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).