Insights on the role of antimicrobial cuffed endotracheal tubes in preventing transtracheal transmission of VAP pathogens from an in vitro model of microaspiration and microbial proliferation

Abstract

We developed an in vitro model to evaluate the effect of different cuffed endotracheal tubes (ETTs) on transtracheal transmission of ventilator-associated pneumonia (VAP) pathogens along external surfaces of ETTs. The model independently assessed the relative contributions of microbial proliferation to the distal tip and microaspiration of contaminated secretions past the cuff by testing in three modes: microaspiration only, microbial proliferation only, and simultaneous microaspiration and microbial proliferation. We evaluated transmission of methicillin resistant Staphylococcus aureus (MRSA) or Pseudomonas aeruginosa (PA) in the presence of a standard ETT; a soft, tapered cuff ETT with subglottic suctioning; and a novel antimicrobial gendine (combination of gentian violet and chlorhexidine) ETT in the model. In the microaspiration only mode, when leakage past the cuff occurred quickly, no ETT prevented transmission. When microaspiration was delayed, the gendine ETT was able to completely disinfect the fluid above the cuff and thereby prevent transmission of pathogens. In microbial proliferation only mode, the gendine ETT was the sole ETT that prevented transmission. With both mechanisms simultaneously available, transmission was dependent on how long microaspiration was delayed. Potent antimicrobial ETTs, such as a gendine ETT, can make unique contributions to prevent VAP when microaspiration is gradual.

Figure 1

Diagram of in vitro model for simultaneous secretion-aspiration and biofilm-colonization as pathogenic routes to contamination of “lungs.”

Figure 2

Diagram of the model run in microaspiration only mode where these is a gap between the distal tip of the ETT and fluid in the lung chamber. A verification experiment with quantitative culture was run showing that microbial proliferation is precluded in this mode. In the verification experiment, a standard ETT was used and the cuff was inflated to 100 cm water to preclude microaspiration. The gap between the distal tip of the ETT and the initially sterile fluid in the lung chamber broke the continuous surface pathway required for MRSA (methicillin resistant Staphylococcus aureus)and PA (Pseudomonas aeruginosa) to proliferate along the ETT to the lung chamber; hence, it remained sterile for 144 hours.

Figure 3

Diagram of the model run in microbial proliferation only mode and quantitative culture showing time required for the lung chamber to become contaminated in this mode. In microbial proliferation only mode microaspiration is precluded by overinflating the cuff to 100 cm H₂O. Dye experiments verified that no leakage occurred over 144 hours at this inflation pressure. Quantitative culture results show times required to contaminate the “lung” chamber using a standard ETT with MRSA (methicillin resistant Staphylococcus aureus) and PA (Pseudomonas aeruginosa) inocula introduced above the cuff.

Figure 4

Time to “lung” contamination with gendine ETT compared to subglottic suctioning ETT. Mean and standard deviation of measured bacterial concentrations (colony-forming units [CFU]/mL) from the upper (above the cuff; (a) and (c)) and lower (“lung”; (b) and (d)) chambers in the in vitro model run in microbial proliferation only mode comparing quadruplicate runs of the ETT with soft-tapered cuff and subglottic suctioning (S/T-C + S-S ETT) and the antimicrobial gendine treated standard ETT (gendine ETT) at various sampling time points. The models were run with ETT cuff inflation pressures maintained at 100 cm H₂O. Inocula of  5 × 10⁵ CFU/mL of PA (Pseudomonas aeruginosa) and MRSA (methicillin resistant Staphylococcus aureus) were pipetted on top of the cuff at time = 0 hrs. (a) above the cuff, PA; (b) below the cuff, PA; (c) above the cuff, MRSA; (d) below the cuff, MRSA. * Statistically significant pairwise differences (P < 0.0001).

Figure 5

Diagram of model run in simultaneous microaspiration and microbial proliferation modes. ETT cuff inflation pressure was 30 cm H₂O. Quantitative culture results using a standard ETT with MRSA (methicillin resistant Staphylococcus aureus) and PA (Pseudomonas aeruginosa) inocula introduced above the cuff indicate that the lung chamber became positive shortly after microaspiration occurred. Dye experiments run with the gendine standard ETT and soft tapered cuff with subglottic suctioning ETT showed that microaspiration in the model occurred within 24 hours at 30 cm H₂O cuff inflation pressures.