ASSOCIATION OF CHEST-WALL MOTION AND TIDAL VOLUME RESPONSES DURING CO2 REBREATHING

The purpose of this study is to investigate the effect of chest wall c onfiguration at end expiration on tidal volume (VT) response during CO 2 rebreathing. In a group of 11 healthy male subjects, the changes in end-expiratory and end-inspiratory volume of the rib cage (Delta Vrc,E and Delta Vrc,I, respectively) and abdomen (Delta Vab,E and Delta Vab ,I, respectively) measured by linearized magnetometers were expressed as a function of end-tidal PCO2, (PET(CO2)). The changes in end-expira tory and end-inspiratory volumes of the chest wall (Delta Vab,E and De lta Vcw,I, respectively) were calculated as the sum of the respective rib cage and abdominal volumes. The magnetometer coils were placed at the level of the nipples and 1-2 cm above the umbilicus and calibrated during quiet breathing against the VT measured from a pneumotachograp h. The Delta Vrc,E/Delta PET(CO2) slope was quite variable among subje cts. It was significantly positive (P < 0.05) in five subjects, signif icantly negative in four subjects (P < 0.05), and not different from z ero in the remaining two subjects. The Delta Vab,E/Delta PET(CO2) slop e was significantly negative in all subjects (P < 0.05) with a much sm aller intersubject variation, probably suggesting a relatively more un iform recruitment of abdominal expiratory muscles and a variable recru itment of rib cage muscles during CO2 rebreathing in different subject s. As a group, the mean Delta Vrc,E/Delta PET(CO2), Delta Vab,E/Delta PET(CO2), and Delta Vcw,E/Delta PET(CO2) slopes were 0.010 +/- 0.034, -0.030 +/- 0.007, and -0.020 +/- 0.032 l/Torr, respectively; only the Delta Vab,E/Delta PETCO(2) slope was significantly different from zero . More interestingly, the individual Delta VT/Delta PET(CO2) slope was negatively associated with the Delta Vrc,E/Delta PET(CO2) (r = -0.68, P = 0.021) and Delta Vcw,E/Delta PET(CO2) slopes (r = -0.63, P = 0.03 7) but was not associated with the Delta Vab,E/Delta PET(CO2) slope (r = 0.40, P = 0.223). There was no correlation of the Delta Vrc,E/Delta PET(CO2) and Delta Vcw,E/Delta PET(CO2) slopes with age, body size, f orced expira tory volume in 1 s, or expiratory time. The group Delta V ab,I/Delta PET(CO2) slope (0.004 +/- 0.014 l/Torr) was not significant ly bPET different from zero despite the VT nearly being tripled at the end of CO2 rebreathing. in conclusion, the individual VT response to CO2, although independent of Delta Vab,E, is a function of Delta Vrc,E to the extent that as the Delta Vrc,E/Delta PET(CO2) slope increases (more positive) among subjects, the VT response to CO2 decreases. Thes e results may be explained on the basis of the respiratory muscle acti ons and interactions on the rib cage.