A system configured to administer at least one therapeutic gas to a patient, comprising: a. at least one reservoir tube, having a proximal and a distal end, wherein the at least one reservoir tube has a volume larger than the tidal volume of the patient breath; b. at least one therapeutic gas inlet at the proximal end of the at least one reservoir tube, wherein a delivery tube is connected to the at least one therapeutic gas inlet and at least one therapeutic gas source; and c. a patient interface fluidly connected to the proximal end of the at least one reservoir tube via a check valve, wherein the patient interface is configured to form a gas-tight seal between the patient and the system, wherein the inhalation side of a check valve is configured to be closed when the patient is exhaling, wherein the at least one therapeutic gas is introduced into the at least one reservoir tube at the proximal end through the at least one therapeutic gas inlet at a time average flow rate greater than the time averaged inhalation rate of the patient, and the at least one therapeutic gas flows along the at least one reservoir tube, from the proximal end to the distal end whilst the patient is exhaling, wherein the volume of the at least one therapeutic gas that is introduced into the at least one reservoir tube whilst the patient is exhaling, is greater than the patient's inhaled tidal volume, and wherein the inhalation side of the check valve is configured to be open when the patient is inhaling, and wherein the inhalation side of the check valve is configured to allow the at least one therapeutic gas to be administered to the patient. a. at least one reservoir tube, having a proximal and a distal end, wherein the at least one reservoir tube has a volume larger than the tidal volume of the patient breath; b. at least one therapeutic gas inlet at the proximal end of the at least one reservoir tube, wherein a delivery tube is connected to the at least one therapeutic gas inlet and at least one therapeutic gas source; and c. a patient interface fluidly connected to the proximal end of the at least one reservoir tube via a check valve, wherein the patient interface is configured to form a gas-tight seal between the patient and the system, wherein the inhalation side of a check valve is configured to be closed when the patient is exhaling, wherein the at least one therapeutic gas is introduced into the at least one reservoir tube at the proximal end through the at least one therapeutic gas inlet at a time average flow rate greater than the time averaged inhalation rate of the patient, and the at least one therapeutic gas flows along the at least one reservoir tube, from the proximal end to the distal end whilst the patient is exhaling, wherein the volume of the at least one therapeutic gas that is introduced into the at least one reservoir tube whilst the patient is exhaling, is greater than the patient's inhaled tidal volume, and wherein the inhalation side of the check valve is configured to be open when the patient is inhaling, and wherein the inhalation side of the check valve is configured to allow the at least one therapeutic gas to be administered to the patient. wherein the patient interface is configured to form a gas-tight seal between the patient and the system, wherein the inhalation side of a check valve is configured to be closed when the patient is exhaling, wherein the at least one therapeutic gas is introduced into the at least one reservoir tube at the proximal end through the at least one therapeutic gas inlet at a time average flow rate greater than the time averaged inhalation rate of the patient, and the at least one therapeutic gas flows along the at least one reservoir tube, from the proximal end to the distal end whilst the patient is exhaling, wherein the volume of the at least one therapeutic gas that is introduced into the at least one reservoir tube whilst the patient is exhaling, is greater than the patient's inhaled tidal volume, and wherein the inhalation side of the check valve is configured to be open when the patient is inhaling, and wherein the inhalation side of the check valve is configured to allow the at least one therapeutic gas to be administered to the patient.