3.Antegrade selective cerebral perfusion (ASCP)
Using this technique which mimics physiological perfusion, some or all
of the cerebripetal arch arteries are antegradilly perfused using
oxygenated cold blood. This is done via balloon-tipped perfusion
catheters introduced during short periods of systemic circulatory arrest
or via angled cannulae introduced through the vessel wall without
arrest. ASCP being propagated first by Bachet (31) and later-on
especially Kazui (32) has been shown to be very successful in outcome
and therefore it is adapted by most surgeons nowadays as there are
almost no longer time constraints (33,34). There is clinical
evidence that ASCP affords superior brain protection compared to HCA
alone or HCA combined with RCP. This is based on the continuation of
aerobic metabolism during the perfusion period and a less marked
hyperaemic response compared to HCA (35). Of course the operative field
is not bloodless and even more crowed due to the presence of different
cannulae, but these minimal drawbacks certainly do not outweigh the
tremendous benefits of a much longer possibility to repair and
reconstruct the arch and side branches. There are now several variants
of ASCP illustrating the unknown and lacking optimal perfusion
characteristics of ASCP, still today despite the fact that the technique
is three decades old. Most surgeons now use separate cannulation
(directly or endoluminally) of the innominate artery (eventually via the
right axillary artery) together with the left common carotid artery (or
clamping of the base of the innominate artery when the right axillary
artery is used as arterial inflow), with occlusion (by a clamp, balloon
or snare) of the left subclavian artery to avoid a steal phenomenon.
Most authors will now use a flow of 10 cc/kg/min corresponding roughly
to normal blood flow under normothermia but this flow rate might be too
high under anaesthesia and profound hypothermia leading eventually to
cerebral oedema. In our centre we advocate the combination of deep HCA
(circulatory arrest at 20 ° C nasopharyngeal temperature) and ASCP at a
flow rate of 10 cc/kg/min in all arch cases especially in every acute
type A dissection because this pathology may yield unexpected surprises
in the arch and in doing so, it allows all surgeons, even those with
limited experience to obtain good results. Bihemispheric antegrade
perfusion via right and left common carotid arteries is now routine in
most centers since in 2% to 15% the circle of Willis is incomplete and
it has been shown that bihemispheric perfusion has a significant
favourable impact on hospital mortality (36). Drawbacks are the
necessity to manipulate and cannulate the cerebral vessels with the
potential risk of dislodging atherosclerotic debris. It is however
important to emphasize that atherosclerosis and acute dissection not
often come about simultaneously. In chronic cases there is more time
available to illustrate the presence of calcifications, atherosclerotic
debris or clots and adapt the cannulation and perfusion strategy.
Surgeons and perfusionists have the possibility to change a number of
parameters such as flow rates, pressures, the number of vessels
perfused, temperature of the perfusate and core temperature and this has
led to a high variability and a lack of standardisation between
different institutes. Also here the avoidance of air emboli is important
as is the use of intraoperative CO2 but this is not
different from HCA. A lot of questions remain unanswered today such as
which is the optimal core temperature before the circulatory arrest can
be instituted (3). Over the last decade we see a trend towards a slow
rise in the temperature at which the circulation is arrested while the
perfusate of the brain is at a lower temperature level. This can be an
explication why the incidence of spinal cord problems and renal failure
is now increasing in clinical series. Continuous corporeal perfusion
through an endoluminal perfusion-occlusion cannula in the proximal
descending aorta or via the femoral artery could minimalize the
deleterious ischemic effects on the spinal cord, kidneys and guts (37).
Other unknown factors, still today are the optimal perfusion pressure,
the optimal temperature of the cerebral/body perfusate and pH
management. Bachet who applied HCA at 25-28° C, perfused the brain at 6
- 12° C (38). Kazui proposed higher perfusion temperatures at 20 – 22°
C (39). For most authors, the basic criterion of sufficient perfusion to
the brain is a restoration of a mean right radial artery pressure of
40-70 mmHg but for technical reasons, in case of axillary cannulation,
the radial artery pressure might not correlate with the right carotid
pressure necessitating additional assessment (3). Apart from the
aforementioned monitoring techniques, measurement of jugular bulb venous
oxygen saturation can be a reliable tool for the estimation of adequate
cerebral perfusion (40).