Terminology and the current limitations of time capnography: a brief review

The carbon dioxide (CO2) trace versus time (time capnography) is convenient and adequate for clinical use. This is the method most commonly utilized in capnography. However, the current terminology in time capnography has not yet been standardized and is, therefore, a potential source of confusion. Standard terminology that is based on convention and logic to represent the various phases of a time capnogram is essential. The time capnogram should be considered as two segments: an inspiratory segment and an expiratory segment. The inspiratory segment is termed as phase ); 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-shaped upswing on the tracing due to mixing of dead space gas with alveolar gas; and phase III, the alveolar plateau, represents CO2-rich gas from the alveoli. The physiologic basis of phase IV, the terminal upswing at the end of phase III, which is observed in capnograms recorded under certain circumstances (such as in pregnant subjects and obese subjects) is discussed in detail. The clinical implications of the alpha angle, which is the angle between phases II and III, and the beta angle, which is the angle between phases III and the descending limb of phase 0, are outlined. The subtle but important limitations of time capnography are reviewed; its current status as well as its future potential are explored.