O2 Saturation in the Intercostal Space During Moderate and Heavy Constant-Load Exercise

To examine the hypothesis that the relationship between minute ventilation (V-E) and deoxygenation from the intercostal space (IC) would be steady regardless of exercise protocols, if an increase in O-2 consumption of the accessory respiratory muscles with an increase of V-E brings about deoxygenation in IC, we measured the relationship between V-E and O-2 saturation in IC (SO2IC) during a constant-load exercise test (CET), and the relationship was compared with that during a ramp incremental exercise test (RIET). Six male subjects performed RIET. On a different day, the subjects performed a moderate and heavy CET (CET_MOD and CET_HVY, respectively). SO2IC decreased from the start of both CET_MOD and CET_HVY and changed little from 2 min. Moreover, SO2IC was significantly lower during CET_HVY than during CET_MOD. In comparison between RIET and CET_HVY at the similar V-E level, SO2IC was significantly higher during CET_HVY than RIET. These results suggest that the decrease in SO2IC was caused not only by an increase in O-2 consumption in IC with an increase in V-E but also by a decrease in O-2 supply.