A method and apparatus for determining a number of important physiologic characteristics of a patient based on a end tidal determinations taken from a single respiratory waveform. In the preferred embodiment, breath detection, end-tidal occurrence, and respiratory information is determined using only the expired CO.sub.2 gas concentration waveform. The system of the present invention is capable of performing the following basic functions: (1) identification of breath-by-breath inspired/expired end-tidal gas concentrations of three or more other expired gases (i.e., O.sub.2, NO.sub.2, and a volatile anesthetic); (2) calculation of the respiratory, inspiratory, and expiratory periods; (3) calculation of the respiratory rate and inspiratory/expiratory ratio; and (4) calculation of the multi-breath and time-trend averages. The preferred embodiment of the invention system comprises a breath detection algorithm which is implemented in two phases--initialization and normal. The initialization phase tests for presence of a respiratory signal presence then determines the parameters necessary for the normal search phase. Initialization of the system requires no prior knowledge of the respiratory rate, end-tidal differences, or breathing type. The search method implemented in the normal search phase determines the occurrence of a breath by identifying the end-tidal inspired value for the next breath. Two independent search methods are used in the normal search phase. Each method identifies a potential candidate inspiration value. The candidate inspiration values are tested for appropriate amplitude. If the candidate inspiration value fails to satisfy predetermined values, the search process continues until a valid candidate is found or the detection search is reinitiated. In a situation where two candidate inspiration values are located, an arbitration procedure is implemented to determine whether one or both of the candidate inspired values is appropriate.
Curtis D. White;Billy L. Carpenter