Defining segments and phases of a time capnogram

The division of a time capnogram into inspiratory and expiratory segments i s arbitrary and results in the inability of a time capnogram to detect rebr eathing instantaneously. Demarcation of a time capnogram into inspiratory a nd expiratory components using gas flow signals will not only facilitate pr ompt detection of rebreathing, but will also allow application of standardi zed and physiologically appropriate nomenclature for better understanding a nd interpretation of time capnograms. A Novametrix(R) CO2-SMO plus respirat ory profile monitor (Novametrix Medical Systems, Wallingford, CT) was used to obtain a simultaneous display of CO2 and respiratory flow waveforms on a computer screen during spontaneous and controlled ventilation using a circ le system with the inspiratory valve competent (no rebreathing) and with th e valve displaced (rebreathing). Because the response time of the CO2 analy zer was similar to the response time of the flow sensor, a comparison was m ade between the two waveforms to determine the inspiratory segment (Phase 0 ) and the expiratory segment of the time capnogram and its subdivisions (Ph ases I, II, and III). The end of expiration almost coincides with the downs lope of the CO2 waveform in the capnograms when there is no rebreathing. Ho wever, in the presence of rebreathing, the alveolar plateau is prolonged an d includes a part of inspiration (Phase 0), in addition to the expiratory a lveolar plateau (Phase III).