Discussion
In our retrospective study, HAP was the main cause of postoperative NI
accounting for 73.13%. it was also a leading cause of mortality as high
as 17.34%. The generation of HAP was associated with the operation of
endotracheal intubation and mechanical ventilation. Intubation destroyed
the normal barrier of epiglottis and weakened the cough reflex and
movement of cilia, which led to the impairment of organism clearance of
airway secretion. Sputum was a good culture medium for bacteria.
Mechenical ventilation also contributed to the development of VAP and
the risk peaked within the first week. The initial step of VAP was the
colonization of potentially pathogenic bacteria in the upper respiratory
tract. Aspiration of these microorganisms either through the
endotracheal tube or a leak around the cuff allowed them to enter the
lower respiratory tract. Accompanied with diminished host immunity, NI
developed.
The predominant bacteria for HAP was
Acinetobacter baumannii. A. baumannii
is a conditional pathogen that may cause NI in critically ill patients.
A. baumannii has simple growth requirments and may survive in desiccated
environment for prolonged periods [10]. Contaminated environmental
sources and transmission via medical personnel may cause outbreaks of NI
[11,12]. A. baumannii has been associated with high mortality and
morbidity [13,14].]Vincent JL and colleagues reported that
infection with A. baumannii was independent associated with a greater
risk for hospital death among 14414 ICU patients [15]. In recent
years, the incidence of A. baumannii infection increased rapidly as well
as its antibiotic resistance. Treatment of A. baumannii is difficult
owing to its resistance to various antibiotics and remarkable ability to
acquire new resistance via different mechanisms, such as plasmids,
transposons, integrons and resistance islands. Antimicrobial resistance
has posed a serious threat to the whole world. In early 1990,
carbapenem-resistant (CPR) strains of
A. baumannii emerged. CPR-A.
baumanniis were often resistant to all classes of antibiotics except for
colistin and tigecycline [16]. Furthermore, a large dose of
sulbactam, fluoroquinolones, aminoglycosides and tetracyclines may also
have bacterial activity against CPR-A. baumanniis [17-20].In our
study, we discovered that the carbapenem-resistance rate of A. baumannii
had reached up to >50%. 90% of A. baumannii were
sensitive to colistin and tigecycline. While the resistance rate of
cefperazone-sulbactam, levofloxacin and minocyline were 34.00%, 42.00%
and 38.10% respectively. Instead of amikacin, we routinely analyzed the
drug sensitivity of gentamicin as a representative of aminoglycosides
but fortunately the resistance rate exceeded 80%.
The principles of treatment for A. baumannii were: ⑴antimicrobial
susceptibility result ; ⑵combination therapy; ⑶enough dose; ⑷enough
period; ⑸personal administration. Optimal therapy was established
according to antimicrobial susceptibility result. But for MDR or XDR A.
baumannii, the recommended therapy was colistin/tigecycline combined
with other agents (i.e., carbapenem, sulbactams, fluoroquinolones or
minocyline).
Except for A. baumannii, Klebsiella pneumoniae ,Pseudomonas aeruginosa
and other G- bacilli were also common pathogens for HAP. As proved in
our study, all of them were clinically sensitive against carbapenem,
extended-spectrum cephalosporins and fluoroquinolones.
BSI is also a serious and common type of NI. It more likely happened in
patients with immunosupression, malnutrition and various invasive
devices. CRBSI is a subset of BSI with the presence of central venous
catheters. Immediately after insertion, the catheter becomes coated with
plasma protein. Bacteria could migrate from the skin along the surface
of catheter. This may happen few hours or more than one week after
insertion. Femoral venous catheter has the highest rate of infection,
followed by internal jugular and subclavian ones [21]. For CRBSI,
once infeciton is suspected, the central catheter should be removed as
soon as possible. In our research, the most common pathogens for BSI
were still G+ cocci (50%), for example Staphylococcus and Enterococcus.
Within the past decades, resistance rate among G+ cocci escalated
obviously. We reported that 78.13% of G+ bacteria were resistant to
methicillin and only 6.25% were resistant to vancomycin. However, all
of them were sensitive to linezolid.
Fungal infection is not unusual after cardiac operation. Risk factors
for fungal infection include immunosupression, malnutrition, diabetes
mellitus and long period use of extended spectrum
antibiotics[22,23]. In our research,
Candida was the most common agent of
fungal infection (88.89%; 8/9), which was concordant with previous
studies [24-26]. Among 8 strains of candida, 6 were Candida albicans
and 2 were Candida tropicalis, with 7 strains isolated from bloodstream.
Other than Candida, Aspergillosis was also a common agent of fungal
infection . Lung was the most frequent site of aspergillosis
infection[27]. Consistent with our result, most candidas were
susceptible to fluconazole and voriconazole but not to candida
glabrata. Aspergillosis was naturally
resistant to fluconazole. Echinocandins was the best choice for definite
and severe fungal infection, because it remianed close to 100%
effective.
Since the distribution of pathogenic bacteria and antibiotics resistance
of NI after cardiac surgery vary distinctly worldwide, our research data
only provide the epidemiological profiles and trends of our institution
and play a certain guiding significance for the prevention and treatment
of NI.