Abstract
Although the partial pressure of end-tidal carbon dioxide (PetCO_2) is a noninvasive, continuous method for measuring respiratory function and is used frequently in applied psychophysiological research, there is much confusion over the relationship between PetCO_2 and psychophysiological relaxation. Some studies have shown that an increment in PetCO_2 reflects relaxation, whereas others have claimed that an elevated PetCO_2 is a sign of stress responses. Therefore, this study was designed to clarify the relationship between PetCO_2 and relaxation. Twelve healthy subjects participated in the following three sessions: 20-min baseline rest (BL), 60-min respiratory self-control, and 5-min post-trial rest (PO). The respiratory self-control session consisted of two tasks, in which the subjects were required to increase (INC) or decrease (DEC) their PetCO_2 level using PetCO_2 and respiration rate biofeedback for 30 min each. All of the subjects succeeded in decreasing their PetCO_2 significantly during the DEC trial. As half of the subjects showed decrements in their PetCO_2 during INC, there was no significant increment in the mean PetCO_2 from BL. Seven of 12 subjects reported that they were more relaxed during DEC than during INC, because they found it easier to decrease PetCO_2 than to increase it. These results suggest that self-control of PetCO_2 can mislead novice subjects to mild hyperventilation, and that PetCO_2 biofeedback-assisted relaxation should not be a general relaxation method, but a specific method used to treat patients with hyperventilation.
