Patent ID: 8459258

Claim:
An apparatus for inducing and maintaining a target end tidal gas concentration of a gas X in a subject, comprising: a) an inlet for at least one component gas of a source gas; b) a source gas outlet configured for connection to a partial re-breathing circuit; and c) means for controlling the end tidal concentration of the gas X based on the rate of flow of the source gas into the partial rebreathing circuit and the concentration of the gas X in the source gas using an algorithm in which: (i) the source gas outlet is assumed to be connected to a partial rebreathing circuit; (ii) the flow rate of the source gas into the partial rebreathing circuit is assumed to be less than the subject's minute ventilation; and (iii) the concentration of the gas X in the source gas is computed from: A) a target end tidal concentration of the gas X; B) the flow rate of the source gas into the partial rebreathing circuit; and C) if the gas X is a gas consumed by the patient, the minute consumption of the gas X, or if the gas X is a gas produced by the patient, the minute production of the gas X; D) wherein if the gas X is a gas consumed by the patient, then the composition of the source gas is selected based on an algorithm derived from the following equation: FG 1 ⁢ X = F T ⁢ ETX + V . ⁢ X V . ⁢ G 1 , where {dot over (V)}X is the subject's minute consumption of the gas X, F T ET X is the target end tidal concentration of the gas X, and {dot over (V)}G 1 is the flow rate of the source gas; E) wherein if the gas X is a gas produced by the patient, then the composition of the source gas is selected based on an algorithm that is derived from the following equation: FG 1 ⁢ X = F T ⁢ ETX - V . ⁢ X V . ⁢ G 1 , where {dot over (V)}X is the subject's minute production of the gas X, F T ET X is the target end tidal concentration of the gas X, and {dot over (V)}G 1 is the flow rate of the source gas.