Risk Factor Of Acinetobacter Baumannii Associated Ventilator-associated Pneumonia: A Nested Case-control Study

Background: Ventilator-associated pneumonia(VAP) is a common complication of hospitalisation that occurs in patients who receive mechanical ventilation(MV). According to the epidemiologic studies worldwide, the incidence of VAP ranges from 6% to 52%, 1.6-52.7 per 1,000 ventilation days. The mortality associated with VAP ranges from 14% to 50%, and is even higher to 76%(Attributable mortality, 20% to 30%) among patients infected by multidrug-resistant(MDR) or pan drug-resistant(PDR) bacteria. In China, the incidence of VAP ranges from 4.7% to 55.8%, 8.4-49.3 per 1000 ventilation days. In addition, the mortality associated with VAP ranges from 19.4% to 51.6%. VAP increases can the time on a ventilator by 5.4 to 14.5 days, length of ICU stay by 6.1 to 17.6 days and length of total hospital stay by 11 to 12.5 days.The most common microorganisms implicated as causative agents of VAP are Gram-negative bacteria such as Pseudomonas aeruginosa and Acinetobacter baumannii. Acinetobacter baumannii is a Gram-negative bacillus that is aerobic, pleomorphic and non-motile. An opportunistic pathogen, Acinetobacter baumannii is widely distributed in nature and the hospital environment such as water, solid and human skin. It has been the most common causative infection, pathogen of hospital-acquired infections, due to its versatile genetic machinery, which allows it to quickly evolve resistance factors, and to its remarkable ability to tolerate harsh environments. Acinetobacter baumannii has emerged as a pathogen frequently incriminated in the patients receives mechanical ventilation, due to the serious illness, accompanied by a variety of basic diseases and immune dysfunction.The strategy of the nested case-control design was introduced by Mantel in 1973 when he described that a large prospective study in which few subjects develop the outcome of interest could be converted into a “synthetic retrospective study” by selecting all of the cases and a random sample of the noncases. It was formally referred to as a “nested case-referent study” by Miettinen in 1982. The nested case-control study is an efficient epidemiological design whereby a case-control approach is employed within an established cohort and it is widely used in the biomedical research. The nested case-control design is particularly advantageous for studies of biologic precursors of disease. Compared with case-control studies, nested case-control studies can reduce ’recall bias’ and temporal ambiguity, and compared with cohort studies can reduce cost and save time.Purpose: The aim of this study was to evaluate risk factors for ventilator-associated pneumonia(VAP) due to Acinetobacter baumannii in critically ill patients and provide the evidence for rational use of antibiotics and hospital-acquired infections treatment and management.Methods: We performed a nested case-control study by individually matching each pneumonia-confirmed case ventilated for more than 48 h in the ICU and Critical Care Medicine Department of Xintai People’s Hospital between January 2014 and December 2014. The exclusion criteria include the patients were younger than 18 years old, death in 3 days after ventilated and patient with severe multisystem dysfunction. The characteristics and clinical data of patients were collected by the questionnaire. The 1:1 control were enrolled with admission department, admission time, gender, age and clinical disease classification. Differences in categorical data were compared using χ2 test. Risk factors were evaluated by univariate regression analysis and multiple logistic regression analysis. All analyses were performed using the statistical software SPSS17.0 and using P=0.05 to indicate statistical significance.Results: Totally 118 ventilator-associated pneumonia(VAP) cases caused by Acinetobacter baumannii was enrolled. Univariate regression analysis identified eight factors to be associated with VAP(P<0.05): length of hospital stay, time on a ventilator, coma, and clinical use of Carbapenems, clinical use of combination antibiotics, smoking, cancer and chronic lung disease. The length of hospital stay more than 30 days, time on a ventilator more than 7 days, coma, clinical use of Carbapenems, clinical use of combination antibiotic more than 3 kinds of antibiotic, smoking and chronic lung disease were identified as independently risk factors to be associated with VAP(P<0.05) by multiple logistic regression analysis.Conclusion: The nested case-control study is an efficient epidemiological design and particularly advantageous for studies of biologic precursors of disease. Compared with case-control studies, nested case-control studies can reduce ’recall bias’ and temporal ambiguity, and compared with cohort studies can reduce cost and save time.The length of hospital stay more than 30 days, time on a ventilator more than 7 days, coma, clinical use of Carbapenems, clinical use of combination antibiotic more than 3 kinds of antibiotic, smoking and chronic lung disease are independently risk factors associated with VAP in our hospital.