Ventilator-associated Pneumonia caused by commensal oropharyngeal Flora: a retrospective Analysis of a prospectively collected Database

The study was conducted at the Maastricht University Medical Centre, a 715-bed hospital with approximately 30,000 annual admissions, 18 mixed surgical-medical ICU beds, and 9 post-cardiothoracic surgery beds. Other elective postoperative patients are rarely admitted, due to a 24-h post anaesthesia care unit. When technically possible and safe, a BAL was performed in all mechanically ventilated patients who met the clinical criteria of suspected VAP. These criteria include (≥2 of the following) a rectal temperature > 38.0 °C or < 35.5 °C, white blood cell count > 10,000/μl or < 3,000/μl, purulent sputum, and a new, persistent or progressive infiltrate on chest X-ray [ 25]. In patients with localized pulmonary lesions, the affected region was sampled, whereas in case of diffuse pulmonary lesions the middle lobe or lingula was lavaged. The BALF was microbiologically evaluated within 15 min after it was obtained. Selective oropharyngeal decontamination (SOD) is used since December 2010, whereas selective digestive tract decontamination (SDD) is used since January 2012. The SOD consists of topical antibiotics (polymyxin E, tobramycin, amphotericin B) applied to the oropharynx, whereas the SDD consists of oropharyngeal and gastric application of the same non-absorbable antibiotics along with a four day course of intravenous cefotaxime. The ethics committee of the institution, the “Medical Research Ethics Committee”, approved the study and informed consent was regarded unnecessary since standard care was provided.

Ventilator-associated pneumonia in clinically suspected cases (for definition, see previous paragraph) was diagnosed if subsequent BALF analysis was indicative for pneumonia: cultures yielding a potentially pathogenic microorganism [ 12] ≥ 10 ⁴ cfu/ml and/or if ≥ 2 % BALF cells containing intracellular organisms (ICOs) [ 10, 26]. In a pneumonia suspected case that was admitted from home less than 3 days prior to diagnosis, community-acquired pneumonia (CAP) was considered, if this case had no recent contact with the healthcare system. A CAP was also considered when the potentially pathogenic microorganism was very unlikely to be nosocomial (e.g. Haemophilus influenzae, Mycoplasma spp.) [ 27]. When a clinical suspected case was admitted in the hospital for more than 3 days and with positive BALF results, but was not mechanically ventilated for ≥ 48 in the 72 h prior to the pneumonia, hospital-acquired pneumonia (HAP) was diagnosed [ 14].

Commensal oropharyngeal flora as the cause of VAP was considered in VAP suspected cases if BALF quantitative cultures revealed COF ≥ 10 ⁴ cfu/ml without significant growth (≥10 ⁴ cfu/ml) of other potentially pathogenic microorganisms. Commensal oropharyngeal flora included (a combination of) the following bacteria: viridans streptococci, coagulase-negative staphylococci, Haemophilus spp. (excluding H. influenzae if not predominant), Moraxella spp. (if not predominant), Corynebacterium spp., Neisseria spp., Peptostreptococcus spp., Stomatococcus spp., and Prevotella spp. [ 28]. Whereas Candida spp. may occasionally cause a pneumonia [ 29], Candida spp. were considered nonpathogenic in this study, consistent with previous studies and guidelines [ 30– 32].

From January 2005 until January 2014, all results of BALF analyses from patients consecutively admitted to the ICU were prospectively collected. From this database, patients with suspected COF as the cause for VAP were included in the present study. Cases lacking a microbiological BAL report were excluded. Retrospectively, the following clinical data were collected or calculated from the included cases: body temperature, C-reactive protein (CRP), white blood cell count, antibiotic administration, ICU length of stay, hospital length of stay, duration of mechanical ventilation, mortality, acute physiology and chronic health evaluation (APACHE)-II score (to calculate standardized mortality ratio [SMR; observed mortality divided by expected mortality]) [ 33], sequential organ failure assessment (SOFA) score (to determine the extent of critical illness) [ 34, 35], clinical pulmonary infection score (CPIS; more than 6 points is indicative of pneumonia) [ 36], and post-mortem examination, if available.

On the advice of the statistical department a reference population was used in order to place the results of suspected cases in perspective. Since June 2013, the hospital participated in the Dutch National Intensive Care Evaluation registry. To experience the possibilities of this registry, the characteristics of all patients admitted from June 2013 to April 2014 in the same ICU were extracted from this database. Post cardiothoracic surgery patients (44 % of all ICU admissions) were excluded. It should be realized that a reference population is not a control group and interpretation of finding should be performed in this perspective.

Bronchoalveolar lavage fluids were initially analysed according to a highly standardized protocol as described elsewhere [ 37]. From each BALF sample, 6 ml was centrifuged (250 g for 10 min), dividing the sample into cells and supernatant. The supernatant was stored in tubes of 1 ml at −80 °C. The cells were re-suspended in 6 ml of a mixture of Eagle’s Minimal Essential Medium with 2 % Dimethyl Sulfoxide and stored in tubes of 1 ml at −80 °C. Oropharyngeal flora was (formerly) reported on the basis of classical bacteriological phenotypic identification tests. In order to confirm and specify these results, included samples were defrosted and quantitatively cultured on blood agar, chocolate agar, and MacConkey agar. The different colony types were identified using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS), a rapid and highly accurate soft ionization technique [ 38]. Antibiotic susceptibility was assessed for all separate strains when quantitative cultures revealed ≥ 10 ⁴ cfu/ml. Results of polymerase chain reactions (PCRs) for viruses and Pneumocystis jirovecii, as well as Grocott’s methenamine silver staining, were collected from electronic patient data organisers using SAP. In patients that were both admitted to the hospital less than 8 days prior to BAL procedure and that lacked PCR results, PCRs for the identification of respiratory viruses were performed on the defrosted BALF. These viruses included influenza virus A and B, human respiratory syncytial virus, human metapneumovirus and parainfluenza virus 1–4. Furthermore, results of endotracheal aspirates (ETA) were analysed, preferably from the day of BAL, otherwise one day before or after. Endotracheal aspirates were obtained twice weekly and in case of clinical suspicion of a pulmonary infection. Samples were immediately microbiologically evaluated when obtained during daytime.

Based on the collected clinical, radiographic, and microbiological data and subsequent microbiological analyses made, the likelihood of presence of COF caused VAP (COF-VAP) was evaluated by 4 researchers (two consultant ICU physicians, one medical microbiologist, one ICU researcher).

Patient characteristics were analysed using descriptive statistics and presented as the mean ± standard deviation, median including interquartile range, or absolute numbers and percentages of patients, where applicable. Demographic and clinical characteristics of the suspected cases were compared with the reference population using the single sample t-test, the paired samples t-test or the Fisher’s exact test, where appropriate. For the comparison of the observed and expected mortality, a Chi-square test was used. A one-way within-subjects ANOVA was used to analyse the course of the SOFA-score, body temperature, leukocytes and CRP. Statistical significance was defined as P < .05. The IBM SPSS Statistics version 20 for Windows (Chicago, IL, USA) was used for analysis.