CARDIOVASCULAR JOURNAL OF AFRICA • Vol 23, No 4, May 2012
214
AFRICA
Discussion
Although cardiac surgery-related mortality has substantially
reduced due to advances in surgical techniques and peri-
operative care, the incidence of pneumonia post cardiac surgery
is still high, varying between 1.5 and 21% in most series.
3-8
Hortal
et al
.
9
reported a 45.9% incidence of pneumonia in the sub-group
of patients needing mechanical ventilation for longer than 48
hours. This wide range in the incidence rates was attributed to
the difference in the characteristics of the study population and
the diagnostic criteria used to define nosocomial pneumonia.
3
Surgical technique plays an important role in the occurrence
of nosocomial infections. For instance, inadequate haemostasis
can lead to hypovolaemia, resulting in an increased need for
blood transfusion, inotropic support, duration of surgery, or even
possible re-operation. However, besides the surgical technique,
several risk factors for pneumonia post cardiac surgery have
been identified: age,
4,9
unnecessary use of broad-spectrum
antibiotics,
5,10,11
duration of mechanical ventilation,
4-6,9,12,13
CPB
time,
3,9
re-intubation,
3,4,9
emergency surgery,
4,5,9
intra-operative
inotropic support,
9
and pre-operative renal dysfunction.
14
In univariate analysis of our study, duration of mechanical
ventilation had a significant effect on postoperative pneumonia,
whereas age, gender, CPB time and need for inotropic support
had no association with pneumonia. Postoperative but not
pre-operative high creatinine and urea levels, indicating renal
dysfunction, were more common among patients with pneumonia
compared to those without pneumonia. However, multivariate
analysis depicted only prior COPD, transfusion of pRBC and
postoperative atrial fibrillation as independent risk factors for
pneumonia.
COPD was reported to cause postoperative pneumonia in
another series.
5
Nosocomial pneumonia is a frequent event in the
course of acute exacerbation of COPD. There is clear evidence
that in up to 50% of stable COPD patients, the lower airways are
colonised by potential pulmonary pathogens. Advanced age and
severity of lung disease are strongly associated with increased
risk for pneumonia.
Cardiac surgery, especially CPB, aggravates COPD. Moreover
the use of inhaled corticosteroids among patients with COPD
significantly increases the risk of developing pneumonia.
15,16
According to Lomas,
17
inhaled corticosteroid use for at least
24 weeks is associated with a 60 to 70% higher relative risk of
pneumonia. Corticosteroid use before elective cardiac surgery
may be limited or at least the dose may be decreased in order to
decrease the incidence of pneumonia. In addition, immunisation
against influenza in older patients with COPD is associated with
a 52% reduction in hospitalisations for pneumonia.
18
The second independent risk factor for the occurrence
of pneumonia following cardiac surgery was the need for
blood transfusion, consistent with previous reports.
3-5,8,9
Blood
transfusions may cause transient immune suppression, thus
increasing the susceptibility to infection. It was found to be an
independent risk factor of deep sternal wound infections.
19
The mechanism of the immunomodulatory effect of allogenic
blood transfusion remains elusive. The infusion of foreign
antigens in either soluble or cell-associated form has been shown
to induce immune suppression, anergy and clonal deletion, most
likely mediated by allogenic white blood cells.
8
Another concern is the altered function of macrophages.
After transfusion macrophages lose migratory ability in response
to chemotactic stimuli and produce more prostaglandin E2,
resulting in decreased activity of antigen-presenting cells and the
production of interleukin 2.
3
There is an association between the length of storage of
transfused red blood cells and the development of postoperative
pneumonia,
3,8
which is seen more rarely among patients with
fresh red blood cell transfusions. Various immunosuppressive
substances are released from white blood cell granules into
red blood cell components during blood storage, contributing
to transfusion-induced immunomodulation.
8
Furthermore, the
deleterious effect of stored blood may be due to depleted levels
of 2,3-diphosphoglycerate and decreased deformability of stored
red blood cells, both impairing oxygen delivery to the tissues.
3
Recently, the involvement of inflammation in atrial fibrillation
has been documented, and high levels of pro-inflammatory
proteins, such as C-reactive protein, have been suggested
to promote the persistence of atrial fibrillation by inducing
structural and/or electrical remodelling of the atria. Atrial
biopsies taken from patients in AF have also demonstrated
evidence of inflammatory infiltrates within the atrial tissue, with
evidence of oxidative damage or occult myocarditis, even among
persons who were thought to have had lone AF.
20
The fact that inflammation plays an important role in the
development of both AF and pneumonia may explain the
concomitance of these complications after cardiac surgery. It
is classic knowledge that pneumonia is one of the non-cardiac
causes of AF, predominantly in elderly patients. However, in our
study, AF was an independent risk factor for pneumonia post
cardiac surgery.
In patients with AF, contraction of the ventricles averages
150/minute. At that rate, the ventricles may not have enough
time to fill maximally with blood before the next contraction,
particularly without the normal contraction of the atria.
21
Moreover, the contractility of the ventricle decreases after CPB.
Therefore AF decreases the amount of blood pumped by the
ventricles and the body begins to compensate by retaining fluid,
resulting in the accumulation of fluid in the lungs. Alveolar
oedematous fluid is a good culture medium for the development
of secondary pneumonia. CPB contributes to this process by
aggravating the pulmonary oedema and inflammation.
22
Also, AF may play an active role in the development
of postoperative pneumonia by prolonging the postoperative
intubation time. However, postoperative AF has not been
suggested as a cause of pneumonia post cardiac surgery in any
previous reports. Our findings must be confirmed in larger series
and it must be clarified whether pneumonia is only a cause or
also a consequence of AF.
The main limitations in our study were the retrospective nature
of the analysis and the small sample size, affecting particularly
the subgroup with postoperative pneumonia. Therefore any claim
about an associative relationship between pre- or peri-operative
variables and outcomes should be viewed with caution. The
study group was also not treated entirely uniformly, as off-pump
CABG patients were included. However, peri-operative care
was standardised, which serves to strengthen conclusions on the
results.
Conclusion
On logistic regression analysis, pRBC transfusion, previous
