Effect of posture on thoracoabdominal movements during CO2 rebreathing

To determine whether the rib cage and abdomen-diaphragm contributions to tidal volume (VT) during CO2 rebreathing are affected by postural change, using respiratory inductive plethysmography, we measured in eight healthy volunteers the compartmental VT responses to progressive hypercapnia in both seated and supine postures. The ventilatory, frequency, and VT responses to CO2 of the total respiratory system were not significantly different between postures. VT responses, corrected for body size, ranged from 1.67 to 3.71% vital capacity (VC) X Torr-1 (mean 2.27) in seated subjects and from 1.08 to 3.79% VC X Torr-1 (mean 2.06), in supine subjects. In both postures, the VT response of the abdominal compartment was nearly uniform among subjects and independent of the total respiratory system VT response (slope = 0.091, r = 0.210 P greater than 0.3 seated; slope = 0.043, r = 0.077, P greater than 0.3 supine), whereas the VT response of the rib cage varied among subjects and was significantly correlated to the total VT response (slope = 0.815, r = 0.84, P less than 0.01, seated; slope = 1.125, r = 0.859, P less than 0.01, supine). Thus high tidal volume responses to CO2 rebreathing are determined largely by recruitment of the rib cage compartment in both seated and supine postures.