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.