In China, more elderly patients are undergoing CABG surgery to treat CHD. These patients often present with complicated co-morbidity profiles. Patients with a history of stroke undergoing CABG are high risk group for post-operative stroke [1]. Recently, to reduce the post-operative complications induced by extracorporeal circulation, off-pump CABG has been extensively adopted. In our study, most of the surgeries were off-pump CABG; only 8 of 430 were on-pump CABG (1.9%).
Post-operative stroke occurred in 32 patients in our study (7.4%). The events can be divided into early stroke and late stroke, according to the time that stroke occurred after surgery. In our study, four patients suffered early stroke, and three of them underwent on-pump CABG. Peel and colleagues found that on-pump CABG showed a higher incidence of early stroke, and off-pump CABG showed a higher incidence of late stroke, which is similar with the results of our study [11].
The risk factors of post-operative stroke can be categorized into pre-operative, intra-operative, and post-operative factors. Pre-operative factors include advanced age, atherosclerosis in ascending aorta, unstable angina, hypertension and history of stroke; Intra-operative factors include the endurance of extracorporeal circulation and aorta clamping, or operation type. Post-operative factors include AF, micro-embolism detachment, and hypotension [12, 13]. Our study selected patients with a history of stroke who were the high risk group for post-operative stroke. We identified risk factors for recurrent stroke, including unstable angina, LVEF≤50%, post-operative AF, and hypotension. Extracorporeal circulation was a statistically significant variable for post-operative stroke in univariate analysis, but was excluded in the multivariate analysis. The limited number of on-pump CABG may account for the lack of significance seen in the on-pump CABG group.
The incidence of post-operative AF is 5%-40%, and in off-pump CABG it has been shown to occur at incidences ranging from 0 to 26% [14, 15]. While AF most often occurs within 2-3 days after surgery, most of these occurrences are self-limited [16, 17]. There are many determinants for post-operative AF, including increasing age, male gender, the incidence of right coronary artery stenosis, excessive manipulation of the heart during surgery and electrolyte imbalance. The correlation between cerebral embolism and AF has been well established. Many studies have proved that post-operative AF is the risk factor of post-operative stroke. Lahtinen and colleagues reported that the average interval from first AF to post-operative stroke was 21.3 hours, and post-operative stroke patients suffered average 2.5 postoperative AF occurrences before stroke [18]. In our study, eleven of 32 patients with post-operative stroke (35.3%) suffered postoperative AF, with most of the AF incidences occurring within three days after stroke, and seven of the incidences occurring before post-operative stroke.
Currently, the interaction between embolism and hypoperfusion is generally considered to be a major cause of post-operative stroke. Hypoperfusion may contribute to embolus retention. Several studies have found multiple emboli in the cortical watershed of patients who died after cardiac surgery, which confirms the synergistic effect of hypoperfusion and embolism on postoperative stroke. Our study suggests that unstable angina, LVEF≤50%, and hypotension are risk factors of post-operative recurrent stroke. All of these factors decrease brain perfusion, leading to stroke. Because the cerebral arteriosclerosis of patients with a history of stroke are more severe than patients without stroke, most of these patients progress to artery stenosis, where a relatively higher blood pressure is needed to maintain cerebral perfusion. When mean arterial pressure decreases 10 mmHg, the incidence of cortical watershed infarction increases by four-fold [19].
In pre-infarction period that occurs before stroke, cerebral circulation reserve cannot compensate for the hypoperfusion status, and this decompensation stage may last for several years. This stage often manifests with headache and difficulty in speaking. [20]. Patients with post-operative stroke may have been in the pre-infarction decompensated stage before surgery. Because of this potential complication, a pre-operative cerebral haemodynamic evaluation, including transcranial doppler sonography, vascular ultrasound, and CT perfusion imaging for high risk patients is important for the prevention of postoperative stroke. [21, 22].
After excluding on-pump patients, multivariate analysis eliminated both UAP and LVEF≤50% from the list of risk factors for post-operative stroke. This suggests that myocardial ischemia and cardiac function may play an important role in post-operative recurrent stroke for on-pump patients, while it influences less those patients that are off-pump. However, due to the limited number of on-pump patients, this concept should be studied in a larger clinical trial.
In conclusion, patients with a history of stroke who undergo CABG are often relatively older and have more complicated co-morbidities. Since our study is a retrospective study, information such as the extent of intracranial and extracranial arteriostenosis is lacking. Future studies, especially prospective studies, are needed to determine the optimum therapeutic strategy for preventing the occurrence of post-operative stroke in patients with a history of stroke. Based on our results, strategies that focus on patients with unstable angina, LVEF≤50% and hypotension may provide improved outcomes.