DISCUSSION
The optimal cerebral protection strategy during aortic arch surgery
remains controversial. The main finding of this network meta-analysis is
that the application of moderate and mild hypothermia in combination
with selective cerebral perfusion is associated with lower incidence of
postoperative stroke, when compared with DHCA alone or in combination
with any selective cerebral perfusion strategy. To the best of our
knowledge, this is the first network meta-analysis on the three most
commonly used hypothermia levels.
Historically, the implementation of DHCA seemed to be safe under 40
minutes 16. The general impression exists among the
surgeons that lower temperatures are safer. Despite well-recognized
complications of deep and profound hypothermia, wide-ranged
implementation of moderate temperatures is not fully adopted. This
emerges from concerns that still exist due to suboptimal organ
protection of non-DHCA approaches, even with the addition of selective
cerebral perfusion 17.
Recently the collaborative efforts of the ARCH registry have resulted in
more published global data on this topic. Keeling and colleagues18 reported the outcomes of 3265 patients from the
ARCH International aortic database who underwent total aortic arch
replacement with either MHCA or DHCA in conjunction with ACP. Comparing
the results of 669 propensity-score matched pairs, despite shorter CPB,
ACC and cerebral perfusion times in MHCA group, no significant
differences in operative mortality and postoperative neurological
complications were found.
The collaborative effort of the same investigators resulted in the
meta-analysis by Tian and colleagues 4 in which they
included 9 studies comparing DHCA with MHCA in conjunction with ACP. In
line with our findings, they reported significantly higher permanent
neurological dysfunction in the DHCA group compared with MHCA in
combination with ACP. A more recent pairwise meta-analysis by Tian et
al. 5 compared “cold” with “warm” hypothermic
circulatory arrest groups in conjunction with ACP. Mean hypothermic
circulatory arrest temperatures were 20.3 ⁰C and 26.5⁰C in the cold and
warm groups, respectively. Significantly more emergent cases were in the
“cold” hypothermia group. At the same time, the proportion of total
aortic arch replacements and bilateral ACP were similar. In line with
our founding, warmer temperatures showed significantly reduced
perioperative mortality. Furthermore, they found a significant reduction
in transient neurologic dysfunction, postoperative dialysis, ventilation
duration and intensive care unit stay compared with colder hypothermic
circulatory arrest. No significant reduction in postoperative stroke was
found.
Our sensitivity analysis of solely RCT and PSM studies revealed that
DHCA is associated with sustained higher postoperative risk of stroke
when compared with MHCA and mild HCA in combination with selective
cerebral perfusion. There was also a sustained significantly higher risk
of operative mortality for MHCA when compared with mild HCA. Though
further differences between DHCA, MHCA and mild HCA for the outcome
operative mortality are not holding up. Reason behind this result could
be underpowered analysis or the adjusted confounding associated with the
network meta-analysis in unadjusted observational studies.
There is sparse published data on the effect of hypothermia level on
postoperative spinal, renal and liver function. Recently, Liang et al.
compared the impact of MHCA with DHCA on postoperative renal function by
performing a pairwise meta-analysis of 14 observational studies6. They concluded that MHCA significantly reduced the
postoperative incidence of renal failure and the need for renal
replacement therapy. Unfortunately, we were unable to find any
differences in the postoperative occurrence of AKI.
The shift towards warmer temperatures persists but is not uniform17. The same restraint exists in the adaptation of
more evolved selective cerebral perfusion techniques, such as unilateral
cerebral perfusion 19. Skeptics advocate that it might
not sufficiently supply the contralateral hemisphere and cause
undetectable transient neurological injuries by conventional imaging
methods 20.
As the debate about optimal cerebral protection continues, this
comprehensive network-meta analysis incorporating the existing data
illustrates that despite the current concerns about warm cooling
temperatures, moderate and mild hypothermic circulatory arrests in
conjunction with selective cerebral perufsion provide improved data are
that may support daily practice as well as direct designs of future
multicentric randomized controlled trials in this regard.