1.- Brain protection: cerebral perfusion demand is crucial. Complications arising from inadequate cerebral protection re- main considerable and often represent the limiting factor for good outcomes. There are surgical techniques that can reduce metabolic demands through deep hypothermic circulatory ar- rest (DHCA), antegrade cerebral perfusion (ACP) or retrograde cerebral perfusion (RCP).

DHCA has remained the cornerstone of effective brain protec- tion strategies and is one of the most widely practiced tech- niques today18. By lowering brain temperatures, intracellular metabolism is reduced, thus prolonging the duration of “safe” circulatory arrest time. Clinically, the duration of DHCA has been associated with the incidence of temporary neurologic deficit. It is recommended that DHCA be limited to less than 30–40min, with prolonged durations resulting in poorer short- and long-term outcomes19. The extent of hypothermia during hy- pothermic circulatory arrest (HCA) (nasopharyngeal tempera- ture), as established by the International Aortic Arch Surgery Study Group (IAASSG)20: Profound hypothermia ≤14°C; Deep hypothermia 14.1–20°C; Moderate hypothermia 20.1–28°C; Mild hypothermia 28.1–34°C.

ACP must be planned beforehand if the DHCA is anticipated to be greater than 30min, ACP is typically used as an extra pres- ervation technique added to the DHCA, by directly supplying warm or mildly cold blood to the brain via the axillary or su- pra-aortic vessels. While DHCA is the preferred method among American surgeons, ACP approach is preferred by most Europe- an and Japanese centers, representing up to 90% of surveyed institutions21. Since most of the time the perfusion of the brain is initiated unilaterally (most likely right-side through the axil- lary artery or innominate trunk), anatomically, this unphysiolog- ic perfusion pattern relies on the integrity of the circle of Willis and secondary extra-cranial collateral networks. Despite scien- tific evidence favoring bilateral ACP, clinical evidence this far has not demonstrated a clear superiority22.

RCP was first described by Ueda and colleagues in 199023, RCP relies on supplying cerebral blood flow through the superior vena cava (SVC) and the associated venous system. In theory RCP provides metabolic substrates, maintains brain hypother- mia, and flushes out embolic debris. Numerous animal and in vivo models have challenged the role of RCP, which explains why in recent decades it is only used in select centers24. Although enough scientific studies on RCP exist, there is little clinical ev- idence to conclusively demonstrate RCP's superior clinical out- comes. Nevertheless, RCP has been shown to offer similar ce- rebral protection to ACP25. It seems that the utility of RCP rests on its ability to provide localized topical cooling of parenchymal tissue as well as its potential for embolic washout.

2.- Brain monitoring:Transcranial Doppler (TCD) ultrasonogra- phy can be used to detect and quantify cerebral micro emboli during surgery (open or endovascular). Ultrasound probes are placed bilaterally on the temple, overlying the middle cerebral vessels. Emboli cause an increase in the reflected ultrasound, causing high-intensity transient signals (HITS). These HITS are the footprints of micro emboli26.

Another method is the Cerebral Perfusion Index: regional cerebral oxygen saturation is monitored noninvasively using a near-in- frared spectroscopy (NIRO or NIRS) which is a representation of brain activity and degree of ischemia. NIRS is simple and non-invasive. It provides real time and continuous monitoring during the entire procedure. A bedside monitor of easy access and reading is connected to bilateral frontal sticky pads with an interior sensor that captures instant cerebral oxygen saturation. This device should be a standard of care and routinely applied in all cases27. More than a single instant value, the fluctuations and the trend in sensor levels can alert surgeons to either main- tain or modify their brain protection approach to avoid potential risks28. Electroencephalography, and jugular venous oxygen sat- urations are other available monitoring methods.

3.- Cannulation: In early series, the femoral artery was used as a cannulation site by Griepp and colleagues29. Over the ensu- ing years femoral cannulation has decreased in use and inter- est since the risks of retrograde embolization have proven to be higher, especially in those patients with a diseased descending aorta. This is particularly important in aortic dissections since intimal flap behavior becomes totally unpredictable after fem- oral retrograde flow is established. Malperfusion of one or more organs can also be increased because of this unpredictable shifting of the intimal flap30. Any type of cannulation allowing forward flow and perfusion (axillary artery or the innominate trunk) is deemed to be favorable in overcoming the limitations of femoral artery cannulation. Lately, central cannulation (R ax- ilary artery or innominate trunk) has become the preferred op- tion in many centers because it reduces the probability of embolic strokes, decreases the disruption of atheroma or calcified plaques, minimizes the risk of malperfusion in dissection cases, and could provide an easy method to rapidly establish ACP if required.

4.- Surgical Considerations: For Type A (Debakey I) aortic dis- section treatment, the traditional model adopts a conservative approach, with resection of the intimal tear within the dissected segment of the aorta. The main purpose of this emergency op- eration has always been to bring a patient alive back from the operating room. Most of the times this is accomplished by re- placing just the ascending aorta and with an occasional exten- sion to the hemiarch. However, with notorious improvements in surgical techniques and the availability of new devices, the first approach is now deemed to be more extensive with regards to aortic resection and reconstruction. It has been recognized that a residual patent false lumen in the arch and descending tho- racic aorta is present in up to 60% of patients with up to 30% of patients requiring reoperations32,33. Considering the long-term complications and the availability of endovascular stent-graft devices, numerous centers have started to advocate for a more extensive initial repair, typically involving a procedure called the frozen elephant trunk (FET). The introduction of the elephant trunk (ET) concept and technique by Borst and colleagues in 1983 promoted the combination of open aortic arch replace- ment with a soft distal Dacron graft hanging down in the true lumen of the descending thoracic aorta for a staged surgical re- pair of a distal residual aneurysm or dissection34. The concept of “frozen” is related to the use of a self-expanding covered stent graft instead of a “floppy” Dacron. This is considered a shift in the treatment paradigm from traditional open repair to the in- novative use of combined open and endovascular techniques (Fig. 3).

Fig. 3.

Elephant Trunk. The classic Elephant Trunk procedure (left picture) is the combination of the classic open aortic arch replacement with a Dacron graft (1) and a free-floating extension of the arch prosthesis into the descending aortic true lumen (2). The Frozen Elephant Trunk (right picture) version is with the forward endovascular deployment of a self-expandable covered stent. (3) in the descending thoracic aortic true lumen. A: Innominate trunk B: Common carotid artery C: Left subclavian artery D: Ascending aorta E: Descending thoracic aorta.

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By exerting radial pressure inside of the true aortic lumen, it promotes false lumen thrombosis. This procedure could obviate the need for second-stage operations to repair the downstream aorta. With promising initial results, many devices combining sewn-in-graft and self-expanding stent graft appeared in the market including, among others, the E-vita Open Plus (Jotec GmBH, Hechingen, Germany), Thoraflex Hybrid (Vascutek, In- chinnan, Scotland, UK), Cronus (MicroPort, Shanghai, China), Chavan-Haverich (Curative GmbH, Dresden, Germany), and the J Graft (Japan Lifeline, Tokyo, Japan)35. Some of these devices are currently going through the FDA approval process for com- mercial use within the United States, hence restricting their use mostly to Europe and Asia.

Some promising mid-term results with the use of FET have been reported. A 10-year follow-up have showed a decent morbid- ity and mortality and high rate of false lumen thrombosis/oc- clusion. There are still some concerns of spinal cord ischemia related to the extent of the aortic coverage36. When it comes to comparison of conventional hemiarch and total arch replace- ment with FET, the latter is technically more demanding and is related with longer periods of circulatory arrests. The cardiovas- cular surgery community demands more extensive and longer follow-up to determine the long-term benefits of FET.