Changes in dyspnea related to decrements in inspiratory reserve volume

During constant work rate exercise (cycling), as tidal volume (V_T) encroaches on inspiratory capacity (IC) reducing the inspiratory reserve volume, the level of dyspnea rises markedly, with a threshold for severe dyspnea around V_T/IC = 70 to 80% (gray zone). The 4 lines (Q1 to Q4) display quartiles of resting lung function (forced expiratory volume in one second [FEV₁] with Q4 being the lowest quartile) and demonstrate how the onset of dyspnea is similar despite varying baseline lung function.