O2 saturation in the intercostal space during moderate and heavy constant-load exercise.

To examine the hypothesis that the relationship between minute ventilation (VE) and deoxygenation from the intercostal space (IC) would be steady regardless of exercise protocols, if an increase in O2 consumption of the accessory respiratory muscles with an increase of VE brings about deoxygenation in IC, we measured the relationship between VE and O2 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 VE 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 O2 consumption in IC with an increase in VE but also by a decrease in O2 supply.