This disclosure describes systems and methods for ventilating a patient with a system that includes an accumulator for storing a gas mixture. The disclosure describes a novel approach for determining the concentrations of gas found in the accumulator by utilizing a sampling chamber. The disclosure further describes a novel approach for a fast delivery of a change in gas mixture to a patient by utilizing a variable-sized accumulator. This disclosure also describes systems and methods for ventilating a patient with a system that includes an accumulator located away from the flow path that reduces/eliminates pockets of an undesirable gas mixture from entering the gas flow path and reaching the patient after a gas mixture change by utilizing at least one of a dip-tube, a purge valve, and a variable size accumulator.