Key words: aorta, dissection, critical care, respiratory
Acute type A aortic dissection (ATAAD) is a clinical emergency and is
associated with significant mortality and morbidity which necessitates
immediate surgical intervention.1 Although surgical
techniques have improved over time, resulting in lower mortality
rates,2 the high incidence of post-operative
complications remains the primary factor influencing the patient’s long
term outlook. One of these complications is acute lung injury (ALI), a
major cause of post-operative mortality with an incidence of 15 to
30%.3 Wang et al. should be commended for
developing a scoring model for severe ALI following ATAAD and
highlighting the predictive risk factors. 4
The results of this study extend the findings of the previous models
that predict ALI following ATAAD and are comparable to what is reported
elsewhere in the literature.5,6,7 However, Wang et al
are the first to incorporate the pre-operative echocardiographic
variables in the predictive model and demonstrated that left atrial (LA)
diameter ≥ 35.5 mm and left ventricular posterior wall thickness (LVPWT)
≥ 10.5 mm were independent risk factors for severe ALI after ATAAD. This
warrants future research regarding potential underlying mechanisms,
particularly the role of left ventricular diastolic dysfunction of which
these are both soft indicators, and the extended role of
echocardiography in predicting complications following ATAAD repair. In
comparison to the previous models, 5–7 the authors
included variables that were available in the emergency, pre-operative
setting. This allows clinicians to identify patients at risk of
developing severe ALI and assist in timely intervention, potentially
leading to better outcomes. This is particularly useful in low-resource
settings, where cost and technology are major limitations for the
management of these high-risk patients. 8
Although the prediction model by Wang et al . uses a very
reasonable method, several significant limitations should be
acknowledged. Firstly, the ALI and non-ALI groups were significantly
different regarding the percentage of obese patients – with 5.8% in
the non-ALI group and three times the value in the ALI
group.4 Prior work shows that obese patients are at
risk of requiring higher inspiratory pressures, and therefore excess
barotrauma subsequently increasing their post-operative ALI
risk.9 Furthermore, obesity is associated with a high
risk of atelectasis during the perioperative period due to the effect of
additional thoracic wall weight and abdominal fat mass impinging
diaphragmatic excursion. This leads to a reduction in functional
residual capacity, arterial oxygenation and the requirement for higher
positive end expiratory pressure (PEEP).9
Another significant limitation is the inclusion of neutrophils as the
surrogate inflammatory marker in the statistical analysis; other markers
such as c-reactive proteins (CRP) and interleukin 6 (IL-6) were not
included despite being validated to be associated with ALI post ATAAD
surgeries. 4 The authors were unable to explain the
exclusion of other white blood cell parameters and the neutrophil to
lymphocyte ratio when assessing the extent of inflammation.
During ATAAD surgeries, blood transfusion is almost ubiquitous due to
the profound coagulopathy associated with a long cardiopulmonary bypass
time and extensive surgery. Therefore, there is an associated risk of
acute respiratory distress syndrome (ARDS) that occurs through several
mechanisms.10 Transfusion-related acute lung injury
(TRALI) is a serious complication that is characterised by a quick onset
of hypoxia and bilateral lung infiltration and a high mortality rate of
5% to 25%.10 Blood products containing leukocyte
antibodies, lipids and cytokines will elevate pulmonary vascular
permeability, thereby causing lung damage and capillary
leakage.10 Blood transfusion should also be considered
as an independent risk factor for postoperative acute respiratory
distress syndrome in patients undergoing ATAAD
repairs.10 Achieving meticulous haemostasis and
improving surgical techniques in order to reduce perioperative blood
transfusions are both parts of a targeted strategy to prevent
TRALI.10 An alternative mechanism for the development
of ARDS in this setting it transfusion-associated circulatory overload
(TACO).10 Patients who develop this complication often
show improvement from diuresis, but it can be difficult to distinguish
from TRALI in its immediate presentation.10 Platelet
activation is also a recognised independent risk factor for ALI in
patients with ATAAD, as they are activated by inflammatory mediators
such as thromboxane A2 (TXA2).11 Platelet count is
reduced in patients with an unruptured abdominal aortic aneurysm, and
thrombocytopaenia has been proposed as a method to predict the increased
risk of in-hospital mortality of patients with
ATAAD.11
Alongside inflammatory markers, certain demographic features should be
considered as an independent risk factor for high mortality in patients
with ATAAD. It is well recognised that ageing increases the risk of
thrombolytic and inflammatory disorders, and therefore mortality in this
cohort.11 Ageing is also associated with intrinsic
platelet activation, aggregation and secretion which increases the risk
of ALI due to increased vascular permeability.11
This is a new prediction model, so there remains a need for long-term
follow-up and further validation of the system in other centres in order
to bolster confidence in it and suitability for more widespread use. The
model holds great promise based on extensive statistical assessment and
a large patient cohort. Various independent risk factors of ALI in
patients with ATAAD, however, have been outlined in other publications,
and these should be carefully considered during the perioperative care
of ATAAD patients to optimise outcomes.
References:
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