Cardiovascular Journal of Africa: Vol 31 No 4 (July/August 2020)

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CARDIOVASCULAR JOURNAL OF AFRICA • Volume 31, No 4, July/August 2020 AFRICA 207
loci of single nucleotide polymorphisms (SNP) related to renal
function, there are two new loci that are specifically associated
with an increased risk in AKI following cardiac surgery.26 This
discovery potentially increases the curiosity in researchers to
further investigate the relationship between AKI and cardiac
surgery at a genetic level.
Risk factors
The risk factors associated with AKI in this patient population
can be grouped into patient-, surgery- and anaesthesia-related
factors.11 Risk factors associated with CSA-AKI can also be
classified into non-modifiable and modifiable risks, enabling the
identification of high-risk patients with modifiable risk factors
that can be optimised for improved outcomes.27For the purposes
of this review, the latter will be discussed.
Modifiable risk factors
The following modifiable risk factors will be discussed: the
duration of CPB time, miniaturised extracorporeal circuit (mini-
CPB), hypothermia, on-pump versus off-pump techniques,
anaemia and blood transfusion, which contribute to the increased
risk of AKI following cardiac surgery.1,28
Duration of CPB
CPB has been reported to induce systemic inflammatory
response syndrome (SIRS), which is a mechanism related to the
development of AKI in cardiac surgery patients.1
In a 2012 meta-analysis by Kumar et al.,29 nine studies were
included, resulting in a cohort of 12 466 patients, where a
total of 756 patients developed AKI following cardiac surgery,
correlating to longer durations of CPB.29 Similarly, Mao et al.1
had indicated a strong association between cardiac surgery-
related AKI with longer CPB times. Mini-CPB has been found
to offer benefits of improved renal function following cardiac
surgery.1 It led to a lesser inflammatory response with reduced
haemodilution when compared to the standard CPB systems.1
Benedetto et al .30 investigated the occurrence of AKI when using
the mini-CPB to that of the conventional CPB in patients for
planned CABG surgery and showed that in patients where the
mini-CPB was used, there were fewer reported cases of cardiac
surgery-associated AKI compared to the conventional CPB.30
Hypothermia
During cardiac surgery, patients are often cooled down to
systemic temperatures below 32°C allowing longer periods of
decreased blood flow.1 In a meta-analysis of 19 randomised,
controlled trials with 2 218 patients, therapeutic hypothermia
was not seen to prevent the development of AKI or the
requirements for RRT.31
It has been postulated that the process of rewarming the
patient post cardiac surgery could be one of the reasons related
to increased ischaemia and reperfusion injury to the kidneys.32
This hypothesis was tested in a study by Boodhwani et al.32
on the effects of mild hypothermia and rewarming on renal
function following CABG surgery. In this randomised control
trial, patients undergoing elective CABG surgery were assigned
to two groups. In the first group, patients were cooled down to
a temperature of 32°C during CPB and then randomly assigned
to be rewarmed to 34°C or 37°C. Results showed elevated serum
creatinine levels in patients who were rewarmed to 37°C. They
concluded that rewarming to 37°C should be avoided as it
contributed to postoperative renal injury.32
On-pump versus off-pump technique
CPB is implicated in inducing a SIRS response that causes
AKI related to cardiac surgery.1, 33 Off-pump coronary artery
bypass (OPCAB) induces less SIRS response when compared to
on-pump CABG surgery.34 Better renal perfusion and decreased
systemic embolisation have been found. 1 There is still however
conflicting evidence in the current literature when comparing
these two techniques, with some authors showing that OPCAB
is superior to the on-pump technique, while other authors have
shown no variation. 35-37
Anaemia
In AKI, peri-operative anaemia contributes to a decrease
in oxygen delivery to the renal tubules, therefore promoting
oxidative stress, especially in the already compromised renal
medulla.38 The oxidative stress is as a result of the reduction in red
blood cells, which also have an antioxidant function.39 Anaemia is
further worsened by cardiac surgery as cardiac output is further
decreased, influencing renal perfusion pressures.38 CPB-related
complications resulting in poor platelet function combined with
anaemia, which requires blood transfusion, was seen as a factor
that contributed to the additional risk of AKI.38
Blood transfusion
The transfusion of packed red blood cells is not without
complications. Koch et al.40 showed that the duration of red
blood cell storage can contribute to complications following
cardiac surgery. They concluded that the transfusion of red
blood cells older than two weeks was associated with an
increased risk of postoperative cardiac surgery complications.
Structural and functional changes to red blood cells were
shown to include the depletion of adenosine triphosphate (ATP)
and 2,3 diphosphoglyrecate (2.3 DPG) in the red blood cell. 41
Alterations in the structural proteins of red blood cells made
them less deformable and contributed to pro-inflammatory
cellular states.41 This abnormal state of the red blood cells causes
injury to target organs such as the kidneys and impairs the
necessary oxygen delivery. 38 Poor oxygen delivery to the kidneys
causes an insult to the renal tissue, a phenomenon termed
‘kidney attack’, 42 resulting in ischaemic injury.16
Hypovolaemia
Hypovolaemia has been recognised as a significant risk factor in
the development of AKI following cardiac surgery.5 A patient’s
volume status plays an important role in the peri-operative period
as it is related to CO.1 A low CO causes activation of the SNS
which ultimately stimulates the RAAS, thus resulting in renal
vasoconstriction.1 The choice between crystalloids and colloids
for intravascular volume expansion is still a topic of debate.5
Diagnosis of AKI
The kidneys receive 20% of the heart’s CO.43 A reduction in
renal blood flow, whether generalised or localised, can cause
a decrease in GFR, thus resulting in AKI. 15 GFR is used