Role of soluble triggering receptor expressed on myeloid cells-1 for diagnosing ventilator-associated pneumonia after cardiac surgery: an observational study
© Matsuno and Carlotti; licensee BioMed Central Ltd. 2013
Received: 5 September 2013
Accepted: 27 November 2013
Published: 1 December 2013
The diagnosis of ventilator-associated pneumonia (VAP) is a challenge, particularly after cardiac surgery. The use of biological markers of infection has been suggested to improve the accuracy of VAP diagnosis. We aimed to evaluate the usefulness of soluble triggering receptor expressed on myeloid cells (sTREM)-1 in the diagnosis of VAP following cardiac surgery.
This was a prospective observational cohort study of children with congenital heart disease admitted to the pediatric intensive care unit (PICU) after surgery and who remained intubated and mechanically ventilated for at least 24 hours postoperatively. VAP was defined by the 2007 Centers for Disease Control and Prevention criteria. Blood, modified bronchoalveolar lavage (mBAL) fluid and exhaled ventilator condensate (EVC) were collected daily, starting immediately after surgery until the fifth postoperative day or until extubation for measurement of sTREM-1.
Thirty patients were included, 16 with VAP. Demographic variables, Pediatric Risk of Mortality (PRISM) and Risk Adjustment for Congenital Heart Surgery (RACHS)-1 scores, duration of surgery and length of cardiopulmonary bypass were not significantly diferent in patients with and without VAP. However, time on mechanical ventilation and length of stay in the PICU and in the hospital were significantly longer in the VAP group. Serum and mBAL fluid sTREM-1 concentrations were similar in both groups. In the VAP group, 12 of 16 patients had sTREM-1 detected in EVC, whereas it was undetectable in all but two patients in the non-VAP group over the study period (p = 0.0013) (sensitivity 0.75, specificity 0.86, positive predictive value 0.86, negative predictive value 0.75, positive likelihood ratio (LR) 5.25, negative LR 0.29).
Measurement of sTREM-1 in EVC may be useful for the diagnosis of VAP after cardiac surgery.
KeywordsTriggering receptor expressed on myeloid cells-1 Ventilator-associated pneumonia Diagnosis Cardiac surgery Postoperative
Ventilator-associated pneumonia (VAP) is the most common nosocomial infection in children following cardiac surgery in Brazil and European countries [1–3]. Its reported incidence ranges from 17.1 to 42.8 episodes per 1000 ventilator-days and it is associated with increased duration of mechanical ventilation, increased length of pediatric intensive care unit (PICU) and hospital stay and higher mortality rates [1, 2, 4].
The diagnosis of VAP is a major challenge in patients undergoing cardiac surgery, because the systemic inflammatory response syndrome elicited by surgical trauma and the interaction of blood with the cardiopulmonary bypass (CPB) circuit may induce symptoms that are similar to those caused by systemic infections . In addition, imaging studies may show pulmonary opacities caused by fluid overload, capillary leak, pulmonary contusion resulting from surgical manipulation, atelectasis or alveolar hemorrhage, which are comparable to those found in patients with pneumonia. Although in critically ill patients with severe bacterial infections early initiation of appropriate antibiotic therapy is crucial for survival, in patients with the systemic inflammatory response syndrome associated with non-infectious processes unnecessary treatment with antibiotics may lead to increased bacterial resistance and increased costs, with no treatment benefit. This highlights the importance of making an accurate diagnosis of VAP.
The gold standard method for diagnostic confirmation of VAP is lung tissue examination and culture [6, 7], but its clinical applicability is limited. The clinical and radiologic criteria recommended by the Centers for Disease Control and Prevention (CDC) have limited specificity whereas the microbiological criteria lack specificity and sensitivity compared with autopsy lung culture . Thus, the use of biological markers of infection as the triggering receptor expressed on myeloid cells (TREM)-1 has been suggested to improve the accuracy of the diagnosis of VAP [8–10].
TREM-1 is a transmembrane glycoprotein expressed on neutrophils, macrophages and monocytes that amplifies the inflammatory response. Its expression by the effector cells is upregulated in tissues infected by bacteria and fungi . The activation of TREM-1 stimulates the production of proinflammatory cytokines and induces neutrophil degranulation and oxidative burst . Moreover, infection induces the release of a soluble form of this receptor from the membranes of activated phagocytes (sTREM-1) which can be measured in body fluids and may be a useful diagnostic tool . It has been shown that plasma sTREM-1 concentrations are increased in patients with sepsis . In addition, elevated levels of TREM-1 have been found in bronchoalveolar lavage (BAL) fluid and in the fluid collected from the ventilator expiratory trap, the exhaled ventilator condensate (EVC), in critically ill adults with VAP [8–10]. However, increased plasma concentrations of TREM-1 have also been reported in patients with the systemic inflammatory response syndrome secondary to cardiac surgery with CPB and after cardiac arrest, without infection . Nevertheless, there are no published data on the role of sTREM-1 in the diagnosis of VAP in patients undergoing cardiac surgery. To address this question, we performed serial measurements of sTREM-1 concentrations in serum, modified non-bronchoscopic bronchoalveolar lavage (mBAL) fluid and EVC samples in intubated and mechanically ventilated children following cardiac surgery. We sought to evaluate whether the use of sTREM-1 could improve the accuracy of VAP diagnosis after cardiac surgery.
Study design and patients
This was a prospective cohort study conducted in a PICU of a tertiary-care university hospital in Brazil. The study was approved by the Research Ethics Board of Hospital das Clínicas of Ribeirão Preto Medical School, University of São Paulo (#1499/2008) and written informed consent was obtained from the patients’ parents. Children with congenital heart disease admitted to the PICU following cardiac surgery from May 2008 to April 2009 and who remained intubated and mechanically ventilated for at least 24 hours after surgery were eligible for the study. Exclusion criteria were a preoperative diagnosis of nosocomial pneumonia and/or sepsis, death in the first 24 postoperative hours and refusal to participate.
Patients received prophylactic antibiotics for 48 hours from the induction of anesthesia with intravenous cefazolin (40 mg/kg every 8 hours) and amikacin (7.5 mg/kg every 12 hours). Patients with a history of hospitalization for more than 72 hours, ICU admission or previous use of antibiotics received intravenous vancomycin (20 mg/kg at the induction of anesthesia followed by 10 mg/kg every 6 hours) and amikacin (7.5 mg/kg every 12 hours).
Patients who underwent CPB received a bolus dose of 30 mg/kg of methylprednisolone or 10 mg/kg of hydrocortisone at CPB start. Modified ultrafiltration was not performed at the end of CPB.
The diagnosis of VAP was made independently by the PICU physician and an infection control practitioner by the 2007 CDC criteria, which considered that no minimum period of time was needed for the ventilator to be in place to diagnose VAP. According to the time of onset, VAP was categorized as early, when it occurred within 1 to 4 days of mechanical ventilation, and late, when it occurred after 4 days of ventilation .
Demographic and clinical data were collected from patients’ health records. Procedure complexity was stratified according to Risk Adjustment for Congenital Heart Surgery (RACHS)-1 score  and severity of illness was assessed by Pediatric Risk of Mortality (PRISM) score between 8 and 24 hours following PICU admission .
Blood, mBAL fluid and EVC were collected daily, starting immediately after surgery (iPO) until the fifth postoperative day (POD5) or until the child was extubated for measurement of sTREM-1 by a sandwich enzyme-linked immunosorbent assay (ELISA – kit Quantikine – R&D Systems, Minneapolis, MN, USA). Blood samples were collected from arterial lines inserted before surgery for blood pressure monitoring. mBAL fluid was collected using a feeding tube, which was introduced down the endotracheal tube until resistance was felt. Then, using a syringe, 1 ml/kg of normal saline (maximum 20 ml) was instilled through the feeding tube followed by a small amount of air to clear the deadspace of the catheter and suction was applied to obtain the fluid . Quantitative cultures of mBAL fluid were considered positive when the growth of >105 CFU/mL was observed . EVC (approximately 1 ml) was collected from the trap located in the expiratory limb of the ventilator circuit . Nasopharyngeal aspirates were collected from all patients for detection of respiratory viruses by real-time PCR using a standard protocol .
Analysis was made using GraphPad Prism 5.0 (San Diego, CA) and SAS 9.2, R 2.15.1 (SAS/STAT® User’s Guide, Version 9.2, Cary, NC: SAS Institute Inc., 2008). Data were expressed as median (range). Patients were grouped according to the presence or absence of VAP. Continuous variables between groups were compared by Mann–Whitney U test and categorical variables by Fisher’s exact test. Variables with repeated measurements (serum and mBAL sTREM-1) were analyzed after logarithmic transformation using a linear mixed-effects model where individual measures over time were included as a random factor and group membership and time as fixed factors. Because there were many undetectable values of EVC sTREM-1 concentrations, they were categorized as positive and negative, and analysed by Fisher’s exact test. Association between variables was assessed by Spearman’s correlation. A Receiver Operating Characteristic (ROC) curve was constructed for determination of cut-off value of sTREM-1 in serum, mBAL and EVC for the diagnosis of VAP, considering values on the day of VAP diagnosis for the VAP group and the largest value over the study period for the non-VAP group, since increased concentrations of sTREM-1 could also be related to the systemic inflammatory response to surgery and CPB, and pulmonary congestion. A p value < 0.05 was considered significant.
Characteristics of the patients
Over the study period, 145 children with congenital heart disease were admitted to the PICU of our institution after cardiac surgery; 63 met the inclusion criteria. Thirty patients were excluded because consent was not obtained and three because of death in iPO. Thirty patients were included in the study. Their median age was 45 days (range 2 to 4015 days) and their median weight 3.4 kg (range 1.8 to 20 kg). Twenty-six (87%) patients underwent CPB. Seven (23%) patients were left with the sternum open for 2 to 5 days after surgery (median 4 days). Twenty-eight (93%) patients had pulmonary congestion in iPO.
Characteristics of the study groups
Patients with VAP (n = 16)
Patients without VAP (n =14)
Use of CPB
Duration of CPB (minutes)
Aortic cross clamp time (minutes)
Duration of surgery (minutes)
Duration of mechanical ventilation after surgery (days)
PICU length of stay (days)
Hospital length of stay (days)
mBAL culture was positive (considering the cutoff of > 105 CFU/mL) in six patients with VAP, with the growth of the following agents: Acinetobacter baumannii (n = 2), Staphylococcus epidermidis (n = 1), Staphylococcus aureus methicillin-sensitive (n = 1), Pseudomonas aeruginosa (n = 1), and Enterococus faecium (n = 1). Blood culture was positive in two patients with VAP, with the growth of Staphylococcus aureus methicillin-sensitive (n = 1; the same microorganism grew in mBAL fluid) and Staphylococcus aureus methicillin-resistant (n = 1). Bocavirus was detected in nasopharyngeal aspirate in one patient with VAP and rhinovirus was detected in five patients in the VAP group and in three patients in the non-VAP group. Three patients with rhinovirus infection in the VAP group were coinfected with the following agents: Acinetobacter baumannii, S. aureus methicillin-sensitive and S. aureus methicillin-resistant; one patient in the non-VAP group was coinfected with respiratory syncytial virus. mBAL and blood cultures were negative in all patients in the non-VAP group.
In the present study, we investigated the usefulness of serial measurements of sTREM-1 in serum, mBAL fluid and EVC for the diagnosis of VAP in children after congenital heart surgery. This is the first study to evaluate the diagnostic value of sTREM-1 in VAP following cardiac surgery. Our results showed that measurement of sTREM-1 in the EVC can be useful for diagnosing VAP in this population. However, sTREM-1 measurements in serum and mBAL fluid did not discriminate patients with VAP from those without VAP following cardiac surgery.
A previous study showed that plasma sTREM-1 concentrations increased after surgery in noninfected patients undergoing elective cardiac surgery with CPB and peak plasma levels of sTREM-1 overlapped with those found in patients with severe sepsis. However, no correlation was observed between sTREM-1 plasma concentrations and CPB duration or aortic cross-clamping time . Our results corroborate these findings which suggest that elevated serum levels of sTREM-1 may also be related to the systemic inflammatory response to surgical stress.
In critically ill mechanically ventilated adults who developed VAP, plasma sTREM-1 levels did not change significantly over time. Nevertheless, a significant increase in mBAL fluid sTREM-1 concentration was observed. A cut-off value of 200 pg/ml combined with an increase of at least 100 pg/ml over the 6-day period before VAP was diagnosed had a sensitivity of 88% and a specificity of 84% for the diagnosis of VAP . In addition, levels of sTREM-1 in BAL fluid were significantly higher in adults receiving mechanical ventilation with community-acquired pneumonia and VAP compared with patients without pneumonia . In contrast, in critically ill adults with acute respiratory failure undergoing BAL for pulmonary infiltrates, the concentration of sTREM-1 in BAL fluid was not significantly greater in those with definite VAP, with the highest sTREM-1 concentrations being observed in patients with alveolar hemorrhage . Accordingly, in our study, sTREM-1 concentrations in mBAL fluid were not significantly different in patients with and without VAP. Moreover, we did not observe significant changes in mBAL sTREM-1 levels from the day before to the day after VAP diagnosis. As more than 90% of patients had pulmonary congestion in the postoperative period, this could have altered sTREM-1 levels in mBAL fluid in our study.
Our data suggest that measurement of sTREM-1 in EVC may improve the accuracy of the diagnosis of VAP, which is in agreement with findings from a previous study in adults in a medical ICU, with a clinical suspicion of VAP . In our study, sTREM-1 measurement in EVC samples had a good discriminative value to differentiate patients with VAP from those without VAP. Indeed, a positive test result rather than a cutoff value of sTREM-1 concentration in EVC was associated with a high probability of VAP being present (likelihood ratio of 5.2). This is especially important for patients following cardiac surgery who can have pulmonary opacities on the chest x-ray caused by multiple noninfectious conditions that may be misdiagnosed as pneumonia. In addition, EVC samples can be easily collected in a noninvasive way.
The major limitation of our study is the lack of a gold standard method for the diagnosis of VAP. However, there is currently no gold standard for the diagnosis of VAP in the clinical setting . Other limitation is the lack of measurements of sTREM-1 concentrations before surgery to determine the baseline patients’ levels.
The detection of sTREM-1 in EVC samples may be useful for the diagnosis of VAP after heart surgery.
Centers for Disease Control and Prevention
Exhaled ventilator condensate
Modified non-bronchoscopic bronchoalveolar lavage
Pediatric intensive care unit
Triggering receptor expressed on myeloid cells-1
Soluble triggering receptor expressed on myeloid cells-1
We thank Ms. Flavia P. Escremim for performing real time-PCR analysis for virus detection and Mr. Davi Casale Aragon for his help with statistical analysis. This study was supported by FAEPA (Fundação de Apoio ao Ensino, Pesquisa e Assistência do Hospital das Clínicas da Faculdade de Medicina de Ribeirão Preto da Universidade de São Paulo).
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