Brief review on systematic hypothermia for the protection of central nervous system during aortic arch surgery: a double-sword tool?

The authors are briefly discussing the fundamental principles of using hypothermia as an adjunct tool of the cardiothoracic surgeon's practice. The relationship between temperature, flow, metabolic requirements and adverse effects is addressed.The methods of brain protection during interruption of its blood supply have been refined and at least the last decade the mortality for complex arch surgery has dropped to less than 10% with an accompanied incidence of neurological damage of 5 to 6%. [1]. More impressively in a large series of total arch replacement with prolong circulatory arrest times, Sasaki et al have quoted a low mortality of 1.9%, and a low incidence of postoperative transient stroke of 6.6% and permanent stroke of 1.6% [2]Few minutes of complete global ischaemia will cause neuronal death in normothermic brain. Systemic hypothermia with circulatory arrest is a clinical term used for nasopharyngeal low temperatures, possibly below 20 degrees. It is known that oxygen consumption decreases by 50% for every 10 degrees decrease in body temperature [3], thus hypothermia reduces the metabolic rate of the central nervous system and lengthens the period of induced or accidental brain ischemia.They have been different school of thoughts regarding optimal body temperature, perfusate temperature and the way of delivery of the perfusate to the brain during hypothermic circulatory arrest, with a reasonable aim by some authorities [4] to achieve electroencephalogram silence as a threshold point of neuro-electrochemical relaxation and also to achieve oxygen saturation at the point of the jugular venous bulb above 90 to 95%.Regarding retrograde delivery of cerebral perfusion during circulatory arrest, most of the efforts to document improvement in cerebral metabolic function as a result of retrograde flow have failed. These studies only observed advantages related to the removal of emboli from the arterial system. [5,6].The nervous system has a high metabolic demands an