TERMINOLOGY AND THE CURRENT LIMITATIONS OF TIME CAPNOGRAPHY - A BRIEFREVIEW

The carbon dioxide (CO2) trace versus time (time capnography) is conve nient and adequate for clinical use; This is the method most commonly utilized in capnography. However, the current terminology in time capn ography has not yet been standardized and is, therefore, a potential s ource of confusion. Standard terminology that is based on convention a nd legit to represent the various phases of a time capnogram is essent ial. The time capnogram should be considered as two segments: an inspi ratory segment and an expiratory segment. The inspiratory segment is t ermed as phase 0; the expiratory segment is divided into phases I, II, III, and, occasionally, IV. Phase I represents the CO2-free gas from the airways (anatomical dead space); phase II consists of a rapid S-sh aped upswing on the tracing due to mixing of dead space gas with alveo lar gas; and phase III, the alveolar plateau, represents CO2-rich gas from the alveoli. The physiologic basis of phase IV, the terminal upsw ing at the end of phase III, which is observed in capnograms recorded under certain circumstances (such as in pregnant subjects and obese su bjects) is discussed in detail. The clinical implications of the alpha angle, which is the angle between phases II and III, and the beta ang le, which is the angle between phases III and the descending limb of p hase 0, are outlined. The subtle but important limitations of time cap nography are reviewed; its current status as well as its future potent ial are explored.