Gravity predominates over ventilatory pattern in the prevention of ventilator-associated pneumonia.

Objective: In the semirecumbent position, gravity-dependent dissemination of pathogens has been implicated in the pathogenesis of ventilator-associated pneumonia. We compared the preventive effects of a ventilatory strategy, aimed at decreasing pulmonary aspiration and enhancing mucus clearance versus the Trendelenburg position. Design: Prospective randomized animal study. Setting: Animal research facility, University of Barcelona, Spain. Subjects: Twenty-four Large White-Landrace pigs. Interventions: Pigs were intubated and on mechanical ventilation for 72 hours. Following surgical preparation, pigs were randomized to be positioned: 1) in semirecumbent/prone position, ventilated with a duty cycle (TITTOT) of 0.33 and without positive end-expiratory pressure (control); 2) as in the control group, positive end-expiratory pressure of 5 cm H2O and TITTOT to achieve a mean expiratory-inspiratory flow bias of 10 L/min (treatment); 3) in Trendelenburg/prone position and ventilated as in the control group (Trendelenburg). Following randomization, Pseudomonas aeruginosa was instilled into the oropharynx. Measurements and Main Results: Mucus clearance rate was measured through fluoroscopic tracking of tracheal markers. Microspheres were instilled into the subglottic trachea to assess pulmonary aspiration. Ventilator-associated pneumonia was confirmed by histological/microbiological studies. The mean expiratory-inspiratory flow in the treatment, control, and Trendelenburg groups were 10.7 +/- 1.7, 1.8 +/- 3.7 and 4.3 +/- 2.8 L/min, respectively (p < 0.001). Mucus clearance rate was 11.3 +/- 9.9 mm/min in the Trendelenburg group versus 0.1 +/- 1.0 in the control and 0.2 +/- 1.0 in the treatment groups (p = 0.002). In the control group, we recovered 1.35% +/- 1.24% of the instilled microspheres per gram of tracheal secretions, whereas 0.22% +/- 0.25% and 0.97% +/- 1.44% were recovered in the treatment and Trendelenburg groups, respectively (p = 0.031). Ventilator-associated pneumonia developed in 66.67%, 85.71%, and 0% of the animals in the control, treatment, and Trendelenburg groups (p < 0.001). Conclusions: The Trendelenburg position predominates over expiratory flow bias and positive end-expiratory pressure in the prevention of gravity-dependent translocation of oropharyngeal pathogens and development of ventilator-associated pneumonia. These findings further substantiate the primary role of gravity in the pathogenesis of ventilator-associated pneumonia.