INTRODUCTION

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Ventilator-associated pneumonia (VAP) is the most frequently occurring nosocomial infection among mechanically ventilated patients, with reported incidences as high as 78% (1, 2). Usually two types of VAP are distinguished: early-onset VAP, when diagnosed within the first 4 d of mechanical ventilation, and late-onset VAP, occurring thereafter. Because VAP has been associated with increased morbidity, longer hospital stay, increased health care costs, and higher mortality rates (3), prevention of this infection is a major challenge for intensive care medicine.

Early-onset VAP is caused by pathogens presumably colonizing the respiratory tract at the time of intubation, such as Streptococcus pneumoniae, Staphylococcus aureus, and Haemophilus influenzae. Late-onset VAP is frequently caused by enteric gram-negative bacteria and Pseudomonas species. These bacteria may be transmitted from exogenous sources (e.g., other colonized patients or contaminated common sources) or endogenous sources (e.g., the stomach or intestine). Because of concomitant colonization of the upper respiratory and digestive tract with these bacteria, the gastropulmonary route of colonization has been considered to be important in the pathogenesis of late-onset VAP (4). Based on the presumed relevance of gastric colonization, modulation of gastric colonization has been attempted as a measure to prevent VAP. However, neither the use of sucralfate for stress ulcer prophylaxis (5, 6), nor modulation of enteral feeding (7) have proven unequivocally to reduce the incidence of VAP. In addition, administration of nonabsorbable antibiotics into the stomach and the intestine reduced colonization at these sites, but did not influence the incidence of VAP (8). Finally, sequential analyses of colonization failed to demonstrate an important role of the gastropulmonary route in several recent studies (5, 7, 9).

Selective decontamination of the digestive tract (SDD) decreases incidences of VAP (10) by eradicating microorganisms from the intestine, the stomach, and the oropharynx by nonabsorbable antibiotics, in combination with systemic antibiotic prophylaxis during the first days of ICU admission. However, the constant threat of selection and overgrowth of antibiotic-resistant microorganisms, lack of formal cost-benefit analyses, and absence of beneficial effects on mortality rates have limited a widespread use of SDD (15). From a conceptual point of view, it has remained unclear which part of SDD prevents VAP. The importance of gastric and intestinal colonization has been questioned, and systemic antibiotics during the first days of intubation may prevent early-onset but not late-onset VAP (16).

We hypothesized that decontamination of the oropharynx, without modulating gastric and intestinal colonization, and without systemic antibiotic prophylaxis, would reduce the incidence of VAP in critically ill intensive care patients.

	METHODS

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Setting

The study was conducted in three intensive care units (ICU) from September 1994 to December 1996. ICU 1 and ICU 2 are located in the University Hospital Maastricht; both harbor a mixed population of medical, surgical, trauma and neurologic patients. ICU 3 is located in the University Hospital Groningen and is a surgical and trauma ICU.

Patients

Adult patients ( 16 yr) admitted to one of these ICUs who were intubated within 24 h of admission and who needed mechanical ventilation with an expected duration of > 2 d could be included. Patients were randomized to receive either topical antimicrobial prophylaxis (TAP), consisting of an Orabase with 2% gentamicin, 2% colistin, and 2% vancomycin or to placebo Orabase without antibiotics. Active medication and placebo could not be visibly distinguished. Orabase was applied in the buccal cavities on a gloved finger every 6 h. The application of Orabase was started within 24 h of intubation. Patients in whom the application of Orabase was not possible or contraindicated were not eligible for the study. No prophylactic antibiotics were administered through the nasogastric tube or systemically as part of the study regimen. Patients were studied until extubation or death. Because  95% of the first episodes of VAP occur within the first 3 wk of ventilation (5, 17), application of Orabase was limited to 21 d. Patients were evaluable if they had been included in the study for > 2 d.

Sucralfate (Ulcogant Suspension; E. Merck, Germany) was used for stress ulcer prophylaxis, unless patients were receiving H₂-antagonists or H⁺K⁺ATPase inhibitors on admission. Stress ulcer prophylaxis was discontinued when enteral feeding was started. Enteral feeding was started as soon as possible, usually when peristalsis was present. All patients had a nasogastric tube. In general, all patients were in supine position during controlled mechanical ventilation and if possible in semirecumbent position during weaning. Moreover, all ventilated patients received daily chest physiotherapy, and endotracheal suctioning was performed by the nursing staff if necessary. When mechanical ventilation was expected to be necessary for > 3 wk, patients received a tracheostomy, usually after Day 14 of ventilation.

Normal oropharyngeal care in our ICUs consisted of rinsing the mouth with water and, if possible, tooth cleaning, once daily. To prevent cross-acquisition, dispensers with disinfectants were present at each bedside, and all staff was regularly enforced to comply with infection control procedures.

Study Design

This study was a prospective, randomized, double-blind, placebo-controlled study. Randomization was conducted per hospital and was executed by the Department of Clinical Pharmacy of the University Hospital Maastricht. The inclusion scheme aimed to create two separate control groups in ICU 1 and ICU 2. One control group (Control A) was studied in the presence of patients receiving TAP, and a second control group (Control B) was studied in an ICU where no TAP was used. After 9 mo of study the inclusion scheme was reversed for another 6 mo (Table 1). To secure the double-blind study design, only the study supervisor and the hospital pharmacist were aware of this inclusion scheme. In ICU 3, all patients were randomized to receive either TAP or placebo during the whole study period (Table 1). This inclusion scheme was chosen to assess whether TAP influences infection rates in control patients treated simultaneously in the same ICU (18). The study protocol was approved by the ethical committee of both hospitals. Informed consent was obtained from the patient or, if this was not possible, from a representative of the family.

Data Collection

Demographic data (e.g., age, sex, medical specialty, preexistent diseases, and length of hospital stay before admission to ICU) and Acute Physiology and Chronic Health Evaluation (APACHE) II scores (19) were recorded on admission. Number of days in ICU and on mechanical ventilation, surgical procedures, parameters of infection (temperature, leukocyte counts and differential counts, chest radiograph interpretations, culture results) and antibiotic use were monitored prospectively. Surveillance cultures were taken on admission and subsequently twice weekly (Monday and Thursday) of oropharynx, trachea, stomach, and rectum. The results of surveillance cultures of oropharynx, stomach, and rectum were not reported to the ICU attendings.

Microbiologic Analysis and Monitoring of Resistance

Semiquantitative or quantitative microbiologic analysis of culture samples was performed according to standard microbiologic methods (20). Antibiotic susceptibility was determined by means of a microbroth dilution method according to the National Committee of Clinical Laboratory Standards (NCCLS) guidelines. Pseudomonas aeruginosa ATCC 27853, Escherichia coli ATCC 25922 and ATCC 35218, S. aureus ATCC 29213, and Enterococcus faecalis ATCC 29212 were used as reference strains. The criteria for susceptibility and resistance, according to the NCCLS guidelines, were used. Colonization was analyzed for Enterobacteriaceae, Pseudomonadaceae and S. aureus (i.e., potentially pathogenic microorganisms [PPMO]), enterococci, and Candida species. Vancomycin susceptibility was tested for all enterococci.

Definitions

Colonization was defined as the isolation of microorganisms (i.e., bacterial or yeast species) in two or more consecutive specimens of one site, in the absence of infection. Colonization on admission was defined as colonization demonstrated within 24 h after admission to ICU. Eradication of colonization was defined as the disappearance of microorganisms in two or more consecutive cultures from a body site that was colonized on admission, and is expressed as the proportion of colonized patients in whom eradication occurred. Acquired colonization was defined as colonization demonstrated > 24 h after ICU admission, in patients without colonization on admission.

All patients were examined daily for the presence of VAP. A clinical suspicion of VAP was defined as the presence of a new, persistent or progressive infiltrate on chest X-ray and  3 of four criteria: rectal temperature > 38.0°C or < 35.5°C; blood leukocytosis (> 10.10³/mm³) and/or left shift or leukopenia (< 3.10³/mm³); > 10 leukocytes per high-power field in Gram stain of tracheal aspirate; and a positive culture from tracheal aspirate. In case of a clinical suspicion of VAP, bronchoscopy with protected specimen brush (PSB) and bronchoalveolar lavage (BAL) were performed and blood cultures were taken. The diagnosis of VAP was established on the basis of positive quantitative cultures from BAL (cutoff point  10⁴ colony-forming units [cfu]/ml) or PSB (cutoff point  10³ cfu/ml), or a positive blood culture unrelated to another source of infection, or a positive culture from pleural fluid in the absence of previous pleural instrumentation. Pneumonia was considered ICU-acquired when diagnosed  48 h after admission to ICU. Pneumonia was classified early-onset when diagnosed within the first 4 d of mechanical ventilation, and late-onset when occurring thereafter.

Nosocomial infections other than VAP were diagnosed according to Centers for Disease Control (CDC) definitions (21). Antibiotic use was analyzed in courses. A course was defined as an episode of clinical or suspected infection in which antibiotics, either consecutively or in combination, were prescribed. A change in antibiotics, for example narrowing after availability of antibiotic susceptibility, was not considered a separate course.

Outcome Variables

The incidence of VAP was the primary outcome variable of the study. Colonization of oropharynx, trachea, stomach, and rectum, number of days in ICU and on mechanical ventilation, other nosocomial infections, antibiotic use, and mortality were secondary outcome variables.

Statistics

The power analysis was performed with an expected decrease in the incidence of VAP from 30% to 10%; it predicted the necessary number of patients per group to be 63 ( 0.80,  0.05). Data are expressed as absolute numbers with or without percentages, as means with standard deviation or as medians with ranges. Chi-square or Fisher exact test were used to compare proportions, t test or Wilcoxon-Mann-Whitney test to compare continuous variables. For each patient the time until event (i.e., diagnosis of VAP or end of study) and death was determined to calculate the probability of remaining without VAP and survival using Kaplan-Meier survival analysis. Groups were compared by log-rank test. Incidence rates of pneumonia were compared by using risk ratios with 95% confidence interval (CI). A probability value < 0.05 was considered to denote statistical significance and all reported p values are two-sided. To accommodate for multiple statistical testing, the Bonferroni correction was used for the secondary endpoints. In these cases a p value < 0.0125 was considered statistical significant. Statistical analysis was performed using the SPSS/PC statistical package (SPSS, Inc., Chicago, IL.)

	RESULTS

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Patients

During the 28-mo study period 213 eligible patients were admitted to ICU 1 and 2. Fifteen patients were not included because of mandibular fixation after facial trauma (n = 2), severe intraoral mucosal hemorrhages due to thrombocytopenia (n = 2), and refusal to give informed consent (n = 11). Seventeen of 198 included patients were ventilated or intubated  2 d or succumbed within 2 d (Table 1). Although 93% of all long-term ventilated patients were included, the number of patients studied simultaneously was lower than expected. Overall, the median daily proportion of all patients being included was 14% (range 0 to 86%), which means that per day on average one patient was included in each seven-bed ICU. The low inclusion rate was a result of an unexpected high admittance rate of children and short-term ventilated postoperative neurosurgical patients, who were not eligible for our study. In addition, several patients remained in ICU long after the 21 d of study. As a result, we were unable to assess whether TAP influences infection rates in control patients studied in an ICU where no TAP was used (Control B).

In ICU 3, informed consent was refused by 10 eligible patients. In this ICU 47 patients were included, two of whom were not evaluable because they were ventilated < 2 d (Table 1). In all, 226 patients were evaluable; 87 study patients, 78 Control A patients, and 61 Control B patients. Baseline characteristics were comparable for the three groups (Table 2). The baseline characteristics of the patients who were not evaluable (n = 19) were comparable to those of evaluable patients (data not shown).

Colonization

Colonization rates on admission in oropharynx, trachea, stomach, and rectum with PPMO, enterococci, and Candida species, were comparable for study and control patients (see Table E1 in online data supplement). TAP eradicated oropharyngeal colonization with PPMO present on admission (75% of study patients versus 0% in Control A patients [p < 0.00001] and 9% in Control B patients [p < 0.00001]) and reduced rates of acquisition of colonization with PPMO at this site (10% of study patients versus 59% in Control A patients [p < 0.00001] and 63% in Control B patients [p < 0.00001]). In addition, TAP was associated with eradication (52% of study, 22% of Control A [p = 0.03], and 7% of Control B patients [p = 0.004]), and with a tendency toward prevention of acquisition with PPMO in the trachea in Control A patients (36% versus 50%; p = 0.06). Importantly, in all three groups colonization rates of the stomach and rectum were not influenced (Table E1).

Acquisition of enterococcal colonization occurred more frequently in control patients in the oropharynx (28% of Control A and 30% of Control B patients versus 3% of study patients; p < 0.00001 for both comparisons), and in the stomach (37% of Control A and 35% of Control B patients versus 18% of study patients; p = 0.01 and p = 0.03 respectively). Acquired enterococcal colonization was comparable in trachea and rectum. Rates of acquired colonization with Candida species were comparable in all groups at all body sites (Table E1).

VAP

During the study period VAP was diagnosed in nine (10.3%) study, 24 (30.8%) Control A patients, and 14 (23.0%) Control B patients (study versus Control A, p = 0.001; study versus Control B, p = 0.04; Table 3; Figure 1), after 9 d (range 2 to 18), after 7 d (range 2 to 19), and after 5 d (range 3 to 18) in study, Control A, and Control B patients respectively. Therefore, comparing study and Control A patients, administration of TAP resulted in a relative risk for VAP of 0.33 (95% CI 0.16 to 0.67), a relative risk reduction of 0.67 (95% CI 0.33 to 0.84), and an absolute risk reduction of 0.21 (95% CI 0.09 to 0.33), which implies that five patients needed to be treated to prevent one episode of VAP.

View larger version (13K): [in this window] [in a new window]   Figure 1.   Probability of remaining free of VAP during study period in study patients and control subjects (study patients versus Control A, p = 0.0004; study patients versus Control B, p = 0.02).

Comparing study and Control B patients, administration of TAP resulted in a relative risk for VAP of 0.45 (95% CI 0.21 to 0.97), a relative risk reduction of 0.55 (95% CI 0.03 to 0.79), and an absolute risk reduction of 0.13 (95% CI 0.004 to 0.25), which implies that eight patients needed to be treated to prevent one episode of VAP.

Approximately 20% of the episodes of VAP were early-onset (2 of 9 study patients, 6 of 24 Control A patients, and 2 of 14 Control B patients). VAP was accompanied by bacteremia in 2 Control A, 1 Control B, and in none of the study patients. A similar distribution of etiologic pathogens was observed (Table 2). The difference in the incidence of VAP persisted after Day 21, demonstrating that ceasing TAP was not associated with a rebound effect on the incidence of VAP (data not shown). Of the patients who were ventilated for more than 21 d, VAP was diagnosed in one study patient, 2 Control A patients, and 2 Control B patients after the application of trial medication was stopped.

A clinical suspicion of VAP with negative or nonsignificant culture results from bronchoscopy occurred with equal frequency in all study groups (24 [28%], 25 [32%], and 15 [25%] in study, Control A, and Control B patients, respectively).

Secondary Outcome Variables

The number of days in study, on mechanical ventilation, in ICU, and in hospital were comparable for study and control patients (Table 3). There were differences in the use of enteral feeding and sucralfate: study patients received enteral feeding more frequently than Control B patients (82% versus 64%; p = 0.02), and Control A patients were more likely to receive sucralfate for stress ulcer prophylaxis compared with study patients (76% versus 61%; p = 0.04). H₂-antagonists or H⁺K⁺ATPase inhibitors were administered to comparable numbers of patients in all groups. Mean durations of enteral feeding, use of sucralfate, and H₂-antagonists and/or H⁺K⁺ATPase inhibitors were also comparable.

Although ICU mortality tended to be lower in study patients (29%) as compared with Control A and Control B patients (35% and 43%, respectively), this difference did not persist during follow-up (Table 3; Figure 2).

View larger version (13K): [in this window] [in a new window]   Figure 2.   Probability of survival during prolonged follow-up in study patients and control subjects (study patients versus Control A, p = 0.7; study patients versus Control B, p = 0.4).

Systemic Antibiotic Use and Resistance

On admission, systemic antibiotics were prescribed for 41 (47%) study, 31 (40%) Control A, and 27 (44%) Control B patients. Twenty-one of the 131 surgical/trauma patients received antibiotic prophylaxis during surgery, 15% of study, 15% of Control A, and 19% of Control B patients. Mean numbers of courses of antibiotics per patient were 0.95 ± 0.68 for study patients and 1.30 ± 0.85 for Control A patients during study period (p = 0.02) and 1.23 ± 0.90 for Control B patients (p = 0.10). During total ICU stay these figures were as follows: 1.33 ± 0.76 for study and 1.82 ± 1.23 for Control A patients (p = 0.02) and 1.69 ± 1.31 for Control B patients (p = 0.34) (Table 3).

No vancomycin-resistant enterococci (VRE) were isolated in either hospital before, during, or after the study. No increase in the number of patients colonized or infected with microorganisms resistant to gentamicin was observed during the study. Separate analysis of the resistance patterns of the pathogens causing VAP did not reveal cases of acquired resistance to the antibiotics used in the oropharyngeal paste.

	DISCUSSION

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The main feature of this study is that modulation of oropharyngeal colonization, without influencing gastric and intestinal colonization and without systemic antibiotic prophylaxis, resulted in a relative risk reduction of 67% in the incidence of VAP. This finding underscores the pivotal role of oropharyngeal colonization in the pathogenesis of VAP, and strongly suggests that modulation of colonization at this site will effectively prevent VAP.

The present study shows that prevention of colonization of the oropharynx, and not the stomach, reduces the incidence of late-onset VAP. The effects of oropharyngeal decontamination have been studied previously in two smaller studies. Rodríguez-Roldán and coworkers (22) used an oropharyngeal paste containing tobramycin, amphotericin B, and polymyxin E. Decontamination of oropharynx and trachea was established in 10 of 13 patients receiving active medication and none developed pneumonia. Eleven (73%) of 15 patients receiving placebo medication developed pneumonia. In a double-blind study, Pugin and coworkers (2) randomized 52 patients to receive either a solution of polymyxin B, neomycin, and vancomycin or placebo in the retropharynx. Colonization with aerobic gram-negative bacteria was significantly reduced in oropharynx and stomach, resulting in a relative risk reduction of VAP of 0.79. Our findings expand the results of both studies. Incidences of VAP in the control groups of both studies (73% and 78% respectively) were extremely high, and not comparable to incidences found in other ICU studies (3, 5, 6). This is probably due to the use of clinical and microbiologic criteria, instead of bronchoscopic techniques, in the diagnosis of VAP, or due to patient selection. Furthermore, because gastric colonization was significantly decreased in the study by Pugin and coworkers, their findings do not contribute to the determination of the relative importance of gastric and oropharyngeal colonization (2).

Our findings demonstrate that VAP can be prevented effectively by modulation of oropharyngeal colonization. Importantly, the relative risk reduction of VAP in the present study is similar to the relative risk reductions reported in the meta-analyses of SDD (ranging from 53% to 78%) (10). This strongly suggests that oropharyngeal decontamination, indeed, represents the effective part of SDD, and that the majority of antibiotic use in SDD is unlikely to add beneficial effects. A similar preventive effect on the incidence of VAP can be achieved with only a fraction of the antibiotics used in SDD. The data, therefore, question the concept of SDD, a method of infection prevention that is used in some ICUs (1). However, antibiotic use bears the constant threat of induction or selection of resistant microorganisms. Absence of resistance problems and very strict control, as in the present study, are mandatory.

Ideally, modulation of oropharyngeal colonization should be established with "nonantibiotic" methods. One potential approach might be the use of chlorhexidine for oropharyngeal decontamination. An oral rinse of 0.12% chlorhexidine reduced the incidence of respiratory tract infections among 353 cardiosurgical patients from 9% in control patients to 3% in patients receiving oropharyngeal decontamination with chlorhexidine (23). The difference was mainly caused by a reduction of infections with gram-negative pathogens. However, how prolonged chlorhexidine use will affect oral, esophageal, and gastric mucosa in critically ill ICU patients is unclear, as is the risk of chlorhexidine resistance after long-term application.

How should our findings be applied together with the results of other studies on prevention of VAP? There is strong evidence that modulation of oropharyngeal colonization will influence the development of late-onset VAP. However, in settings where early-onset VAP represents a bigger problem, other strategies, such as subglottic secretions drainage (24) or targeted systemic prophylaxis during the first 24 h (16) may be more appropriate.

In a previous study in our hospital, topical antimicrobial prophylaxis of the oropharynx and stomach with colistin, tobramycin, and amphotericin B was associated with overgrowth and infections caused by gram-positive species, such as E. faecalis and coagulase-negative staphylococci (25), findings that have been reported by others as well (26, 27). Because our aim was to perform a conceptual study to determine the effects of modulation of oropharyngeal colonization on the incidence of VAP and to elucidate the pathogenesis of VAP, we included vancomycin in our prophylactic regimen. We are well aware of the emergence of VRE in many countries and the Hospital Infection Control Practices Advisory Committee recommendations to avoid the prophylactic use of vancomycin (28). However, neither VRE nor methicillin-resistant S. aureus (MRSA) had been isolated on a regular basis in the Netherlands or in the ICUs of both hospitals, at the start of the study, and systemic vancomycin was used only sporadically (29). In addition, the duration of topical antibiotic prophylaxis was limited to a maximum of 21 d to minimize the risk of induction of resistant strains. Finally, susceptibilities for vancomycin were determined for all enterococci isolated during the study. A plan of enforced infection control had been developed when vancomycin-resistant strains would have emerged. With all these control measures, we felt assured to use vancomycin as a prophylactic agent in a setting with low vancomycin use, an overall low incidence of antibiotic resistance, and complete absence of VRE and MRSA. Although acquired resistance in gram-negative bacteria is also a potential threat of topical antimicrobial prophylaxis, the susceptibility of gram-negative bacteria to gentamicin was approximately 90% in our ICU and this did not decrease during the study.

Several potential insufficiencies of our study must be addressed. Incidences of VAP may have been influenced by several variables that were not equally distributed in the study groups. For instance, more control patients (Control A as well as Control B) received sucralfate and study patients were more likely to receive enteral feeding. However, because sucralfate has been associated with lower incidences of VAP in some studies (17, 30) and enteral feeding has been assumed to be a risk factor for VAP (31), both of these discrepancies would have favored the control groups rather than the study patients. If so, the true beneficial effects of oropharyngeal decontamination would have been even higher. Another concern is whether aspiration of antibiotics into the lungs influenced the diagnostic yield of bronchoscopic samples. However, negative bronchoscopic results obtained because of a clinical suspicion of VAP occurred with equal frequency in both control groups and study patients, suggesting that leakage of antibiotics in the lower respiratory tract did not occur. Moreover, from a number of study patients, samples of tracheal aspirate were obtained for determination of gentamicin and vancomycin concentrations during the study period (Days 3, 7, 14, and 21). All gentamicin concentrations were below the threshold of detection (i.e., < 0.03 mg/L). Vancomycin concentrations were below the threshold of detection (i.e., < 0.05 mg/L) in most samples and very low concentrations in the remaining (median 0.25 mg/L) (data not shown).

In conclusion, our study demonstrated that modulation of oropharyngeal colonization, without influencing gastric and intestinal colonization, effectively reduces the incidence of late-onset VAP. This finding proves the pivotal role of oropharyngeal colonization in the pathogenesis of this infection. When compared with preventive strategies for VAP which aim to modulate either gastric or intestinal colonization, prevention of oropharyngeal colonization is by far the most effective.