1.Hypothermic circulatory arrest (HCA)
This technique was introduced in the 1950s and further popularized in the 1970s, even today a lot off centers have very positive clinical results with only HCA (15). It is based on the ability of hypothermia to reduce the metabolic rate of all tissues including the central nervous system. During cardiopulmonary bypass core temperature can vary from normothermia (37° C), mild hypothermia (32-35° C), moderate hypothermia (28-32° C) to deep hypothermia (< 28° C). Profound hypothermia suppresses cerebral metabolism enough to allow a short period of total circulatory arrest during which the aortic repair can be performed. Despite many negative critiques still today a lot of surgeons throughout the world use HCA as their preferred method of choice because it is simple and allows for a complete bloodless surgical field also free of cannulas. Despite its simplicity, it demands for attentive perfusion and monitoring techniques with slow, gradual and thorough cooling (> 30 minutes) to avoid oesophageal-blood gradients of > 10 ° C. Rewarming must also be gradual with a temperature gradient of less than 10 ° C to avoid damage and to optimize oxygen delivery to the vulnerable brain intra- and post-operatively. It is necessary to flood the operative field continuously with CO2 . Randall Griepp, the promotor of HCA, advises to cool thoroughly to prevent a temperature rise of the brain during the HCA interval and to pack the head in ice in order to prevent undesirable rewarming (16). To obtain maximal metabolic suppression Griepp recommends cooling until the oesophageal temperature reaches 10° to 13° C (16). This level of deep cooling is nowadays still used by a lot of surgeons in the US but it is our impression that in Europe and Japan, the temperature at which the body circulation is arrested, is higher (20 – 25 ° C, even 28 ° C) also obtaining good clinical results (17). Even tepid HCA (32° C) seems to compare favourably with deep HCA (20-25° C) (18). Immediately after a period of HCA follows a period of reactive hyperaemia or cerebral hyperperfusion followed by several hours of increased cerebrovascular resistance during which brain oxygen extraction is increased but cerebral blood flow may be inadequate (19).
Certainly the main drawback of HCA is that the safe duration of the arrest period is limited to about 20 to 25 minutes despite meticulous perfusion and monitoring approaches (20). Hypothermia will never reduce the cerebral metabolism to nil. The arrest duration is the single largest operative determinant of neurologic outcome. If the arrest time is longer than 20 to 25 minutes, it will result in an increase in neurological dysfunctions and when it is longer than one hour, the risk of stroke and mortality increases sharply (21). In our opinion this time restriction of 20 minutes is sufficient only to perform an open distal anastomosis at the base of the innominate artery or a hemi-arch replacement, even in the hands of the most technically skilled and experienced surgeons. If a more extensive reconstruction is necessary because of additional arch tears, aneurysmal aortic arch or when the surgeons wants to perform a prophylactic repair in view of future dilatations (e.g. in connective tissue disorders or in young patients), adjunctive brain protection techniques are absolutely mandatory. It is a false idea that the safe arrest duration of 20 minutes can be prolonged with pharmacologic adjuncts such as barbiturates.
Due to the obligatory slow cooling and slow warming the second drawback of HCA is the increased cardiopulmonary bypass time. This may lead to a higher evidence of neurologic complications in the early postoperative period and a decrease in quality of life in the long-term period (22). Coagulation disturbances are also related to the use of HCA. There are three mechanisms that contribute to the temperature-related coagulopathy: platelet and clotting factor dysfunction and the fibrinolytic activity. Nowadays these coagulopathies are dealt with more easily since we have at our disposal more effective pharmacological agents as well as platelet and plasma transfusions and other local hemostatic agents.