The effect of subglottic secretion drainage on the incidence of ventilator associated pneumonia

Background. Ventilator-Associated Pneumonia (VAP) is an undesired side effect of mechanical ventilation in intensive care units (ICUs). Aim. We evaluated whether endotracheal tubes with subglottic secretion drainage (SSD) would reduce the incidence of VAP among patients undergoing mechanical ventilation in an ICU. Methods. The analysis of medical records of patients undergoing mechanical ventilation exceeding 48 h who were hospitalised in ICUs between 2007 and 2014 led to separating two groups of patients: those in whom no subglottic drainage was applied (NSSD) (records dating from 2007–2010) and those whose treatment involved endotracheal tubes with subglottic secretion drainage (SSD) (records dating from 2011–2014). Results. Analysis of 1807 patients hospitalised in ICUs (804 NSSD patients and 1003 SSD patients). A difference was found in the frequency of VAP incidence between the groups (P<0.001). In the NSSD group as many as 84 cases were reported (incidence: 10.7%), and in the SSD group – 43 cases (incidence: 5.2%). The odds ratio (OR) and relative risk (RR) was 2.5. The probability of VAP was significantly higher in the NSSD group. The risk factors of VAP incidence (P<0.001) included the correlation between reintubation (R=0.271), tracheostomy (R=0.309) and bronchoscopy (R=0.316). Conclusion. Use of endotracheal tubes with subglottic secretion drainage in patients in the ICU on mechanical ventilation significantly reduced the incidence of VAP.


INTRODUCTION
It is typical in intensive care units to apply advanced treatment techniques, which include invasive monitoring and mechanical support of vital functions in cases of organ or system failure.The most frequently applied therapeutic procedures in intensive care units are intubation and tracheostomy, which allow for mechanical ventilation 1 .Numerous scientific societies and associations, such as the American Thoracic Society/Infectious Diseases Society of America (ATS/IDSA), Centers for Disease Control (CDC), and the American Association of Critical-Care Nurses (AACN), recommend the application of Subglottic Secretion Drainage (SSD) in order to limit the risk of developing Ventilator -Associated Pneumonia (VAP) by means of draining the secretion remaining over the sealing cuff.The tubes need frequent cleaning as they often get blocked, especially the draining duct.This may be the reason why they are not commonly applied 2,3 , VAP infections in intensive care units are a phenomenon with a range of health issues.They are the reason behind prolonged hospitalisation, increased mortality and significant increase in treatment costs 4,5 .According to the literature, 20-50% of all hospital infections are developed in ICUs, with a significant portion of those consisting of VAP infections [6][7][8][9] .Increased risk of infection development in ICU patients is associated with the necessity to use invasive procedures, the aim of which is to support ineffective systems 10,11 .In VAP prevention, the importance of the types of endotracheal tubes (ETT) and cuff pressure monitoring is emphasized 12,13 .
According to the studies of European supervision -European Centre for Disease Prevention and Control (ECDC) 2011-2012 -the incidence density rate for VAP in ICUs is 10.5 per 1000 days of intubation.In the annual ECDC epidemiological report of 2014, pneumonia was found in 5.3%, of which 93% was related to intubated patients, and the incidence density rate associated with VAP was 10.1 per 1000 days of intubation 14,15 .In Poland, the incidence density rate for VAP reaches between 10.2 and 16.5 per 1000 days of mechanical ventilation 8,9,16 .
The research was aimed at evaluating the effect which the application of endotracheal tubes with SSD would have on the incidence of VAP among patients undergoing mechanical ventilation.

MATERIALS AND METHODS
The study involved a retrospective analysis of patients undergoing mechanical ventilation lasting more than 48 h, who were hospitalised in Św.Łukasz Provincial Hospital in Tarnów between 2007 and 2014.The patients were observed in two separate groups: the first group consisted of patients hospitalised between 2007 and 2010 in whose cases subglottic secretion drainage was not applied; No Subglottic Secretion Drainage Group (NSSD), and the second group hospitalised between 2011 and 2014 consisted of patients whose treatment involved endotracheal tubes with SSD.The intubation tube, with subglottic suctioning capability, was placed in each patient in whom the necessity to perform mechanical ventilation for over 2 days was expected, as well as in those in the case of whom the score according to the American Society of Anesthesiology (ASA) scale exceeded 3 points.Patients who did not meet the criteria described above were excluded from the study.
The research methods included analysis of the medical records of the patients undergoing mechanical ventilation who were hospitalised in the ICU wards of Św.Łukasz Provincial Hospital in Tarnów.
Preventive measures defined by standard treatment procedures were applied in the examined hospital ward before the research started in 2007 and also during the research.They included: patients' reclining position, disposable intubation tubes if required, application of open system of airway secretion drainage, tubes with polyurethane cuffs, sustaining the cuff pressure at 20-30 cm H 2 O, disposable heat and moisture exchanges (HME) which were close-coupled with antibacterial filters in the respirator tube system.I ntubation tubes were removed only when necessary, meaning a situation of unexpected gradual narrowing of the tube lumen until its complete closure.The consistency of the application of the preventive measures mentioned above was systematically supervised by monitoring the amount of materials used and confirmed by appropriate documents.In the case of any suspicion of VAP infection the patients had microbiological tests done.The material for microbiological tests was collected from the respiratory tract by means of bronchoalveolar lavage (BAL), a brush, trachea aspirate or, alternatively, a qualitative culture of the patient's sputum was examined.Cases of VAP were detected by means of active methods by the Infection Control Unit cooperating with ICU staff.Information about patients' treatment was obtained from the electronic data base in the InfoMedica hospital system.8][19] ).
In order to carry out the evaluation of the epidemiological situation the following epidemiological indica-tors were applied: VAPs cumulative incidence (number of ventilator-associated pneumonia episodes / number of patients x 100); VAPs /1000 patient days (number of ventilator-associated pneumonias episodes / number of patient days x 1000; VAPs incidence density (number of ventilator-associated pneumonias episodes / number of ventilator-days x 1000); VAPs mortality (% death of VAPs=number of deaths caused by ventilator-associated pneumonias/number of ventilator-associated pneumonias x 100); Device-associated rates and device utilization (DU ratio = number of ventilator-days / number of patient-days).
Per mission to conduct the study was obtained from the Director of Św.Łukasz Hospital in Tarnów, and all analyses of medical documentation were performed according to ethical standards and the principles of the Declaration of Helsinki.
The statistical analysis of collected material used IBM SPSS STATISTICS (SPSS -Statistical Package for the Social Sciences) statistical software, and Microsoft Excel from Microsoft.The description of the entire study population was prepared using basic statistical parameters.In the case of features of ordinal or dichotomous character, the information about the number and percentage share of specific variants was used.For calculating the measure of the relation between risk exposure and VAP development, odds ratio (OR) and relative risk (RR) were used, determining the probability ratio for event occurrence in comparable groups.In order to establish which of the relations were statistically significant, a number of statistical tests were used (P) including the chi-squared test (χ²) and Spearman's rank correlation coefficient (R).

patients took part in the research between 2007
and 2014.There were 804 patients in the "no subglottic secretion drainage" group and 1003 patients in the "subglottic secretion drainage" one.A difference was observed in VAP incidence frequency between both groups (P<0.001).In the NSSD group 86 VAP cases were reported (cumulative incidence 10.7%), whereas in the SSD group, in which the tubes with subglottic secretion drainage were applied, there were only 43 VAP cases (4.3%).The odds ratio (OR) and relative risk (RR) was to 2.5, which meant that the probability of VAP cumulative incidence was higher in the NSSD group.The mortality rate was similar -33.7% and 37.2% (Table 1).
The average age of hospitalized patients was 56 in the NSSD group and 59 in the SSD group.VAP infections were more common in men than in women, however the difference was not statistically significant.In the group in which subglottal drainage was not applied as many as 60 VAP cases were reported (cumulative incidence 14%) vs 26 cases (9%) in SDD group.In all age categories VAP incidence was higher in the NSSD group and lower in the SSD one.A statistically significant determinant of VAP cumulative incidence (P<0.001) was the situation when the time spent on the ward exceeded 20 days.Among the patients with mechanical ventilation and hospitalized longer than 20 days the VAP cumulative incidence was the highest in both groups and amounted to 54 cases in the NSSD group (cumulative incidence 91.5%) vs 23 cases in the SSD group (47.9%) (Table 2).
Both in the NSSD and the SSD group VAP infections were most frequent in patients whose primary reason for hospital admission included multiple organ trauma, respiratory system disease and central nervous system disease.Other diseases which were the reason for patients' hospitalization were also analyzed (Table 3).Statistically significant treatment-associated risk factors for developing VAP (P<0.001) which proved to be statistically moderately correlated included the correlation between reintubation (R=0.271),tracheostomy (R=0.309) and bronchoscopy (R=0.316).Aver age positive correlation means that the performance of invasive procedures in a patient, such as: tracheostomy, reintubation and bronchoscopy increases the risk of VAP development (  nosa and Escherichia coli.Acinetobacter baumannii, which is dominant in the etiology of VAP, proved to be 96% resistant to carbapenems and two other groups of medicines, and in the case of Pseudomonas aeruginosa the resistance applied to 100% of isolates (Table 5).

DISCUSSION
Both medical objectives and patients' comfort can be achieved by applying proper care based on "SMART" strategy: Specific, Measurable, Achievable, Realistic, Timely.Therefore, in the case of providing proper care of respiratory system of patients undergoing mechanical ventilation it is advisable to apply such methods which reduce the frequency of complications and can be applied easily and quickly.Thus, evidence-based medicine (EBM) plays an important role in the optimal choice of methods which allow for minimization of the risk of complications.
VAP infections in ICUs may be the reason for prolonged hospitalisation, increased mortality rate and a significant increase in the costs of treatment 4,5 .Scientific publications point out that as much as 20-50% of all hospital infections originate in ICUs, and VAP makes up a large percentage of them 6,8,20 .Therefore, replacing traditional methods by application of endotracheal tubes subglottic secretion drainage may be a significant factor which would reduce the risk of complications including infections.
According to research carried out in 2011-2012 by the ECDC, the indicator of VAP incidence density in ICUs reached 10.5 for 1000 days of intubation 20 .The annual epidemiological report prepared by the ECDC in 2014 pointed out that pneumonia was detected in 5% of patients, 93% of whom were intubated, and the VAP incidence density reached 10.1 for 1000 days of intubation 8 .In Poland VAP incidence density ranges between 10.2 and 16.5 for 1000 days of mechanical ventilation 8,9,16 .
In the population of 1807 patients examined during the research in the ICU of Św.Łukasz Provincial Hospital in Tarnów, VAP incidence reached 8% (the figures differed for specific groups: NSSD -11% vs SSD -4%), and the incidence density quotient reached 14 for 1000 days of mechanical ventilation (NSSD group -18.9 vs SSD group 9.1).Following the analysis of microbiological flora which was responsible for the incidence of hospital pneumonia the conclusion can be drawn that the dominant microorganism found in the material examined which accounts for VAP incidence was Acinetobacter baumannii, and then Pseudomonas aeruginosa.According to the ECDC.Pseudomonas aeruginosa was the most frequently isolated microorganism 15 .
In the research carried out in the ICU of Św.Łukasz Provincial Hospital in Tarnów a 50% decrease in the number of VAP infections was reported in the group  25 ).Dezfuliani et al. in their meta-analysis of 5 randomized controlled trials from 2005 also discovered positive effects of the application of subglottic secretion drainage 23 .Hospitalisation time and mortality rate were comparable in both groups.The results obtained in Św.Łukasz Provincial Hospital in Tarnów and based on the analysis of 1807 patients treated there, confirm the observations of the aforementioned authors.In some publications authors point out the higher efficiency of subglottic secretion drainage among patients whose treatment involves a long time of mechanical ventilation 2 .Mujica-Lopez et al. emphasize that the diameter of a suction catheter in endotracheal tubes affects the suction efficiency and cause blockages 26 .Other research implies that the high efficiency of the application of endotracheal tubes with subglottic secretion drainage in VAP prevention might be influenced by other factors, such as particle structure of the secretion, suction force, or the type of material from which the sealing cuff is produced [27][28][29] .
The caus e of patient hospitalisation may be an independent risk factor of VAP development.Multiple organ trauma is the most frequent cause of hospitalisations in ICUs.In studies conducted by Różańska et al. the VAP incidence among patients with trauma was 29% (ref. 30).In another study, VAP incidence in trauma was 9% (ref. 31).In this study, VAP incidence in patients admitted to ICUs with multiple trauma was 15% in NSSD patients, with a subsequent decrease in incidence in SSD to 5%.
Risk factors related to invasive diagnostic and treatment methods may contribute to VAP development.In this study, tracheostomy, reintubation and bronchoscopy were more often performed in patients with VAP and increase in intervention frequency was positively correlated with VAP incidence.

CONCLUSION
Use of endotracheal tubes with subglottic secretion drainage in patients from an ICU ward significantly reduced the incidence of VAP.
The time of mechanical ventilation was shorter in the patients with subglottic secretion drainage but the mortality rate was comparable for both groups.The results need to be tested further.

Table 4
Comparison of the number of patients, patient-days of hospitalisation, numbers of VAP incidence, cumulative incidence, incidence density, mortality rate in "no subglottic secretion drainage group" (NSSD) and "subglottic secretion drainage group" (SSD) with mechanical ventilation.

Table 2 .
Comparison of mechanically ventilated patients by sex, age and the time of mechanical ventilation with a division into "no subglottic secretion drainage" (NSSD) and "subglottic secretion drainage" (SSD) groups.

Table 3 .
Independent factors of VAP presence related to the patient and his/her primary disease (the reason for hospital ICU admission) with a division into "no subglottic secretion drainage" (NSSD) and "subglottic secretion drainage" (SSD) groups.

Table 4 .
Independent factors of VAP presence related to the treatment applied on ICU with a division into "no subglottic secretion drainage" (NSSD) and "subglottic secretion drainage" (SSD) groups.
NSSD -No subglottic secretion drainage, SSD -subglottic secretion drainage, P -significance level of Chi-square test of independence for VAP, R = Spearman's rank correlation coefficient