Transcranial doppler cerebral embolus detection during carotid and cardiac surgery: Successes and challenges

Abstract

Transcranial Doppler ultrasound (TCD) has proven to be a very sensitive tool for detecting emboli in the cerebral circulation. The basic principle of detection is extremely simple: if an embolus backscatters more power than the blood in which it is moving, then the transitory increase in power can be detected and measured. In general even relatively small gas bubbles will be detected, but some larger solid emboli may be missed. TCD has been used to study spontaneous embolism in patients with various conditions including carotid artery stenosis, cardiac disorders, and acute stroke and in many types of interventional procedures including carotid endarterectomy and stenting, catheter ablation for atrial fibrillation, coronary artery bypass grafting and valve replacement surgery. The outcomes of these studies have been of mixed success: embolus detection during carotid surgery seemingly leading to improvements in surgical and medical treatment, whilst the value of embolus detection during cardiac surgery is still very much in question. In the case of carotid artery surgery intraoperative monitoring has alerted surgeons to manoeuvres during the dissection phase of the operation that are causing embolization to occur and thus change their technique, and has aided in the development of pharmaceutical strategies to dramatically reduce the incidence of stroke in the immediate post-operative period. In the case of cardiac surgery the picture is much less clear. A common problem after cardiac surgery is postoperative cognitive decline (POCD) which has been been attributed to a number of factors including the presence of new ischaemic lesions in the brain due to emboli entering the cerebral circulation during surgery, but there is no clear evidence of this and in a recently published systematic review of 18 studies, 9 studies with a total of 647 patients showed a positive association between POCD and embolic load, whilst in the other 9 with a total of 434 patients there appeared to be no association. There are several reasons why, even if embolization is an important factor, studies have so far failed to prove an association. Firstly although vast numbers of emboli occur during cardiac surgery it is likely that most of them are gaseous and our studies have suggested probably insignificant. Secondly emboli may only be a part of the story as some studies have shown associations between POCD and type of anaesthesia, blood pressure variations, impaired autoregulation, biomarkers associated with inflammation, use of neuroprotective agents, and the use of hypothermia.