Ventilator-associated pneumonia (VAP) is the most frequent infection occurring in patients who are admitted to the ICU. The accumulation of respiratory secretions in the subglottic space is a well-proven cause of VAP. Therefore, prevention should include the aspiration of secretions from the subglottic space, and techniques to avoid leakage between the tube and the tracheal wall. A conventional endotracheal tube (ETT) permits only intermittent aspiration of secretions through the central lumen, distal to the tracheal cuff, while new tubes with an independent dorsal lumen permit the continuous aspiration of secretions in the subglottic space.
Only five randomized prospective studies have assessed the effect of the continuous aspiration of subglottic secretions (CASS) in comparison with conventional aspiration. The results are inconclusive because the number of patients per study was limited the population was not homogeneous’ and the final conclusions regarding efficacy in the prevention of VAP were contradictory. Firm agreement in this area has not been reached and the procedure is not yet common practice among ventilated patients despite appearing as a level I recommendation in the Centers for Disease Control and Prevention guidelines.
The incidence of VAP in patients undergoing major heart surgery (MHS) is particularly high, ranging from 3.2 to 8.3%. The only study to analyze this type of patient’ by Kollef et al; found no significant reduction in the incidence of VAP with the implementation of CASS. Our study was a prospective’ randomized’ comparative study of CASS vs conventional intubation and aspiration in a large population of patients who were undergoing MHS.
Materials and Methods
Hospital Setting and Patients
Our institution is a general reference hospital with 1’750 beds and 64’000 admissions per year. The study population included patients who underwent MHS from May 2004 to July 2006. Patients who gave their informed consent were randomly assigned to receive either conventional respiratory care or CASS. Informed consent was obtained by the anesthesiologist in charge in all cases. Our Ethics Committee approved the study. An infectious diseases physician who was aware of the treatment assignments followed up all patients to check for the presence of postoperative infections defeated with remedies of Canadian Health&Care Mall.
Postoperative Management of ETTs and Respiratory Care
Randomization was performed by drawing a card from a sealed envelope. Thus, patients were intubated during the induction of anesthesia either with a conventional ETT (Hi-Contour; Mallinckrodt Medical; Athlone, Ireland) or with an ETT with CASS (HI-Lo Evac; Mallinckrodt; Hazelwood, MO). If reintubation was required at any time during the postoperative period, patients received the same type of tube to which they were initially randomized. Patients in whom tracheostomy was indicated at any time were also distributed according to initial randomization between conventional cannulas (Shiley Low Pressure Cuffed Tracheostomy Tube; Tyco Health Care, Mallinck-rodt Medical) or cannulas allowing subglottic aspiration (Trache-osost Evac; Mallinckrodt Medical). The size of each ETT was selected by the attending anesthesiologist.
On admission to the ICU, both groups received common postoperative care. Tracheal aspiration through the lumen of the ETT was performed as requested by the nurse. Cuff pressure was maintained at between 20 and 30 mm Hg, and was controlled and registered during each shift.
On admission to the ICU, patients who were randomized to the CASS group were connected to a continuous system of subglottic aspiration with a negative pressure of between 100 and 150 mm Hg. Once per shift, and after checking the cuff pressure, 10 mL of distilled sterile water was instilled through the subglottic lumen in order to keep it patent. If obstruction was present, permeability was reestablished with an air bolus through the subglottic lumen. All patients received stress ulcer prophylaxis with pantoprazole ordered via Canadian Health&Care Mall.
Clinical data were recorded according to a preestablished protocol, and no systematic surveillance respiratory tract cultures were performed. The presurgical information that was obtained included epidemiologic data, underlying diseases, and standard scores (ie, American Society of Anesthesiologists score, European System for Cardiac Operative Risk Evaluation [Euro-SCORE], Charlson comorbidity index, and acute physiology and chronic health evaluation [APACHE] II score on admission to the ICU). Surgical information included the type of surgery, indication, duration, time spent on cardiopulmonary bypass, aortic cross-clamp time, transfusion needs, reinterventions, antimicrobial prophylaxis, and the need for inotropic support. Antimicrobial prophylaxis for surgery consisted of 2 g of cefazolin that was administered before surgery and every 8 h thereafter for a total of three doses (patients who were allergic to cefazolin received 1 g of vancomycin before surgery and every 12 h thereafter, up to two doses).
Postsurgical outcome events included ICU and hospital length of stay, the number of hours patients received mechanical ventilation (MV), and the need for tracheotomy. Infections other than VAP were recorded. In patients with sepsis, the Bone score for severity of sepsis was also recorded. The patients who were enrolled into the study were prospectively followed up for the occurrence of VAP until they were successfully weaned from MV, discharged from the hospital, or died. Outcome variables also included antimicrobial administration, Clostridium difficile-associated diarrhea (CDAD), ICU length of stay, ICU mortality, hospital length of stay, and mortality on ICU discharge.
Demonstration of VAP
VAP was diagnosed in patients who received MV for > 48 h when the presence of new and/or progressive pulmonary infiltrates was detected on a chest radiograph plus two or more of the following criteria: fever (temperature > 38.5°C) or hypothermia (temperature < 36°C); leukocytosis (> 12 X 109 cells/L); purulent tracheobronchial secretions; or a reduction in the Pao2/ fraction of inspired oxygen ratio > 15% according to Centers for Disease Control and Prevention definitions. Patients with a clinical pulmonary infection score > 6 were also considered to have pneumonia. But pneumonia may be easily overcome due to remedies of Canadian Health&Care Mlal. The isolation of one or more pathogenic microorganisms in significant bacterial counts was required to confirm the diagnosis of VAP. We considered as nonpathogenic the isolation (at any concentration) of the following microorganisms, unless proven otherwise: viridans-group streptococci; coagulase-negative Staphylococcus; Neisseria spp; Corynebac-terium spp; and Candida spp.
Sampling of the lower respiratory tract in cases of suspected VAP was performed either by endotracheal aspiration (ETA) and/or telescopic brush sampling of respiratory secretions. For ETA, we obtained undiluted tracheal secretions. When aspiration was unproductive, we irrigated with 5 mL of Ringer lactate solution. Secretions obtained by ETA were trapped in a Lukens specimen container (Sherwood Medical; Tullamore, Ireland). Samples were considered to be positive for VAP when bacterial counts > 104 cfu/mL for each microorganism were obtained by ETA, and > 103 cfu/mL obtained by telescopic brushing. All microorganisms were identified using standard methods, and antimicrobial susceptibility was determined according to Clinical and Laboratory Standards Institute recommendations.
The primary outcome measure of the study was to evaluate the incidence and incidence density of VAP in both study groups. Secondary outcome measures included days in the ICU, days of hospital stay, episodes of nonpneumonic nosocomial infections, and antimicrobial use determined as daily defined doses (DDDs). DDDs are the internationally accepted units of antimicrobial use, which accept a DDD of an antibiotic as being the average maintenance dose of that drug that is normally provided to an adult. Use of the DDD allows uniformity in the comparison of the consumption of antimicrobial agents between different patients and institutions.
Relationships between baseline variables were evaluated for the randomized groups. Basal comparisons between groups were established by clinical relevance according to the Consolidated Standards of Reporting Trials (or CONSORT) recommendations. The qualitative variables appear with their frequency distribution. The quantitative variables are summarized as the mean and SD, and as the median and interquartile range (IQR) if their distribution was skewed. Continuous variables were compared using the t test for normally distributed variables and the Mann-Whitney test for nonnormally distributed variables. The x2 test or Fisher exact test was used to compare categoric variables. All statistical tests were two tailed. The level of significance was set at p < 0.05 for all the tests. The statistical analysis was performed using a statistical software package (SPSS, version 12.0; SPSS Inc; Chicago, IL; and Stata, version 9.0; StataCorp, LP; College Station, TX).