Tidal volume amplitude affects the degree of induced bronchoconstriction in dogs

When isolated constricted airway smooth muscle is oscillated, muscle tone d ecreases. We investigated whether changing tidal volume (VT) would affect i nduced bronchoconstriction in an in vivo canine model. Open-chest dogs were intubated with a double-lumen endotracheal tube, which isolated each main bronchus, and mechanically ventilated with a dual-cylinder ventilator. Bron chial pressure (Pbr) and flow were measured separately in each lung. Resist ance and elastance were calculated by fitting the changes in Pbr, flow, and volume to the equation of motion. After baseline measurements at the same VT (150 ml), the two lungs were ventilated with different VT (50 vs. 250 ml ) at a constant positive end-expiratory pressure. A continuous infusion of methacholine was begun, and measurements were repeated. The two lungs were then ventilated with the same VT (250 ml), and measurements were again repe ated. A similar protocol was performed in a second group of dogs in which m ean Pbr was kept constant. Bronchoconstriction was more severe in the lung ventilated with lower VT in both protocols. When VT was reset to the same a mplitude in the two lungs, the difference in bronchoconstriction was abroga ted. These results demonstrate that large VT inhibits airway smooth muscle contraction, regardless of mean Pbr.