[Dyspnea sensation and chemical control of breathing]

In order to estimate the role of peripheral chemosensitivity in dyspnea sensation, we performed BH experiment under the acute or chronic hypoxic condition. The former was simulated by a given rate (0-3.2 mg/kg/hr) of doxapram infusion. The latter experiment was carried out during sojourn in Lhasa (3700 m), China. Subjects conducted BH by inhaling 7% CO2 in O2 and assessed dyspnea sensation by visual analog scale (VAS) while repeatedly measuring PCO2 at breaking point (BP). Lowering of resting PETCO2 by augmented ventilation was derived by doxapram infusion and during acclimatization at high altitude. The effect of PCO2 on VAS was enhanced by doxapram. However, altitude acclimatization resulted in attenuated effect of PCO2 on VAS despite of further development of hypocapnia. The rate of PCO2 elevation during doxapram infusion was reduced and it might be attributed to decreased body storage of CO2. On the other hand, its rate was tended to recover to sea level value after acclimatization at high altitude and it may have cancelled the mitigated dyspnea sensation. Thus, BHT almost comparable period in both acute hypoxia and during altitude acclimatization. These results suggest that CO2 storage in the body contributes to modify dyspnea sensation as well as augmented peripheral chemosensitivity.