Chest wall kinematics during chemically stimulated breathing in healthy man

Chest wall compartment kinematics and respiratory muscle coordinate activity, during either hypercapnia or hypoxia, have not been comparatively assessed in healthy humans. We assessed the displacement volume of the chest wall (Vcw) in 5 normal subjects during hypoxic-normocapnic and hypercapnic-hyperoxic rebreathing by using linearized magnetometers. Vcw was divided into displacement volumes of the rib cage (Vrc) and the abdomen (Vab). Esophageal (Pes) and gastric (Pga) pressures were simultaneously recorded and transdiaphragmatic pressure (Pdi) was calculated by subtracting Pes from Pga. Pressure swings (sw) from end expiration (EE) to end inspiration (EI) were also calculated. During both hypoxia and hypercapnia, from quiet breathing to 40 L/min VE, Vrc,EI increased consistently but Vrc,EE, and Vab,EI did not. Moreover, Vab,EE decreased significantly during hypercapnia and remained unchanged during hypoxia. PesEI decreased (more negative values) and PesEE increased (less negative values) during either stimulus, while PgaEE increased with hypercapnia. Pdisw, calculated as the difference between PdiEE and PdiEI, increased significantly with both hypercapnia and hypoxia ( p = 0.002 for both). On the plot of Pes vs Pga, the slope of a line from end expiratory to end inspiratory lung volume between 20 and 40 L/min VE progressively increased during hypercapnia indicating increasing rib cage muscle (RCM) contribution to inspiratory pressure swings relative to the diaphragm. From these results we conclude that in healthy man: (i) with both chemical stimuli RCM contribution accounts for increase in Vrc displacement; (ii) with hypercapnia, the decrease in Vab,EE displacement indicates abdominal muscle (ABM) contribution to tidal volume; (iii) RCM and ABM assist the diaphragmatic function during hypercapnic stimulation.