Tube Modifications
A number of approaches have been investigated to reduce VAP incidence through modifications of the ETT. These efforts have focused on systems for drainage of subglottic secretions, coating of the ETT with anti-bacterial materials, and the sealing capacity of the cuff.
Subglottic Secretion Drainage
Despite tracheal suctioning, secretions tend to accumulate around the ETT cuff, where they cannot be removed. Subglottic secretion drainage systems usually consist of an accessory aspiration conduit opening above the ETT cuff and a vacuum source. Secretions may be continuously or intermittently removed from the subglottic space. Continuous aspiration has been shown to cause mucosal injuries in animal models, therefore intermittent aspiration systems are generally preferred. Use of an ETT equipped with a subglottic secretion drainage system has been associated with a reduction in VAP incidence. A multicenter study of more than 300 patients showed a decrease in VAP rate in the group treated with intermittent secretion drainage. The beneficial effects of secretion drainage lasted over time, with no significant adverse events. A recent meta-analysis including nearly 2500 patients confirmed the efficacy of ETTs equipped with subglottic secretion drainage systems in preventing VAP, shortening ICU stay and reducing days of ventilation.
A different method of secretion drainage is the Mucus Slurper, a modified tube equipped with multiple aspirating holes opening on the tip of the ETT. Secretions are aspirated intermittently in the early expiratory phase, keeping the ETT lumen and the proximal trachea free from secretions. However, no clinical data are available and the effectiveness of this device in VAP prevention remains to be determined.
ETT Coating
The inner surface of coated ETT tubes may be covered by a thin layer of anti-microbial agent(s), to prevent the formation of biofilm and bacterial colonization. Among several coating agents, silver seems to be feasible: It is biologically compatible, easy to employ, and effective in reducing tracheal colonization and bacterial growth in animal models. The silver coating has bacteriostatic properties: Silver ions penetrate into the microbial membrane interfering with DNA synthesis, thereby preventing cell replication. Other agents have shown even greater in vitro antibacterial activity as compared to silver, but their clinical use remains to be investigated.
Several clinical trials have been conducted evaluating the efficacy of VAP prevention using silver-coated ETTs. In the North American Silver-Coated Endotracheal Tube (NASCENT) study, the use of a silver-coated ETT was associated with lower rates of VAP and of late-onset VAP in more than 2000 patients. Hence, the use of anti-bacterial coating seems suitable to treat patients expected to be ventilated for more than 48 hours, possibly resulting in VAP prevention and cost-effectiveness.
Cuff Seal and Shape
Modified cuffs have been proposed to improve tracheal sealing and reduce secretion drainage. Traditional hi-volume/low-pressure cuffs are made of polyvinylchloride (PVC). The surface of a traditional ETT cuff folds when inflated in the trachea, creating potential channels through which secretions can drain and reach the subglottic space. In vitro models have shown passage of fluids around a traditional ETT cuff already 1 minute after the beginning of the experiment, whereas modified cuffs provided significantly better sealing. The principal cuff modifications involve changes in cuff shape and materials employed. The tapered shape seems to provide better sealing as compared to the classical cylindrical shape, possibly because the tapered cuff maintains better contact with the tracheal wall, resulting in less folding of the cuff surface. The use of materials other than PVC, such as polyurethane, lycra, silicone or latex, also result in better sealing in vitro; however, no definitive clinical data are available regarding the material of choice to prevent VAP. ETTs equipped with polyurethane cuffs protected against early postoperative pneumonia in a population of cardiac surgery patients, but the effect on VAP rate was not investigated. A retrospective study on more than 3000 patients showed an association between the use of a polyurethane cuff and a decrease in VAP incidence. A randomized clinical trial showed VAP rate reduction for patients intubated with an ETT equipped with both a subglottic secretion drainage system and a polyurethane cuff, but the contribution of the polyurethane cuff to preventing VAP in this combined system is unclear.