Cerebral Complications after cardiac surgery

Journal Reviewed: Journal of Cardiothoracic and Vascular Anaesthesia Issues: Vol 13 Nos 1,2,4,5 (relevant articles) Reviewer: Dr J Swanepoel MB BCh (registrar) Summary This brief overview looks at articles within the Journal that address two potential cerebral complications of cardiac surgery: Brain injury, and Awareness during surgery 1. Brain injury Consider the following statements: "In adults the incidence of neurological morbidity is between 7 to 87% with stroke in about 2 to 5%, whereas the neurological morbidity increases to 30% in infants and children undergoing cardiopulmonary bypass." [1] "Although the majority of survivors do not have obvious cerebral sequelae, there is increasing disquiet about the high incidence of acute neurological events in the immediate postoperative period as well as evidence that at long-term follow-up there are subtle cognitive and motor deficits in many." [2] "Brain injury remains a significant and potentially devastating outcome of cardiopulmonary bypass (CPB). The reported post CABG stroke rate ranges from 0.9% to 5.4% and the incidence of neurophysiologic impairment ranges from 28- 79%, with persistent impairment at 6 months in 19-57% of the cases. These outcomes are associated with increased mortality, longer hospital stays and increased use of intermediate or long term care facilities." [Stump, 1999] Clearly, these statements reflect an unsatisfactory outcome of cardiac surgery. There is a need for better understanding of the underlying mechanisms for post CPB neurological deficits, improved intraoperative and postoperative monitoring of cerebral function and development of techniques to limit or prevent such complications. Several articles in recent copies of the Journal address these issues: A confusion of monitoring methods (Article 1) A small study assessing near infra-red spectroscopy (Article 2) Retrograde cerebral perfusion and lignocaine (Article 3) Assessment of retrograde cerebral perfusion (Article 4) Article 1: Neuro physiological Monitoring and Outcomes in Cardiovascular Surgery Journal: The Journal of Cardiothoracic and Vascular Anaesthesia 13 (5) 600-13 Article type: Review Author: David A Stump (Wakeforest University School of Medicine, Winston-Salem) This article gives a good summary of the available methods of monitoring intra-operative cerebral function and postoperative neurophysiological assessment techniques. If one is new to the subject, there may be one or two concepts which may require further clarification by reading up other references, for example Near Infrared Spectroscopy. What comes through strongly, is that there is no single method which can quantify cerebral injury and that most, if not all the methods in use, are in the process of being refined and validated. One is therefore still left with some confusion as to which techniques are the most reliable. Unfortunately this may lure some of us into the trap of complacency and lead to resistance to implementing these new methods. However, to be ignorant of these potential means of improving the poor neurophysiological outcome in these patients, will become increasingly indefensible. Click here for article details Article 2: Cerebral Oxygenation During Cardiopulmonary Bypass Measured by Near-Infrared Spectroscopy: Effects of Haemodilution, Temperature and Flow Journal: The Journal of Cardiothoracic and Vascular Anaesthesia 13 (5) 544-8 Article type: Clinical study (n=14) Authors: Andrea Lassnigg et al (University Clinic of Vienna) This study looked at the effects of starting cardiopulmonary bypass and hypothermia on cerebral oxygenation as assessed by Near-Infrared Spectroscopy (NIRS), in 14 adults undergoing elective cardiac surgery. The authors found a significant drop in oxygenated haemoglobin concentration and oxidated cytochrome aa 3 levels occurred at 3 minutes after initiating bypass. The proposed mechanisms for this drop were: haemodilution (lower limits below which cerebral injury occurs thought to be between 6 and 11g/100ml) microembolism (high incidence occurs at the onset of CPB) changes in cerebral circulation resulting from nonpulsatile flow (vasoconstricting substances released). This study is based on a small sample, but confirms some previous research findings by Nollert et al. [4] It is valuable from the point of view that it highlights some of the current issues surrounding NIRS, namely: NIRS seems to be a better monitor of regional decreases in cerebral oxygenation which are not picked up by a global measurement of brain oxygenation, such as jugular venous bulb oxygenation. Although many studies try to play the two off against each other for ability to assess the oxygenation of intracerebral blood, it seems that the two should actually complement each other instead, as they are probably giving different information, i.e. regional cerebral oxygenation versus global oxygenation. Monitoring cytochrome oxidase aa 3 may be a valuable feature in the early, noninvasive detection and prevention of cerebral hypoxia. Although a decrease in oxidated cytochrome aa 3 levels of 3.8umol/L has been suggested to be the level at which postoperative neurophysiological dysfunction starts occurring, it seems that monitoring the trend of cytochrome oxygenation is useful in detecting regional hypoxia. This may be particularly relevant because numerous variables may interfere with NIRS readings and therefore invalidate the absolute measurements made. In terms of improving cerebral protection during CPB, Retrograde Cerebral Perfusion (RCP) seems to be topical in the literature. The April 1999 edition of the Journal of Cardiothoracic and Vascular Anaesthesia has two articles on the subject. Article 3: "Effect of Lignocaine on Improving Cerebral Protection Provided by Retrograde Cerebral Perfusion: A Neuropathologic Study". Journal: The Journal of Cardiothoracic and Vascular Anaesthesia 13 (5) 549-554 Article type: Research study (dogs) Authors: Dongxin et al (First School of Clinical Medicine, Beijing) It is probably worth mentioning in that this article provides a short but interesting background history of retrograde cerebral perfusion. Whether the results will be of practical relevance remains doubtful, as it seems as if there could be some potentially deleterious side effects and the study was done on dogs. Article 4: "Assessment of Arteriovenous Blood Flow during Retrograde Cerebral Perfusion" Journal: The Journal of Cardiothoracic and Vascular Anaesthesia 13 (2) 173-5 Article type: -- Author: Paul G. Loubser (Baylor College of Medicine, Houston) This study discusses some technical issues of RCP, including the presence of internal jugular valves and alternative routes by which the perfusate may return to the systemic circulation. 2. Awareness The following article is quite an eye opener! It raises a couple of issues which may make you change your approach to intraoperative awareness. Some of the article's points are re-inforced by another article in Anaesthesiology [3] from last year. Article 5: "Awareness During Cardiac Surgery" Journal: The Journal of Cardiothoracic and Vascular Anaesthesia 13 (2) 214-9 Article type: Review Authors: Deepak K. Tempe et al, (G.B. Pant Hospital, New Delhi) Tempe could perhaps have gone into greater depth about the methods used to monitor depth of anaesthesia, especially the Bispectral Index, which is the first FDA-approved means of measurement of the hypnotic effects of drugs. However, a good reference for more information on these methods is the article: "Can We Measure Depth of Anaesthesia?" by Carl E. Rosow (No. 246 of the 1998 ASA refresher course lectures). Click here for article details References Pua and Bissonnette: Cerebral physiology in paediatric cardiopulmonary bypass , Canadian J Anaesthesia 1998 Oct;45(10):960-78 Kirkham FJ: Recognition and prevention of neurological complications in paediatric cardiac surgery , Paediatric Cardiology 1998 Jul-Aug; 19(4):331-45 Domino, K.B. et al. Awareness during Anaesthesia Anaesthesiology 1999: 90: 1053- 61, Nollert et al. Postoperative Neurophysiological Dysfunction and Cerebral Oxygenation during Cardiac Surgery , Thorac Cardiovasc Surg 43: 260-264, 1995. Editorial pointer Readers looking for a fairly comprehensive overview of the neurological dysfunction in cardiac surgery should read the following moderately well-written review: WC Boyd & GS Hartman New Horizons 1999 7 504-513 In summary dysfunction may be due to macroemboli, microemboli or hypoperfusion, with emboli probably being more important than hypoperfusion. Most emboli appear to arise from the ascending aorta, and crunchy atheromatous aortas massively increase the risk of ischaemic stroke (Odds ratio 9:1). Critical perfusion pressures are controversial, previous recommendations being based on totally inadequate studies. Strategies designed to prevent neurological complications include: alternative aortic cannulation sites and clamping techniques; CABG without CPB (mid-CAB); maintaining higher perfusion pressures (MAP > 50mmHg) in elderly patients at risk (well covered in this review); in-line filters; and pulsatility of flow. Apart from filters, none of these has consistently shown benefit so far. Strategies employed to minimise the impact of ischaemia/embolism are legion: hypothermia (supported by most but not all studies) and , importantly, avoidance of cerebral hyp er thermia during rewarming; maintenance of euglycaemia (no demonstrated benefit); pH management (controversial); haemodilution (experimentally but not clinically validated); 'pharmacoprotection' (aprotinin appears to help, glutamate antagonists show promise, burst suppression with thiopentone or propofol of no value, and no other agents as yet); maintaining higher perfusion pressures (as mentioned above); Points that are not well addressed by Boyd and Hartman are What percentage of patients with neurological dysfunction following cardiac surgery have significant aortic atheroma; The role of microemboli (dismissed in two lines); Mechanisms of cognitive dysfunction; What percentage of patients undergoing (for example) CABG are having unnecessary operations - an important strategy for decreasing the incidence of an operative complication is to decrease the incidence of inappropriate surgery! Clearly we have a long way to go in minimising the risks of brain injury following cardiac surgery. Ed       Up Date of First Publication : 2000-01-24 Date of Last Update : 2006/10/23 Web page author: Click here