Background

Ventilation-Associated Pneumonia (VAP) occurs in patients who have been mechanically ventilated via endotracheal intubation or tracheostomy for 48 h or more. Pneumonia occurring within 48 h after extubating also belonged to VAP [1, 2]. VAP is one of the most common hospital-associated infections (HAIs) in the pediatric intensive care unit (PICU) [3, 4], second only to bloodstream infections, accounting for approximately 20% of HAIs in the PICU [5]. The occurrence of VAP lead to a prolonged hospital stay, an increase in medical costs and clinical mortality, and an impact on children’s prognosis [6]. In recent years, as healthcare professionals have gained a deeper understanding of VAP, more intensive care units have adopted standardized operating procedures (SOPs) for VAP prevention. The SOPs encompassed key practices such as oral hygiene, elevation of the head of the bed, sterile endotracheal suctioning, minimal ventilator circuit adjustments, daily sedation interruption, assessment for extubation readiness, intubation and equipment management, and the use of H2-receptor antagonists [7, 8], which was considered to be VAP bundle. In Spain, VAP bundle proved effective in reducing VAP rates, with Peña-López [9] reporting a decrease from 8.16 to 0.65 per 1,000 tracheostomy ventilation-days, and Esteban [10] noting a reduction from 28.3 to 10.6 per 1,000 ventilation days. A multinational study by Rosenthal [11] covering Colombia, Turkey, El Salvador, India, and the Philippines showed a 31% reduction in VAP rates from 11.7 to 8.1 per 1,000 ventilator days following bundle implementation. These findings indicate that bundles are highly effective in reducing VAP incidence across diverse regions and healthcare settings. Although a decrease in the incidence has been observed, it is still high, and once it occurs, the mortality and economic burden of the patient have increased significantly. Antibiotic exposure has been reported as a risk factor for the development of VAP, but it is unclear which antibiotic causes it.

The use of broad-spectrum antibiotics remains a crucial treatment for critically ill pediatric patients. Antibiotic treatment regimens and durations were influenced by various clinical factors, including the infectious pathogen, site of infection, and the host’s immune status. Timely and appropriate antimicrobial therapy can improve the prognosis of critically ill pediatric patients [12, 13]. However, prolonged exposure to antibiotics can lead to the growing of drug-resistant bacterial strains, alterations in the host’s microbial ecology, and even disruption of the immunity. Prior antibiotic exposure has been identified as a risk factor for VAP development [14], making it a growing concern among clinical practitioners. Moreover, although SOPs were applied to prevent the occurrence of VAP in clinical practice, some children still developed VAP, and many pathogens have been shown to originate from microorganisms colonizing the intestines. Therefore, we speculated that the use of broad-spectrum antibiotics in critically ill children may be a key factor in the development of VAP, but there is a lack of systematic research on the types of antibiotics and the duration of exposure before VAP occurrence in children. A systematic analysis of VAP patients was conducted in this study, aim to find factors that may affect the occurrence of VAP.

Methods

Subjects

This study was a three-year, multi-center retrospective study conducted in accordance with the principles of the Helsinki Declaration and approved by the Clinical Research Ethics Committee of the First People’s Hospital of Lianyungang (Approval No. LW-20231024003-01) and the Children’s Hospital of Soochow University (Approval No. 2023CS034). Written informed consent was obtained from patients or their guardians.

A total of 278 children admitted to the Pediatric Intensive Care Units (PICU) at the Children’s Hospital of Soochow University and the First People’s Hospital of Lianyungang and received invasive mechanical ventilation (MV) between January 2020 and December 2022 were included in this study. Among these patients, 171 met the inclusion criteria, which required a minimum MV duration of ≥ 48 hours and excluded patients who had undergone MV before admission to the PICU, and patients who developed pneumonia within 48 hours after MV initiation (Fig. 1). VAP was diagnosed according to the diagnostic criteria outlined in the " Pneumonia (Ventilator-associated [VAP] and non-ventilator- associated Pneumonia [PNEU]) Event [2]. During the observation period, 61 cases developed VAP and were classified as VAP group, while the remaining 110 cases did not develop VAP and were classified as non-VAP group.

Fig. 1
figure 1

The algorithm of inclusion and exclusion of patients in the study

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Definitions

The prior antibiotic exposure refers to the period of one month before the occurrence of VAP. The observation endpoint was 48 h after of mechanical ventilation (MV) withdrawal. Disease severity was assessed on admission using the Pediatric Critical Illness Score (PCIS) and the Pediatric Risk of Mortality Score III (PRISM III) [15].

Antibiotic information

The daily dosage and duration of all antibiotics used by the enrolled patients during the observation period of antibiotic exposure (if the duration was less than 1 day, it was not included in the statistics) were collected and analyzed. Prior exposure to antibiotics includes carbapenems (including meropenem and imipenem), cephalosporins (such as cefotetan, ceftazidime, ceftriaxone, and cefuroxime), beta-lactam/beta-lactamase inhibitor combinations (referred to as beta-lactam/lactamase inhibitor combinations, such as piperacillin-tazobactam and ampicillin-sulbactam), vancomycin, linezolid, azithromycin, trimethoprim-sulfamethoxazole, tigecycline, polymyxin B, amikacin, and metronidazole. In 2016, the Infectious Diseases Society of America recognized the duration of treatment for pneumonia associated with ventilators and medical institutions as seven days [16], and the same recommendation was made for the diagnosis and treatment of sepsis and septic shock in children [17].Therefore, we set a 7-day cutoff for carbapenem exposure.

Collection of clinical and laboratory data

According to the aim of the research, the case report form was designed to track the electronic medical records of all the selected subjects. Data entry was carried out after the training of data entry clerks, and the double entry method was adopted, which was completed by two people independently. General information, outcomes, the choice of endotracheal intubation, mental status and underlying disease were collected. Factors potentially related to the occurrence of VAP during the antibiotic exposure were also collected. These factors included the use of intravenous immunoglobulin (IVIG), enteral/parenteral nutrition, proton pump inhibitors, corticosteroids, nasogastric tube placement, central venous catheterization, hemodialysis, tracheostomy status, repeated intubation, and prior use of non-invasive ventilation.

Statistical analysis

Data analysis was conducted using SPSS 27.0 statistical software. Continuous data was represented by the median and quartile interval [M (P25, P75)], and the Mann-Whitney U test was used for comparison between the two groups. Categorical data were presented as frequencies and percentages (%), and the chi-squared test or Fisher’s exact test was used to compare these two groups. Initially, a univariate analysis was conducted, then variables with clinically determined risk factors for incidence of VAP and a p-value < 0.2 [18, 19] were included in the multivariate regression. Multivariate Cox regression model was used to determine the independent risk factors for the occurrence of VAP, which was the primary outcome in this study. P < 0.05 was determined as statistically significant.

Results

Clinical characteristics

There were no significant differences of gender (p = 0.613), age (p = 0.276), PCIS score on admission (p = 0.219) and PRISM III score (p = 0.206) between the VAP group and the non-VAP group. The clinical characteristics of the VAP group and the non-VAP group were shown in Table 1. Patients in the VAP group had significantly higher mortality (36.07% vs. 21.82, p = 0.044), length of hospital stay (32.00 vs. 20.00 days, p<0.001), length of PICU stay (25.00 vs. 10.00 days, p<0.001), duration of mechanical ventilation (16.00 vs. 6.00 days, p<0.001), and hospitalization costs (16,6100 vs. 6,3400 yuan, p<0.001) compared to the non-VAP group. After intubation, the median time of VAP was 7 days (4.4,13) for the VAP group.

Table 1 Clinical characteristics of the VAP group and the non-VAP group
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Analysis of antibiotics exposure

The antibiotic exposure rate for the 171 mechanically ventilated (MV) patients was 98.83% (169/171). The comparison of antibiotic exposure in pediatric patients between the VAP group and the non-VAP group was shown in Table 2. The VAP group had a significantly higher rate of exposure to ≥ 3 types of antibiotics, the duration of exposure ≥ 7 days, and a longer duration of antibiotic exposure compared to the non-VAP group (Table 2).

Table 2 Comparison of antibiotic exposure in pediatric patients between the VAP group and the non-VAP group
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In order to analyze the relationship between the type of antibiotic and the duration of exposure and VAP, we did further analysis and the results showed that there were no differences of cephalosporin, β-lactamase inhibitor combination, vancomycin and/or linezolid, azithromycin and trimethoprim-sulfamethoxazole between these two groups, but the VAP group had significantly higher rates of exposure to carbapenem antibiotics, vancomycin and/or linezolid, and longer exposure durations for these classes of antibiotics, as well as a higher rate of exposure ≥ 7 days, compared to the non-VAP group (Table 3).

Table 3 Comparison of exposure to different classes of antibiotics between the VAP group and the non-VAP group
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Carbapenem exposure was an independent risk factor for the occurrence of VAP

To identify risk factors that may be associated with occurrence of VAP, we first performed univariate analysis. The results revealed that underlying diseases (such as hematological malignancies, genetic metabolic diseases, neurological diseases, congenital heart defects, congenital respiratory system developmental abnormalities, immunodeficiency, malnutrition, rheumatic autoimmune diseases, etc.), a history of repeated intubations, tracheostomy status, total antibiotic exposure ≥ 7 days, exposure to ≥ 3 types of antibiotics, exposure to carbapenem antibiotics for ≥ 7 days and exposure to vancomycin and/or linezolid for ≥ 7 days were significantly associated with the occurrence of VAP.

To find risk factors that could independently predict the occurrence of VAP, multivariate regression analysis was performed with p value less than 0.2 and clinically determined possible risk factors. The results showed that exposure to carbapenem antibiotics for ≥ 7 days (OR = 5.156; 95% CI, 1.881–14.137; P = 0.001), a history of repeated endotracheal intubations (OR = 3.575; 95% CI, 1.449–8.823; P = 0.006), tracheostomy status (OR = 5.767; 95% CI, 1.686–19.729, P = 0.005), and prior use of IVIG (OR = 0.426; 95% CI, 0.185–0.981; P = 0.045) were significantly associated with the occurrence of VAP. Therefore, exposure to carbapenem antibiotics for ≥ 7 days, a history of multiple endotracheal intubations (≥ 2 times), and tracheostomy status were independent risk factors for VAP, while the use of IVIG was an independent protective factor against VAP (Table 4).

Table 4 Univariate and multivariate logistic regression analysis of factors associated with VAP.
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Distribution of pathogens detected in the VAP patients

According to the diagnostic criteria for VAP, there were 61 cases with VAP, of which 63.93% (39/61) of the children were detected with pathogenic bacteria in respiratory specimens. There were 11 cases isolated with mixed pathogens. A total of 55 strains were isolated, mainly Gram-negative bacteria (72.73%,40/55), Gram-positive bacteria (18.18%,18/55) and fungi (9.09%, 5/55). Moreover, the detail pathogens for the VAP patients were also listed (Table 5). From the results, we found 67.5% (27/40) Gram-negative bacteria, 40% (4/10) Gram-positive bacteria and 100% (5/5) were presented in patients with prior carbapenem. For Gram-negative bacteria, Acinetobacter baumannii was the most common isolated pathogen, next was Pseudomonas aeruginosa. Few fungi have been isolated, but 100% of patients were indicated with prior carbapenem (Table 5), indicating dysbiosis caused by prior carbapenem exposure made fungi opportunistic pathogens.

Table 5 Distribution of pathogens verified in the VAP patients
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Discussion

The incidence of VAP in pediatric patients varied significantly worldwide, ranging from 2.00 to 38.4% [20,21,22,23]. This variability can be attributed to multiple factors, including underlying disease, the etiology of MV, and the quality of infection control implementation. In this study, the VAP incidence was 21.94%, which may be related to the high proportion of immunocompromised patients with hematologic malignancies admitted to our center. For these patients, there were high-risk factors for infection, and once fever with neutropenia occurs, antibiotic was initiated. In addition, due to the difficulties in the duration of antibiotic because of the complicated conditions, long-term use of antibiotics may occur, and whether these measures are risk factors for VAP is worth consideration.

This research identified prolonged exposure (more than 7 days) to carbapenem as an independent risk factor for VAP occurrence among various antibiotic classes. Carbapenem have been associated with an increased risk of nosocomial infections, including Clostridioides difficile infections and VAP [24,25,26]. As a class of broad-spectrum antimicrobial agents, carbapenems exhibit excellent antibacterial activity against most aerobic and anaerobic bacteria. Under the pressure of carbapenem exposure, colonizing bacteria in the respiratory tract may develop resistance due to metallo-β-lactamase production, high-level cephalosporinase production, extended-spectrum β-lactamase production, and alterations in binding sites. This can lead to multidrug-resistant strains capable of cross-resistance to carbapenems, β-lactams, and aminoglycosides [27]. These antibiotic-resistant strains can expand and become dominant in the respiratory tract, serving as potential pathogens for VAP. In our study, the main pathogenic pathogens of ventilator-associated pneumonia was Gram-negative bacteria, accounting for 72.73%, and the cases with prior carbapenem exposure were about twice as high as those without prior carbapenem exposure. The most prevalent Gram-negative bacteria were Acinetobacter baumannii, Pseudomonas aeruginosa, and Klebsiella pneumoniae, all of which exhibited carbapenem resistance, consistent with previous studies. Additionally, the use of carbapenem can alter the gut microbiota, disrupting the normal host-microbiota interactions in both the gut and the lungs, which is known as the gut-lung axis. This disruption can contribute to lung injury and pulmonary infections [28]. These factors collectively increase the risk of VAP in MV patients following exposure to carbapenem antibiotics.

The study also found a higher proportion of exposure to vancomycin and/or linezolid in the VAP group. However, the multivariate regression analysis did not identify it as an independent risk factor for VAP occurrence. Vancomycin and linezolid belong to the glycopeptide and oxazolidinone classes of antibiotics, respectively, and are used to treat severe Gram-positive bacterial infections, especially methicillin-resistant Staphylococcus aureus infections. Previous experiments in mice have shown that vancomycin and linezolid administration, either subcutaneously or orally, can suppress anaerobic bacteria and Enterococci in the gut microbiota and promote the colonization of extended-spectrum β-lactamase-producing Klebsiella pneumoniae [29]. Vancomycin has also been shown to reduce gut microbiota diversity in mice, leading to a decrease in Gram-positive bacteria (e.g., Firmicutes) and an increase in Gram-negative bacteria (e.g., Proteobacteria) [30]. These changes may explain why MV patients are more prone to developing VAP after exposure to vancomycin and/or linezolid. Furthermore, a significant correlation between prior antibiotic exposure (≥ 3 types or ≥ 7 days) and VAP occurrence was revealed in this study. Several studies pointed out that the use of multiple antibiotics has a greater impact on the microbiota [31]. Additionally, Pérez-Cobas et al. found that antibiotics can influence the composition of the gut microbiota early in treatment, with microbiota diversity reaching its lowest point after 11 days of treatment [32]. The use of multiple antibiotics or prolonged antibiotic treatment may have a more significant impact on the microbiota and could be a contributing factor to the development of VAP.

This study has some limitations. Firstly, there is an inevitable selection bias in the design of the retrospective study, and the influence of unmeasured variables and unknown confounding factors cannot be ruled out in this study. Secondly, although this study is a multi-center study, the sample size was small, which may lead to lack certain generalization. In the future, we will continue to work on the potential threats of antibiotic exposure, especially for carbapenem antibiotics. In the next step, we will plan a multicenter and prospective study in this area, thus further testing our conclusions. Additionally, molecular biology and genomics methods can be used to study the association between antibiotic exposure and bacterial resistance, to reveal the specific mechanism of antibiotic resistance in the development of VAP in children, and to use microbiome technology to compare the intestinal and respiratory microbial composition of children with VAP and healthy controls to understand the effect of antibiotic exposure on the structure and function of microbial communities, and then explore how the gut-lung axis promotes the occurrence of VAP.

In summary, carbapenem exposure was an independent risk factor for the occurrence of VAP, especially when the duration was more than 7 days. For critically ill children, reducing carbapenem use and duration as much as possible should be considered.