• Gianluigi Li Bassi, Alberto Zanella, Massimo Cressoni, Mario Stylianou, and Theodor Kolobow.
• Pulmonary and Critical Care Medicine Branch, Section of Pulmonary and Cardiac Assist Devices, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, MD, USA. libassig@aim.com
• Crit. Care Med. 2008 Feb 1;36(2):518-25.

ObjectivesCritically ill intubated patients are positioned in the semirecumbent position to prevent pneumonia. In tracheally intubated sheep, we investigated the effects of gravitational force on tracheal mucus transport and on bacterial colonization of the respiratory system.DesignProspective randomized animal study.SettingAnimal research facility at the National Institutes of Health.SubjectsSixteen healthy sheep.InterventionsSpontaneously breathing or mechanically ventilated sheep were randomized to be positioned with the orientation of the trachea above (40 degrees, trachea-up) or below (5 degrees, trachea-down) horizontal.Measurements And Main ResultsTracheal mucus velocity was measured through radiographic tracking of radiopaque tantalum disks, insufflated into the trachea. After 24 hrs, sheep were euthanized, and samples from the airways and lungs were taken for microbiological analysis. The proximal trachea was colonized in all sheep. In trachea-down sheep, all mucus moved toward the glottis at a mean velocity of 2.1 +/- 1.1 mm/min. When mucus reached the endotracheal tube, it either entered the endotracheal tube or was lodged at the inflated endotracheal tube cuff. In all trachea-up sheep, abnormal tracheal mucus clearance was found. Mucus, mostly on the nondependent part of the trachea, moved toward the glottis at an average velocity of 2.2 +/- 2.0 mm/min and constantly accumulated at the inflated endotracheal tube cuff. From the proximal trachea, mucus eventually moved toward the lungs on the dependent part of the trachea, leading to an "intratracheal route" of colonization of the lungs. Pneumonia was found in 6/8 of trachea-up sheep and the same microorganisms were isolated from the lungs and the proximal trachea. No pneumonia was found in trachea-down sheep (p = .007).ConclusionsThe study indicates that following tracheal intubation gravitational force influences tracheal mucus clearance. When the trachea is oriented above horizontal, a flow of mucus from the proximal trachea toward the lungs is highly associated with bacterial colonization of the airways and pneumonia.

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