Respiratory drive and pattern during inertially-loaded CO₂ rebreathing: implications for models of respiratory mechanics in obesity

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 CO₂ 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_CO2 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 H₂O at Pet_CO2 = 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_CO2 = 65 mm Hg increased significantly (8.3 ± 2.0 to 10.5 ± 2.9 cm H₂O·(L·sec^-1)^-1, P < 0.025) illustrating the strategy of maintaining similar ventilation by increasing inspiratory force against the load.