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: