Effects of body orientation on the development of ventilator-associated pneumonia in mechanically ventilated swine

Introduction

Ventilator-associated pneumonia (VAP) is a frequent nosocomial infection with an average incidence of 20% of ICU patients undergoing mechanical ventilation. Elevation of the head of the bed to >30° (semirecumbent position) is a recommended strategy to reduce gastric reflux, and subsequent aspiration of colonized gastric contents. However, the efficacy of this strategy to prevent VAP remains controversial. We studied the relationship between gravity and VAP in a swine model, an omnivore with gastrointestinal physiology similar to humans.

Methods

Twenty-six female Yucatan minipigs were randomized into four groups: (A) eight pigs were mechanically ventilated with an orientation of the trachea approximately 45° above horizontal for 72 hours. In the remaining three groups (B to D) the head of the bed was oriented 10° below horizontal (Trendelenburg position): (B) six pigs were mechanically ventilated for 72 hours; (C) six pigs were mechanically ventilated for 72 hours with enteral feeding; and (D) six pigs were mechanically ventilated for 168 hours with enteral feeding. At the end of the study period, pigs were electively sacrificed and quantitative lung microbiological cultures performed.

Results

All eight pigs kept in a semirecumbent position developed pneumonia and respiratory failure (PaO₂/FiO₂ = 132 ± 139 mmHg vs. 479 ± 42 mmHg, P < 0.0001) with a median of 5.5 lobes out of six colonized. Sixteen pigs kept in the semirecumbent position had sterile lungs and two pigs ventilated in the Trendelenburg position for 7 days developed a low level of colonization. Body orientation was the only significant predictor of lung colonization and pneumonia (P < 0.001).

Conclusion

The semirecumbent position is uniformly associated with lung colonization and respiratory failure by 72 hours. In contrast. the positioning of the trachea and the endotracheal tube below the horizontal prevented the development of VAP.