An apparatus and method of using an end-tidal gas value taken from an non-intubated patient that includes measuring a plurality of gas concentration values, determining an end-tidal gas value from the gas concentration values, and reporting the end-tidal gas value for a breath as the greater of either: (a) a maximum gas concentration values observed during such a breath, or (b) the end-tidal gas value reported for a previous breath decreased by a maximum allowable breath-to-breath percent change.
