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.