**9. Hypothermic circulatory cardiac arrest and cerebral protection strategies**

Because of the low threshold tolerance to ischemia, brain protection is of paramount importance during aortic arch procedures. Hypothermia is an option to increase the ischemic time. However, there are limitations of hypothermia and hypothermic circulatory arrest. Protective effects of hypothermia last no more than 9 minutes at 30°, 14 minutes at 25°, 21 minutes at 2°, 31 minutes at 15° and 45 minutes at 10° [28]. Neurological deficits are seen in elderly patients subjected to hypothermic circulatory arrest exceeding 25 minutes.

Several cerebral perfusion techniques have been introduced to extend the safe period of arch repair without residual neurological deficits. Retrograde cerebral perfusion (RCP) in tandem with hypothermic circulatory arrest was introduced in 1990 by Ueda et al. [29] Because the cerebral venous sinuses have no valves, RCP was proposed to provide retrograde perfusion and cooling of central nervous system (CNS). It offered to back-flush air emboli and debris from the cerebral circulation. Neuroprotective effects were most likely related to cooling rather than true nutritive flow [30]. It was also found to provide limited benefit in patients with significant carotid stenosis and vascular anomalies (e.g. an incomplete Circle of Willis) [31].

Antegrade selective cerebral perfusion (SCP) was introduced by Jean Bachet and Daniel Guilmet in Europe [32] and by Teruhisa Kazui in Japan in 1986 [33]. This new perfusion method of "cold cerebroplegia" in combination with hypothermia significantly reduced neurologic complications. Antegrade selective perfusion can be established either by direct cannulation or by anastomosing a prosthetic graft. The options for locating such are (i) right subclavian artery, (ii) innominate artery, (iii) right common carotid artery. These may be combined with left common carotid artery cannulation to provide bilateral antegrade cerebral perfusion.

Direct cannulation is limited by high risk of embolism due to plaque mobilisation from manipulation or by jet flow [34].

Axillary artery cannulation can be used to provide unilateral antegrade SCP during hypothermic arrest without manipulation of the arch vessels. This can be combined with balloon occludable perfusion catheter to left carotid artery to provide bilateral antegrade SCP. To avoid steal, an occlusive balloon catheter is inserted in left subclavian artery.

Unilateral antegrade SCP is sufficient for majority of patients with no pathology of the arch vessels and cerebral vessels. Adequate backflow from the contralateral carotid artery suggests good collateralisation. Near-infrared spectroscopy (NIRS) monitoring can also help to exclude contralateral malperfusion. Bilateral cerebral perfusion may be useful in patients with carotid artery stenosis, previous stroke or cerebrovascular anomalies (incomplete Circle of Willis). Malvindi concluded in his review that "While both unilateral and bilateral ASCP are acceptable, bilateral antegrade cerebral perfusion is safer, when the antegrade SCP time is more than 40-50 minute" [35].

Cerebral perfusion is performed at a rate of 8–12 cc/min/kg body weight, perfusion pressure of 40–60 mmHg at 23–28°C. Alpha stat pH management compared to pH stat management prevents "luxury perfusion" by marinating cerebral autoregulation decreasing the risk of embolization [35].
