Described here are closed-circuit breathing devices and methods for their use. In general, the closed-circuit breathing device is configured to achieve a steady-state equilibrium, whereby therapeutic gas is introduced into the breathing circuit in small, controlled volumes until a steady state concentration of the therapeutic gas is reached. During this time, the closed-circuit breathing device is operated in a true closed circuit, such that the therapeutic gas is not lost to the atmosphere. Safety measures are built into the closed-circuit breathing device so that a hypoxic mixture is not delivered to the subject. The therapeutic gas may be xenon.
