Brain protection techniques in type A aortic dissection
General aspects
We need to clarify first some important general aspects like temperature monitoring, clamping of the aorta or not and brain monitoring.
During cardiopulmonary bypass temperature monitoring of the core of the body including the head and brain, is used in general because this core compartment will undergo fastest temperature changes due to the high flow rate into the mediastinal organs. It can be measured at different reliable sites such as tympanic membrane, nasopharynx and oesophagus (4). Nasopharyngeal temperature is now accepted by most surgeons as a good representative of the brain temperature. Pulmonary artery catheter can also be used to measure the core temperature but due to its invasiveness and costs it is not routinely used. Rectal and bladder temperature, often used in general cardiac and aortic surgery lag behind central monitoring sites and they certainly do not indicate accurately the cerebral temperature. Moreover they are affected by the presence of stools and bacteria that generate heat or by the production of urine. Skin temperature is not related to core temperature and is confounded by several factors such as ambient temperature. Blood temperature in the arterial line from the oxygenator is a also very good reference of cerebral temperature (5). The best indicator of cerebral cortical temperature is measurement of jugular bulb temperature (5) though it is rarely used in practice. Most surgical centers use a combination of the aforementioned temperature monitoring sites. In the literature there is a lack of uniformity making the interpretation of the results difficult and sometimes confounding. We share the opinion of Saad H et al (6) that temperature management strategies during cardiopulmonary bypass and more specific using hypothermic circulatory arrest (HCA), rely primarily on personal or institutional preferences rather than solid scientific basis.
A second technical important question is whether ascending aortic clamping during the surgical repair of type A aortic dissection is safe and mandatory. In chronic and acute cases, most surgeons will clamp the ascending aorta and proceed with cooling and meanwhile perform the root repair. However because clamping can pressurize the false lumen in the acute setting leading to partial or even total malperfusion of the brain (7), clamping can only be done if adequate and appropriate intra-operative monitoring allows for rigorous and reliable real-time evaluation of the brain perfusion to eventually adapt perfusion strategy immediately. On the other hand many experienced aortic surgeons counsel against clamping in acute type A and proceed directly with cooling to do the open distal reconstruction first, then followed by the proximal repair during the rewarming phase (8). In the chronic cases aortic clamping is less risky but it also requires adequate monitoring of the brain.
Minimal intra-operative monitoring during surgery for type A aortic dissection encompasses bilateral radial artery pressure (or cannula-based perfusion pressure monitoring if axillary cannulation is used) and forehead regional near infrared spectroscopy (NIRS). We have to keep in mind that NIRS does not provide information in the basilar artery region. Some exceptional centers also use transcranial doppler monitoring indicating the flow through both middle cerebral arteries. Another option is the use of electroencephalography to demonstrate electrical brain silence and represent neural-cell arrest during HCA. Iatrogenic brain malperfusion can be avoided by switching from femoral to axillary artery although in the context of acute type A dissection initial femoral cannulation remains reasonable even in the 21st century (8).
There has been much debate about the ideal arterial cannulation site related to acute type A dissection. Nonrandomized studies have shown that the axillary cannulation is related with better neurological outcome (9,10, 11) but some authors including ourselves consider femoral artery cannulation as safe in aortic dissection (12): it is quick and reliable but absolutely necessitates adequate intra-operative monitoring as said before to allow for a prompt switch from cannulation site if signs of brain malperfusion occur. Urbanski et al. demonstrated that even with direct cannulation and perfusion of one or both carotid arteries plus distal aortic perfusion, e.g. through a femoral artery, perfect results can be obtained (13). Direct aortic arch cannulation of the ascending aorta can be performed in chronic as well as acute dissection (14). In certain conditions transapical cannulation can be performed as a last resource aiming at the introduction of the cannula directly into the true lumen in emergent cases.
Because there is clinical consensus that an open distal anastomosis in acute type A dissection with inspection of the arch and arch vessels is an absolute minimum (8), there is a need to interrupt partially or totally the cerebripetal blood flow. Therefore the brain has to be protected. In general, the ideal brain protection technique should be simple to applicate and learn (very often acute dissection operations are performed by the youngest surgeons), effective and reliable. We have at our disposal three basic techniques: