The rate of carbon monoxide (CO) elimination is increased by CO 2 -stimulated ventilation in CO poisoned, unconscious dogs. However, most conscious humans are unlikely to tolerate prolonged breathing of 5-10% CO 2 . We proposed to determine: 1) the effect on the half-time of carboxyhemoglobin (COHb) elimination (Tj/ 2 ) of a voluntary increase in ventilation of approximately 5 times from resting levels with FjO 2 = 100%); 2) whether this level of hyperpnea is sustainable long enough to provide

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... herapeutic benefit when the PCO 2 is maintained at control levels; and 3) the effect of minute ventilation on the half time of elimination of carboxyhemoglobin. Methods: After obtaining institutional board approval, seven normal male volunteers were exposed to CO until their venous [COHb] reached 10%. They then breathed 100% O 2 at resting ventilation or approximately 5 times the resting level of ventilation (~96% O 2 , balance CO 2 ) for 1.5 h on separate days. A non-rebreathing circuit (Eur. Respir. J. 1998;12(3):698.) was used to prevent changes in P E^C O 2 during hyperpnea. The Ty 2 was calculated from plots of [HbCO] versus time. Results: 1) The Tjy 2 significantly fell from 78 minutes at resting ventilation to 31 minutes with hyperpnea (p <0.01)( Figure 1) ; 2) All subjects sustained the hyperpnea without difficulty; and 3) There was a hyperbolic relation between minute ventilation (normalized for body weight and a [Hb] of 15 g/L) and the Tjy 2 ( Figure 2) . P E T CO 2 during hyperpnea did not differ from that during resting ventilation.