MAIN TEXT
We read with great interest the manuscript by Mousavizadeh and Daliri
titled ”Hypothermic Circulatory Arrest Time affects neurological
outcomes of Frozen Elephant Trunk for Acute Type A Aortic Dissection: a
systematic review and meta-analysis”.1 The authors
explained with profound clarity the emergence of, and advancements
associated with, the frozen elephant trunk (FET) prosthesis as a method
of surgical management for complex pathologies of the proximal aorta. In
particular, the use of total arch repair (TAR) with FET for rectifying
acute type A aortic dissection (ATAAD). The authors highlighted that
although the incidence of adverse outcomes following this procedure vary
from centre to centre, postoperative complications such as spinal cord
ischaemia (SCI), stroke, and in-hospital mortality remain omnipresent
risks that could possibly be associated with time spent under
hypothermic circulatory arrest (HCA).1 Building on
this, Mousavizadeh and colleagues undertook a systematic review and
meta-analysis to evaluate the association between pre- and
intraoperative patient characteristics and post-operative complications,
with a view of elucidating any relationship between HCA time and
neurological outcome.
Surgeons tasked with repairing ATAAD can elect, depending on myriad
preoperative and intraoperative factors, to proceed conservatively or
aggressively; that is, whether to execute a hemiarch (HAR) or total arch
repair (TAR).2 Whereas HAR is typically carried out
prior to addressing the many downstream sequelae of ATAAD, the more
aggressive TAR often involves deployment of an FET prothesis to enable
control of intimal tears and encourage false lumen (FL) remodelling in a
single procedure.2 Though the TAR approach with FET is
purportedly associated with improved FL thrombosis and higher rates of
5-year freedom from rupture, reintervention, and death than HAR, it
remains technically demanding and is associated with especially
debilitating postoperative complications.2 The risk of
causing spinal cord ischaemia (SCI), stroke, acute kidney injury, or
in-hospital death are particularly pertinent – these are
well-documented, common complications associated with arch
repair.3, 4 It is therefore prudent that Mousavizadeh
and colleagues have opted to carry out a meta-analysis and systematic
review into this relationship.
The investigation benefits from robust and appropriate inclusion and
exclusion criteria, which are clearly stated in the manuscript. Notably,
by choosing to exclude studies describing non-TAR, non-ATAAD cases, as
well as those that do not provide individualised pre- and post-operative
data on ATAAD/TAR patients, the authors ensured a high degree of focus
and specificity, while still managing to capture a relatively large
sample size (3,211 patients across 35 studies).1 The
investigation also benefits from the execution of suitable and reliable
analytic tests. The results obtained via univariate meta-regression
corroborate the theory that the risk of SCI (p=0.05) and postoperative
stroke (p=0.04) is associated with increased HCA
time.1 Further, the investigators also highlighted
pooled estimates for incidence of in-hospital mortality, acute kidney
injury, postoperative bleeding, stroke, and SCI that seem to suggest an
association between prolonged HCA duration and increased risk of
postoperative complications.1 The authors highlight
that there exist discrepancies between their data and that which is
already reported, and indeed this is likely due to differences in the
overall cohort: Mousavizadeh and colleagues’ meta-analysis was carried
out on a notably homogenous cohort of only patients with ATAAD
undergoing TAR with FET.1 Yet, it should be
highlighted (as in the manuscript), that the data analysed was subject
to significant publication bias. This is apparent in both subjective
visual analysis of the Funnel plots and objective analysis via Egger’s
test used for each outcome.1 Further, the HigginsI2 values reported for each outcome suggest
high degrees of heterogeneity across the 35 studies included – these
values ranged between 63.93% (postoperative stroke) and 86.65%
(AKI).1, 5 An exception is post-operative SCI:I2 is reported as 19.56% suggesting relative
homogeneity across the studies included.1 Finally, the
authors note that all studies included were retrospective and reported
findings based on non-randomised observations, hence considerable
selection bias is therefore implicated.1 Yet, it
should be highlighted that carrying out prospective, randomised trials
are particularly challenging, impractical, and possibly unethical in the
field of cardiac surgery. Moreover, as emphasised in the manuscript,
there lacks a standard reporting method for TAR with FET, hence a degree
of heterogeneity across the 35 studies is inevitable.1,
5
Choudhury et al. estimate that SCI occurs in up to 7.3% of
patients undergoing FET implantation for ATAAD, and a meta-analysis by
Rezaei et al. reported a pooled estimate of stroke and paraplegia
occurring in 7% and 3.5% of cases respectively.6, 7Prolonged cardiopulmonary bypass (CPB), HCA, and anterograde cerebral
perfusion durations are well-documented determinants of adverse
postoperative events, neurological damage included.8Jiang et al. suggest that executing TAR with FET at moderate
(rather than deep) hypothermia (28ºC) can help reduce overall HCA
duration, and attenuate the risk of neurological
injury.8 Selective perfusion via the left subclavian
artery (LSA) has also been proposed as a means of improving spinal cord
perfusion via collateral vasculature during HCA, thereby further
lowering the risks of HCA-associated SCI and stroke.9Furthermore, the distal landing zone and graft length (longer than 15
cm, extending beyond T8) of the FET have previously been identified as a
potential risk factor for postoperative SCI; indeed, Jiang et al.suggest that decreased occlusion of the intercostal vessels, achieved
via proximalisation of FET stent to Zone 0, may contribute do decreased
SCI risk.8, 9, 10
10, 11
To conclude, the findings in this manuscript do seem to point towards
HCA duration as one of the key contributing factors towards causing
postoperative neurological complications, such as SCI and
stroke.1 The investigation was carried out with robust
methodology and analytic approaches, and all potential limitations
thereof are reported by the authors.1 This
meta-analysis and systematic review therefore provides valuable insight
into the relationship between HCA duration and postoperative
neurological complications, and recommends further prospective research
into the area to fully ascertain whether HCA duration is the true
culprit in causing SCI and stroke.