Respiratory drive and pattern during inertially-loaded CO 2 rebreathing: implications for models of respiratory mechanics in obesity

Abstract

Chest wall inertance ( Iw) constitutes a mechanical load which is small in normal individuals, but increases in the morbidly obese. The effect of increased Iw on respiratory drive and pattern has not previously been investigated. We studied this effect by measuring rebreathing CO 2 response in 10 normal subjects with and without a 45.5 kg inertial load. Changes in inspiratory occlusion pressure (P 0.1), ventilation, respiratory rate, mean inspiratory flow ( Vt Ti) duty cycle ( Ti Tt), inspiratory and expiratory times, and [P 0.1/( Vt Ti)] were assessed. The P 0.1 vs. Pet CO 2 response shifted to the left during inertial loading, while the slope remained unchanged (x-intercept = 36.7 ± 4.9 mm Hg unloaded vs. 32.7 ± 7.5 mm Hg loaded, P < 0.05), increasing P 0.1 from 18.5 ± 8.0 to 21.0 ± 7.6 cm H 2O at Pet CO 2 = 65 mm Hg (P < 0.005) . Respiratory pattern was unchanged with the inertial load except for a slight decrease in tidal volume. The inspiratory transfer characteristic [ P 0.1/( Vt Ti] at Pet CO 2 = 65 mm Hg increased significantly (8.3 ± 2.0 to 10.5 ± 2.9 cm H 2O·(L·sec −1) −1, P < 0.025) illustrating the strategy of maintaining similar ventilation by increasing inspiratory force against the load.