that post-operative atrial fibrillation associated with stroke after CABG occurred in around one-third of patients, and the meantime was 21 h.21 In another study by Kollar et al. from 6 CABG patients with AF, 3 strokes happened within 1 week after conversion to normal sinus rhythm. One patient with pre-operative and AF who underwent emergency CABG woke up with a stroke.22

In a study by Anwar Shah et al., diabetes mellitus was related to the increased occurrence of stroke after CABG.17 In another study by de Oliveira et al., it was shown that diabetes mellitus is an important factor of higher risk of stroke during the first 24 h after CABG.23 Casto et al. also showed that diabetes was a statistically significant (P = .04; OR 2.61) risk factor for stroke after CABG.12 Baker et al.’s study on 4380 patients who underwent CABG, diabetes was demonstrated as a risk factor for stroke after CABG.24 In another study by Bucerius et al., diabetes was determined as a predictor of stroke after cardiac surgery.4

The occurrence of post-operative stroke in patients who underwent CABG and had a history of stroke was significantly higher than in patients who had no prior history of stroke.25 Mao et al. review study on 14 studies showed that prior cerebrovascular disease or stroke is a consistent independent predictor for stroke after CABG.11 Cao et al. showed that unstable angina, post-operative AF, post-operative hypotension, and left ventricular ejection fractions (LVEF)≤50% are independent risk factors for post-CABG recurrent stroke in patients with a former history of stroke.26

Cardiac surgery commonly includes manipulation of the ascending aorta by arterial cannulation, cross-, and side-clamping and these may raise the chance of embolization of the atherosclerotic substances to the brain.27 Linden et al. demonstrated that patients who had atherosclerosis in the ascending aorta had an 8.7% occurrence of stroke after cardiac surgery and the risk is associated with the existence, place, and size of the disease.28 It was reported that atherosclerosis of the ascending aorta (ascending aortic wall thickness of ≥4 mm) was a risk factor for post-CABG strokes.29 In the Asenbaum et al.’s study, all patients with CABG-related stroke had atherosclerotic disease of the ascending aorta and the stroke rate was significantly related to the existence and degree of atherosclerotic disease.30

In the Lee et al.’s study, it was found that cerebral atherosclerosis was closely associated with the incidence of stroke after CABG. It is an independent risk factor for strokes and the reason for a substantial proportion of it. They showed that intracranial, extracranial, and extracranial/intracranial cerebral atherosclerosis were significantly related to stroke.31

Low cardiac output syndrome is a significant predictor of post-CABG stroke.32 In Likosky et al.’s study on 11 825 patients who underwent CABG, it was shown that low cardiac output syndrome augmented the stroke risk with an OR of 3.32 In Stamou et al.’s study on 16 528 patients who underwent CABG during 1989–1999, low cardiac output was associated with post-operative stroke.33

Dashe et al. in their study revealed that the post-CABG stroke risk was raised in patients with severe extracranial carotid stenosis or occlusion and the stroke incidence had a positive association with the degree of carotid stenosis.34 Lescan et al. also suggested that asymptomatic contralateral carotid stenosis is a risk factor for stroke after cardiac surgery in patients who had carotid occlusion before cardiac surgery.35 Naylor et al. in a review study reported that the stroke risk raised to 3% in CABG patients with unilateral carotid stenosis, 5% in patients with bilateral carotid stenosis, and 7%–11% in those with carotid occlusion.36

In Likosky et al.’s study on patients who underwent CABG, it was shown that the long use of inotropes augmented the stroke risk with an OR of 5.32

In de Oliveira's study, it was revealed that systemic arterial hypertension is a determinant factor of a higher risk of stroke during the first 24 h of the post-operative period after CABG.23 In another study by Kangasniemi et al., hypertension was reported as a risk factor for stroke after CABG.37 In Bucerius et al.’s study also, hypertension was defined as a predictor of stroke after cardiac surgery.4

In a study, it was shown that cardiopulmonary bypass (CPB) for 90–113 min (OR = 1.59, P = .022) and for 114 min or more (OR = 2.36, P < .001) were a significant predictor for stroke after CABG.38 A systematic review by Mao et al. reported that prolonged cardiopulmonary bypass time, 2 h, had a consistent positive correlation with post-CABG stroke.11 Santos et al. reported that cardiopulmonary bypass for more than 110 min is a risk predictor for stroke after cardiac surgery.39 In another study by Bucerius et al. on 16 184 patients who underwent cardiac surgery, CPB time >2 h is a predictor for stroke after surgery.4

In a Karhausen et al. study, the occurrences of post-CABG stroke were 1.09%, 1.50%, and 3.02%, respectively, for platelet counts more than 150 × 10/L, 100–150 × 10/L, and less than 100 × 10/L. Each 30 × 10/L reductions in platelet counts resulted in a 12% increase in the stroke rate. Also, 2-folds increase in the stroke rate was observed in patients with moderate to severe thrombocytopenia. In studies, it was reported that a significant relation between post-operative nadir platelet counts and the mortality rate after CABG surgery.40

In a recent study by Arnan et al., acutely raised post-operative BUN appeared as the most potent predictor of strokes after cardiac surgery including CABG (OR, 2.37 for every 25% rise in BUN, with ROC AUC of 0.855).41

Table 1 shows the risk factors for stroke after CABG according to the search in the review, systematic review, and original published studies.

Atherosclerotic debris, air, fat, and surgical debris are considered as the origin of embolism in cardiac surgery. It was shown that micro-emboli that are defined by transcranial Doppler in the middle cerebral arteries are associated with neurological injuries after cardiac surgery. It is also shown that the quality of embolism has a role in brain injury and not its amount.47

Recently indirect pieces of evidence show that small capillary arteriolar dilations (SCADs), small lipid emboli, that found in the autopsy of patients who died after cardiac surgery, may be related to the neurological injuries after cardiac surgery. SCADs can be reproduced in canine models of cardiopulmonary bypass. SCADs are supposed to be originated from primarily lipid emboli during cardiopulmonary bypass procedure.48

Macro-emboli which were produced as a result of aortic atherosclerosis and atrial fibrillation suggested being associated with post-operative stroke.49

Significant blood pressure variations during surgery can raise the probability of post-operative neurological injuries and the risk of stroke. It was shown that a decrease in systolic blood pressure to ≤50 mm Hg for ≥10 min increased neurological complications after cardiac surgery.50 Since the cerebral perfusion pressures >50 mm Hg are tolerable for the brain.51 However, this may not be correct for patients with intra- or extracranial stenosis or in those with chronic hypertension. Pulse pressure, which is the vascular stiffness marker, was also recognized as a stroke predictor after cardiac surgery.52

It was theorized that embolization and hypoperfusion have a synergistic effect in neurological injury. Since hypoperfusion reduces the emboli washout, which leads to injury in the watershed areas of the brain.53

It is believed that epiaortic ultrasonography reduces the neurological outcomes after cardiac surgery by decreasing cerebral embolization.54 Some studies have demonstrated a decrease in the incidence of stroke in patients who experienced surgical technique alterations according to the consequences of epiaortic scanning.55 Rosenberger et al. reported that in the CABG patients managed with Epiaortic ultrasonography showed a lower occurrence of stroke (1.4% vs. 1.6%).56 Zingone et al. also showed that the total and early stroke rate reduced in a CABG cohort after using the epiaortic imaging.57

Studies have recommended a relation between off-pump CABG and reduction in stroke reduction. It is assumed that off-pump coronary artery bypass graft surgery reduces the risk for brain injury via decreasing cerebral embolization and inflammatory responses.54 In a study on more than 42 000 patients, a 35% decrease was seen in the stroke risk in patients treated with off-pump CABG in comparison to patients who were treated with on-pump CABG.58 Nishiyama et al.‘s study showed that off-pump CABG decreased the occurrence of stroke mostly by reducing early strokes; but, the delayed stroke risk was not different in patients who underwent off- and on-pump CABG.59 In contrast, some studies showed no difference between the risk of stroke in off- and on-pump CABG.60 Therefore, the result is controversial.