Introduction
Neurologic injury has been a major concern in cardiac surgery due to the
atherosclerotic burden in the vasculature, cross clamping, and the use
of extracorporeal circulation, leading to a significant increase in
morbidity, mortality, and cost.1-3 Furthermore,
disrupting blood flow to the brain will compound on the potential for
neurologic complications.
The first documented aortic replacement dates back to the 1950s with
Denton A. Cooley, Michael DeBakey, and E. Stanley
Crawford.4 In both 1951 and 1955, aortic arch
replacements with the use of bypass shunts and moderate hypothermia were
fraught with failure. Only after DeBakey’s concept of an early form of
antegrade cerebral perfusion in 1957, did surgeons successfully enter a
new era of aortic surgery. This was followed by Griepp’s introduction of
total circulatory arrest under deep hypothermia (up to 14 °C) in
1975.5-7 With these advances, the risk of stroke and
mortality with aortic surgery has greatly improved over the past 70
years.
The main strategies that exist today to mitigate the risk of brain
injury include deep hypothermic circulatory arrest (DHCA), antegrade
cerebral perfusion (ACP), retrograde cerebral perfusion (RCP), as well
as a hybrid approach.8 The choice in cerebral
protection is dependent upon the surgeon’s preference and experience
with a specific technique, so it is important to recognize the potential
advantages or disadvantages of each strategy.
Brain protection strategies
Dating back to the 1970s, the DHCA strategy works by reducing
injury-inducing pathways by lowering the cerebral
metabolism.9 The body is usually cooled to
temperatures of 20ºC or greater. Longer times of DHCA have been
correlated to an increased rate of neurological dysfunction, as
described by Yan and colleagues (Fig. 1),10 so there
is usually a need for the use of additional cerebral protection
strategies (ACP and RCP).
RCP requires connecting the arterial tubing to a cannula placed in the
superior vena cava (SVC) and then reversing blood flow to the brain in a
retrograde fashion. This approach has been successful in reducing both
mortality and morbidity.11 An advantage of this
strategy is that it provides cold retrograde blood flow to the brain
during DHCA and therefore allowing for a decreased risk of
embolization.12Moderate hypothermic circulatory arrest (MHCA) with temperatures ranging
from 24ºC to 28ºC can be combined with selective
ACP.13 Once circulatory arrest is established, one or
multiple arch vessels are cannulated and perfused with cold blood in an
antegrade fashion. Some studies have suggested that ACP leads to better
neurological outcomes when compared with RCP.14The debate between RCP and ACP has been ongoing since the 1990s. Despite
the fact that RCP has proven to be effective, many surgeons still prefer
using ACP.15 RCP neuroprotective effects are explained
by its continuous cerebral cooling, and not from antegrade nutritive
blood flow.16 RCP’s efficacy may also be reduced by
the presence of venous valves that require an elevated perfusion
pressure, which could cause increase intracranial pressure and
subsequently worsen neurological outcomes.17 However,
RCP does not require exposure or manipulation of the aortic arch
vessels, whereas ACP requires insertion of a catheter into the arch
vessels, vessel clamping or snaring; which can increase the potential of
vessel injury or embolization. Nonetheless, there has not been a
clinical study demonstrating that one approach is superior to the
other.18 RCP provides improved exposure of the arch
vessels, although there is a constant flooding of venous blood return
into the operative field, whereas ACP yields a dryer field with
catheters potentially obscuring visibility.
Surgical techniques
1. Antegrade cerebral perfusion
Antegrade cerebral perfusion can be safely achieved unilaterally through
the innominate or right common carotid artery, as well as combined with
the left common carotid artery to achieve bilateral ACP. When comparing
unilateral and bilateral ACP, most published series quote use of
unilateral ACP only if adequate collateralization is demonstrated
through backflow from the contralateral carotid artery or if
contralateral perfusion is evident by near-infrared spectroscopy
(NIRS).4 Bilateral ACP may be more advantageous when
considering existing conditions such as arterial stenosis, stroke, and
other vascular anomalies. Many surgeons prefer bilateral ACP to ensure
bihemispherical perfusion due to its documented efficacy and safety
beyond 40-50 mins of circulatory arrest.19Alternative cannulation strategies can be utilized to establish
antegrade cerebral perfusion. A prosthetic graft can be anastomosed to
the right subclavian, innominate, or right common carotid artery.
Additionally, balloon occlusion perfusion catheters can be utilized to
provide antegrade cerebral perfusion without vessel clamping or
snarring. Peripheral cannulation of the axillary artery can be performed
to establish cardiopulmonary bypass and subsequent ACP with concomitant
insertion of a balloon occlusion perfusion catheter into the carotid
artery without directly cannulating supra-aortic
vessels.4
2. Retrograde cerebral perfusion
Retrograde cerebral perfusion takes a different approach as cold
oxygenated blood is delivered retrograde through a cannula placed in the
superior vena cava.19 After the patient is cooled to
achieve deep hypothermia and circulatory arrest established, RCP can be
employed. If a bicaval venous cannulation technique is employed for
cardiopulmonary bypass, Y-connectors with limbs are utilized to connect
the arterial to the venous lines. These limbs are clamped during the
conduct of cardiopulmonary bypass. When beginning to initiate retrograde
cerebral perfusion, the SVC cannula is snared and the limb connecting
the arterial cannula to the SVC cannula is
unclamped.20 This enables cold blood to run retrograde
into the SVC to perfuse the brain (Fig. 2). A cardiotomy suction is
placed in the open thoracic aorta to suction the blood return from the
arch vessels.21 Ice packs are also placed on the scalp
of the patient for additional topical brain cooling. RCP, as well as
ACP, has been historically performed via a median sternotomy, upper
hemisternotomy, or other partial sternal splitting variations. However,
currently, RCP is the only cerebral protection strategy that can be
employed through a right mini-thoracotomy minimally invasive ascending
aorta and hemiarch surgery.22A hybrid approach to these techniques include utilizing 2 separate
heater coolers, one for the systemic circulation and one for the
cerebral circulation. With this approach, the right carotid artery is
exposed in the neck or the innominate artery is exposed from within the
chest. Each vessel is directly cannulated and clamped with perfusion
coming from a separate tubing connected to the heart lung machine with
its own respective heater cooler. Since there are 2 heater coolers,
perfusion to the brain can be maintained at 10 degrees C, while
perfusion of the body can remain a mild to moderate hypothermia (28
degrees C). This not only allows continuous brain perfusion, but avoids
the potential coagulopathy associated with systemic hypothermia.
Discussion
Improvements to cerebral protection strategies have enabled longer
circulatory arrest times when compared to DHCA alone, which limits the
safe circulatory arrest time to only 30 minutes.18 By
incorporating ACP and RCP into the cerebral protection strategy, stroke
and mortality rates have been reduced. Although using MHCA with ACP has
been the primary neuroprotective strategy in many high-volume aortic
centers worldwide, the question of which strategy provides better
outcomes in ascending aorta, hemiarch or arch replacement remains an
area of significant controversy and debate. Thus, several studies have
attempted to answer which cerebral protection strategy offers the best
patient outcomes (Table 1).
In a prospective study, Abdelgawad and colleagues demonstrate excellent
results utilizing MHCA with ACP. Although, these results were comparable
to those of DHCA with RCP. ACP was correlated with both shorter clamp
and ischemic times with no statistically significant difference in
mortality and stroke rates.23 Other studies have also
advocated for ACP as the strategy of choice. In a 2008 study,
Apostolakis and colleagues investigated outcomes in 48 patients
undergoing urgent aortic surgery for type A dissection. Their results
concluded that ACP had a lower rate of stroke, earlier extubation,
shorter ICU stay, and reduced costs.24 Wiedemann and
colleagues analyze 329 patients undergoing aortic surgery for type A
dissection and also found ACP to be associated with better short and
long-term survival rates.25However, several studies suggest RCP with DHCA to be equivalent, if not
better, to ACP with MHCA. In a prospective, randomized study of 20
patients undergoing hemiarch replacement, Leshnower and colleagues used
magnetic resonance imaging (MRI) to evaluate the incidence of brain
injury. Despite no significant difference in clinically evident
neurologic injury between patient who had MHCA with ACP and DHCA with
RCP patients, the former had a higher incidence of radiographic
neurologic injury (n=9,100% vs n=5, 45%; p=0.01).26Moreover, an STS database analysis of 7353 patients found that
retrograde cerebral perfusion demonstrated a low risk of acute stroke in
patients who underwent type A dissection repair.27The literature is further obscured by multiple studies advocating one
technique over the other. A large Society of Thoracic Surgeons database
analysis evaluated 7830 patients undergoing hemiarch or total arch
replacement from 2014 to 2016. The study excluded aortic dissections
from the analysis. It concluded that in patients requiring more than 30
minutes of circulatory arrest, optimal strategies for cerebral
protection include deep hypothermia with either antegrade or retrograde
cerebral perfusion, or moderate hypothermia with antegrade cerebral
perfusion.28 This conclusion essentially suggests that
either strategy is acceptable as long as some form of cerebral perfusion
is used. Furthermore, in a large study of 4,128 patients undergoing
ascending aortic repair for Type A dissection in Japan, RCP had similar
results in both mortality and neurologic outcomes when compared to
ACP.29Recently, minimally invasive surgical techniques have been applied to
replacement of the ascending aorta and hemiarch at experienced centers.
The majority of minimally invasive procedures reported are performed via
an upper hemisternotomy (Fig. 3).30-32 In a
mini-thoracotomy approach, cerebral perfusion is exclusively performed
via RCP.33 The initial outcomes of the first reported
minimally invasive, right mini-thoracotomy replacement of the ascending
aorta and aortic valve in 20 patients was excellent; with no strokes,
reoperations for bleeding, conversions to sternotomy or 30-day
mortality.34 In 2018, Lamelas and colleagues further
described their experience with a sternal-sparing replacement of the
ascending aorta with or without a concomitant aortic valve replacement
via a right mini-thoracotomy in 74 patients. A propensity score-matched
analysis was performed between 63 patients undergoing a sternotomy and
an equivalent number of patients undergoing a minimally invasive
non-sternotomy approach. The 30-day mortality was 3.2 % in both groups,
while stroke incidence was 0% in the minimally invasive group and 1%
in the sternotomy group. Despite a longer circulatory arrest times in
the minimally invasive group, stroke rate, as well as the transfusion
requirements, ventilation times, as well as the ICU and hospital stays
were less than the sternotomy group.35
Conclusions
The literature regarding the preferential use of antegrade versus
retrograde cerebral perfusion during aortic surgery remains
controversial. Analysis of many large databases in thoracic surgery have
yielded excellent results for both cerebral protection strategies. The
minimally invasive hemisternotomy or right mini-thoracotomy approach to
replacement of the ascending aorta has also demonstrated excellent
results. Future studies will be needed to compare sternotomy and
minimally invasive approaches utilizing both antegrade and retrograde
cerebral perfusion techniques.