Abstract: A gas exchange testing system has an apparatus for delivery of an auxiliary gas during a gas exchange test. Measurements representing physiological parameters of a patient are made during the gas exchange test. The auxiliary gas is delivered so that a gas exchange test can be performed on a patient who requires auxiliary gas. Additionally, the auxiliary gas is delivered so that gas exchange testing can determine if the auxiliary gas is beneficial and which concentration of the auxiliary gas is most beneficial. A testing process compares measurements under a condition where the auxiliary gas is delivered and under a condition where it is not delivered. Another testing process compares measurements under a condition where one auxiliary gas is delivered and under a condition where a different auxiliary gas is delivered.