Stroke is the most distressing complication after a bypass surgery, which can result in permanent disability, increased risk of mortality, and a longer stay in the hospital. Although there have been advancement in the surgical and medical management, the risk of stroke after bypass surgery has not come down. The reason could be because now older and sicker people have been deemed suitable to undergo a bypass.
Although the causes for a stroke could be many, two variables are believed to play a major role – cerebral embolisation and hypo perfusion during surgery.
Cerebral embolism is certainly the most common cause of stroke after bypass. Cerebral emboli occur from either the ascending aorta during surgery or from the heart due to atrial fibrillation.
During cannulation of the aorta for establishing cardiopulmonary bypass, when aortic clamp is applied or released, there is a chance of embolisation of debris from the aorta to occur. Cerebral emboli usually co-exist with intra-operative hypoperfusion impairing the clearance of microemboli. Also cerebral hypoperfusion may be exacerbated in case of carotid artery stenosis, which is another risk factor for intra-operative stroke.
Chronic atrial fibrillation is a risk factor for cerebral embolism and so patients who are undergoing bypass are at increased risk of stroke in the peri-operative period as blood thinners dosage is regulated. Recent studies also reveal that new onset atrial fibrillation during the peri-operative state also carries a risk factor for stroke post the bypass, and at least 15-30% of patients undergoing bypass tend to develop atrial fibrillation.
Hence, it is utmost essential to identify the patients that are at increased risk of stroke before proceeding to surgery and also obtaining informed consent.In order to manage the risk of stroke during bypass, the key determinants are atherosclerosis of the aorta and pre-existing cerebrovascular disease. Atherosclerosis of the aorta can be determined using various imaging studies such as transesophageal echocardiography, CT, and MRI, but intra-operative ultrasound scanning is considered to be the best diagnostic tool.
Various factors such as age, diabetes, hypertension, peripheral vascular disease, renal failure, and left ventricular dysfunction have been reported as risk factors for perioperative stroke in patients undergoing bypass. As these risk factors can be assessed before the surgery itself, the information can help the patients, their family, and the physicians in decision making.
Also the standard practice of cooling and then rewarming the patient during bypass to prevent organ damage could impair the body’s mechanism that controls the blood flow to the brain, which potentially increases the risk of stroke. Although the reasons are unclear, nevertheless rewarming is found to be responsible for the body’s autoregulation mechanism failure. This increases the chance of the brain not getting adequate supply of blood and oxygen, increasing the risk of brain injury.
A recent study reveals that about 5% of cardiac patients have been found to awake from surgery with significant loss of controlled movement as well as speech difficulties, and the reason interrupted blood flow to the brain – stroke. Although the reasons are unclear, the researchers suggest that the reason could be breakdown of the blood-flow regulation mechanism.
Hence, it must be understood that bypass is not the end of treatment, but just the beginning. Although bypass does help in treatment of heart disease, it comes with its own risk of complications in the months and years following surgery. Also, one must remember that bypass is not a cure for heart disease and so work with the care team to prevent worsening of the condition which will help in reducing the risk of stroke.