Cerebral Autoregulation in Cardiac Surgery

 

Each year cardiovascular disease causes over 4.3 million deaths in Europe. It accounts for nearly half of all deaths in Europe (48%). Around 1 million adults undergo cardiac surgery each year worldwide[1],[2]; coronary – artery bypass grafting (CABG) surgery is the most common major surgical procedure. Adverse neurological events with serious impact on morbidity and mortality remain one of the major clinical problems in cardiac surgery.[3] [4]

Four main neurologic and cognitive complications are observed after cardiopulmonary bypass (CPB): stroke (the most serious, with an incidence of 1.5 to 5.2%), postoperative delirium (10 to 30%), and short-term (33 to 83%) as well as long-term cognitive changes (20 to 60%). Even in low – risk patients undergoing CABG surgery, more than one third of patients had cognitive decline after 5 years[5],[6],[7]. Delirium and cognitive changes is a frequently occurring but often under-diagnosed and under-treated problem in the ICU by the fact that this complication is often followed by additional symptoms, especially when no verbal contact with the patients is possible, e.g. in cases of mechanical lung ventilation, hypoactive delirium or reduced consciousness are caused by consumption of sedatives, opiates analgesics.

Brain injury during cardiac surgery results primarily from cerebral embolism and/or reduced cerebral blood flow (CBF). The latter is of particular concern for the growing number of surgical patients who are aged and/or who have cerebral vascular disease. Normally, CBF is physiologically regulated (or kept relatively constant) by cerebrovascular autoregulation (CVA) system. CBF is relatively constant within a physiological range of cerebral prefusion pressure (CPP) allowing for stable cerebral O2 supply commensurate with metabolic demands. Cardiac surgery could lead to brain injury since current practices of targeting low mean arterial blood pressure (MAP) in a range of 50-60 mmHg during CPB may expose patients to impairment of CVA and thus - cerebral hypoperfusion. The selected low value of MAP is based on data supporting a MAP of 50 mm Hg as the lower limit of CVA.  In normotensive adults CBF remains relatively constant at MAPs between 50–150 mm Hg.  Above and below this limit, autoregulation is lost and cerebral blood flow becomes dependent on MAP. When CPP falls below the lower limit of CVA, cerebral ischemia ensues. More recent investigations have demonstrated that the lower limit of CVA is individual dependant and may be much higher than 50 mmHg6,[8],[9],[10]. Studies in awake, normotensive adults have demonstrated that the mean lower limit of CVA is 73–88 mmHg9. Furthermore, the autoregulatory curve may be shifted to the right in the patients with hypertension, diabetes, pre existing cerebral vascular disease or advanced age.

Targeting MAP during CPB to a level above an individual's lower autoregulatory threshold could reduce the risk for brain injury and prevent the late cognitive functional decline in patients undergoing cardiac surgery. Understanding changes in haemodynamics, metabolism and CVA holds the key to individualised, optimised therapy, however currently there are no commonly accepted reliable techniques for non-invasive CVA monitoring during the cardiac surgery.



[1] Jean – Claude Tardif, Coronary artery disease in 2010, Eur Heart J Suppl (2010) 12

[2] European Cardiovascular Disease Statistics 2008

[3] Roach G, Kanchuger M, Mangano CM, et al. Adverse cerebral outcomes after coronary artery bypass surgery. N Eng J Med 1996; 335: 1857–63

[4] Zimpfer D. et al., Neurocognitive deficit following coronary artery bypass grafting: a prospective study of surgical patients and nonsurgical controls, Ann Thorac Surg 2004; 78:513–9

[5] B. Phillips – Bute et al., Association of Neurocognitive Function and Quality of Life 1 Year After Coronary Artery Bypass Graft (CABG) Surgery, Psychosomatic Medicine  2006 68:369–375

[6] Hogue C. W., Palin C.A.  Arrowsmith J. E., Cardiopulmonary Bypass Management and Neurologic Outcomes: An Evidence - Based Appraisal of Current Practices, Anesth Analg 2006;103:21–37

[7] McKhann G.M. et al., Stroke and Encephalopathy after Cardiac Surgery: An Update, Stroke. 2006; 37:562-571

[8] Siepe M. et al., Increased systemic perfusion pressure during cardiopulmonary bypass is associated with less early postoperative cognitive dysfunction and delirium, Eur J Cardiothorac Surg (2011) 40(1): 200-207.

[9] Murphy G. S. et al., Optimal Perfusion during Cardiopulmonary Bypass: An Evidence-Based Approach, Anesth Analg 2009; 108:1 394 –417

[10] McKhann G.M. et al., Stroke and Encephalopathy after Cardiac Surgery: An Update, Stroke. 2006; 37:562-571