A portable oxygen concentrator (10) including a reservoir (26) for storing oxygen-enriched gas and a delivery line (41) for delivering the oxygen-enriched gas from the reservoir to a subject. An oxygen delivery valve (36) communicates with the reservoir via the delivery line. A sensor (48) is in communication with gas flowing through the delivery line and generates a signal related to a breathing characteristic of the subject. A controller (21) operates in a first mode wherein the controller opens the oxygen delivery valve for continuous delivery of the gas to the subject and a second mode wherein the controller selectively opens and closes the oxygen delivery valve responsive to the signal of the sensor to deliver the gas in pulsed durations. A relief valve (46) is associated with the delivery line and opens responsive to the pressure within the delivery line exceeding a predetermined threshold so as to decrease pressure within the delivery line.
