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Page 12 of 21 Calafiore et al. Vessel Plus 2023;7:18 https://dx.doi.org/10.20517/2574-1209.2023.42

[88]
P < 0.001, in matched patients). The problem is not yet completely solved .

The left subclavian artery and spinal cord injury
The progressive increase in the temperature of hypothermic CA raised the problem of spinal cord
protection. The question was if CP during MHCA was enough to avoid SCI or if DHCA was the most
reliable tool against spinal cord ischemic damage.

The vascularization of the spine depends on several arteries. Among the epiaortic trunks, the most
important is the subclavian artery by means of the anterior spinal artery (branch of the vertebral artery), the
deep cervical artery (branch of the costocervical trunk), the anterior intercostal arteries (branches of the
internal thoracic artery) that fully anastomose with the posterior intercostal arteries. Not all the posterior
intercostal arteries come from the descending aorta, as the first two comes from the supreme intercostal
artery, the first branch of the costocervical trunk. In general, cord vascularization is assured by many
arteries of different caliber that anastomose each other to maintain a sufficient nutrient flow. However, the
possibility of steal exists if a low resistance pathway is opened. In the case of CA, the LSA can be not
perfused and not clamped, can be clamped and not perfused, or can be perfused. In all these circumstances,
steal happens. In the first option, it is evident. In the second one, there is a modest flow to the cervical cord,
but the thoracic and lumbar cord remain progressively without flow (the flow from the internal thoracic
artery to the posterior intercostal arteries via the anterior intercostal arteries goes into the empty descending
thoracic aorta). In the third option, vascularization of the cervical cord is assured, but steal still exists for the
thoracic cord. Maintaining the LCA open can not only reduce spinal cord perfusion, but also cause
hypoperfusion of the left cerebral hemisphere due to the steal of blood through the left vertebral artery into
the empty arch.

In 18 patients undergoing total aortic arch replacement and SACP of the three epiaortic trunks, brain and
spinal cord oxygen saturation levels using near-infrared spectroscopy at the forehead and along the mid-line
[51]
of the back at the T3 and T10 levels were investigated . When CA started (tympanic temperature lower
than 25 °C), a rapid decline of the oxygen saturation in all levels was observed. After ACP was instituted, the
oxygen saturation levels increased in the forehead and remained partially elevated at the upper thoracic level
(T3), but continued to decline without recovery at the lower thoracic level (T10). The authors concluded
that during CA, the upper thoracic spinal cord via collateral circulation from vertebral arteries is partially
perfused by ACP, but the lower thoracic cord is not. Then cooling is more protective against SCI than is
ACP. Even if surgeons are aware that both the circle of Willis and the feeding arteries of the spinal cord are
highly variable, detailed preoperative imaging to identify anomalies is rarely performed, although
postoperative SCI remains a possible complication after complex aortic repair , as nowadays we have no
[89]
accurate modality to monitor spinal cord perfusion both intraoperatively and postoperatively when warmer
temperatures during hypothermic CA are used for long intervals.

Retrograde cerebral perfusion
RCP can provide only partial perfusion of the brain and is therefore insufficient to sustain cerebral
metabolism. The presence of valves in the internal jugular vein prevents a significant amount of blood from
[90]
reaching the brain . Then, it is not safe to increase the temperature of CA over 20 °C. RCP can help flush
solid particles and air bubbles from the arteries, thus reducing embolic phenomena. However, even during
normothermia, most RCP blood is shunted away from capillaries, and this shunting is increased during
deep hypothermia as arteriovenous and veno-venous shunts open [91,92] . It has been demonstrated
experimentally that most of the blood was shunted to the inferior vena cava and less than 1% returned to the
aortic arch. Moreover, in humans, more than 20 direct veno-venous anastomoses have been described .
[92]