A method is disclosed to maintain isocapnia when breathing exceeds baseline breathing and a circuit therefore. The circuit has a non rebreathing valve, a source of fresh gas, a fresh gas reservoir and a source of gas to be inhaled when minute ventilation exceeds fresh gas flow. The flow of fresh gas is equal to minute ventilation minus anatomic dead space, wherein additional inhaled gas exceeding fresh gas flow has a partial pressure of CO2 equal to the partial pressure of CO2 of arterial blood. Also disclosed is a method to identify the anatomic dead space and alveolar dead space by using a breathing circuit consisting of a non rebreathing valve, a source of fresh gas, a fresh gas reservoir and a source of gas with a partial pressure of CO2 substantially equal to that of arterial blood.