Terminology of capnograms
Bhavani Shankar Kodali MD
Over the last two decades, time capnography has
become a standard of monitoring in anesthesia practice in many countries. Along
with the acceptance of this technology, however, there has also been a
considerable proliferation of terminology representing the various components of
a time capnogram. This ambiguity in terminology has been a source of confusion
For instance, numerous terms, such as
PQRS, ABCDE, EFGHIJ, and phases I through IV, have been used to depict the various components
of a time capnogram.1-12 Some
have used "phase IV" to
designate the terminal upswing at the end of phase III, which is occasionally
observed in capnograms recorded in pregnant and obese subjects.
Others have used "phase IV" to designate the descending limb of
a time capnogram. In much the same way that the nomenclature of the various
segments of the ECG have been standardized, it is necessary to define and
standardize the nomenclature used to designate the various components of a time
capnogram. A standard terminology facilitates teaching, comprehension,
communication, and research A terminology representing various phases of a time capnogram, and based on
logic, convention, and tradition, has been described
Bhavani-Shankar et al several
terminology is currently being adapted by several authorities including
Nunn's Respiratory Physiology.13
The logical and conventional basis of this terminology is summarized as
In 1949, Fowler described SBT-N2 (single-breath test for nitrogen) to study uneven ventilation in lungs where instantaneous nitrogen concentrations are plotted against expired volume.14 The resulting nitrogen curve is divided into four phases: phase I, phase II, phase III, and phase IV. When the instantaneous CO2 concentration is plotted against expired volume , the resulting curve resembles an SBT-N2 curve in shape and is called an SBT-CO2 curve. An SBT CO2 curve is also traditionally divided into three phases: I, II, and III, and, occasionally, a phase IV, if present.1,2,15. Phase IV does not occur normally, but may be seen under certain circumstances, as described in the physiology section. The physiologic mechanism responsible for phases I, II, and III is similar in SBT-N2 as well as in SBT-CO2 curve. However, the mechanism resulting in phase IV in SBT-CO2 may be different from that in an SBT-CO2 curve, as explained in the physiology section.
Unlike an SBT-CO2 trace, a time capnogram has an inspiratory segment in addition to the expiratory segment. There is no inspiratory segment in an SBT-CO2 curve, as, by definition, a SBT-CO2 trace is a plot of PCO2 and expired volume. However, the expiratory segment of a time capnogram resembles an SBT-N2 curve and an SBT-CO2 curve in shape. Furthermore, the physiologic mechanism responsible for the shape of the expiratory segment is similar to that in either SBT-N2 curve, or SBT-CO2 curve.2 Hence, it is prudent conventionally and logically to also consider the expiratory segment of time capnogram as three phases: I, II, and III as in SBT-N2/SBT-CO2 curve. Occasionally, at the end of phase III, a terminal upswing (phase IV) seen in an SBT-CO2 curve or an SBT-N2 curve, may occur in a time capnogram. The details of phase IV are discussed in the physiology section.
terminology is summarized as follows.
A time capnogram can be divided into inspiratory
(phase 0) and expiratory segments. The expiratory segment, similar to a single
breath nitrogen curve or single breath CO2 curve, is divided into
phases I, II and III, and occasionally, phase IV, which represents the terminal
rise in CO2 concentration. The
angle between phase II and phase III is the alpha angle.
The nearly 90 degree angle between phase III and the descending limb is
the beta angle.
Limitation of time capnogram:
The assumption that expiration ends at the commencement of down-stroke is not necessarily true all the time. In a time capnogram, the beginning and the end of an inspiratory segment, and the beginning and the end of expiration (expiratory time) cannot be delineated accurately without superimposing the simultaneously recorded respiratory flows. Expiration begins somewhere in the horizontal line before the actual upstroke, and ends somewhere on the phase III, reminder of phase III being expiratory pause. For further understanding of this concept, please refer to the references 2 and 3 below, and the 'Capno-pitfalls' section of this website.
Bhavani-Shankar K, Moseley H, Kumar AY, Delph Y.
Capnometery and Anaesthesia: Review article.
Can J Anaesth 1992;39:6:617-32.
Bhavani-Shankar K, Kumar AY, Moseley HSL, Ahyee-Hallsworth R.
Terminology and the current limitations of time capnography: A brief
review. J Clin Monit 1995;11:175-82.
Fletcher R, Jonson B. Deadspace and the single breath test for carbon
dioxide during anesthesia and artificial ventilation.
Br J Anaesth 1984;56:109-19.
Moon RE, Camporesi EM. Respiratory monitoring. In Miller RD, ed.
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Fundamentals of current clinical practice.
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Adams AP. Capnography and
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intensive care. London: Churchill Livingston, 1989:155-175.
Sweadlow DB. Capnometry and capnography:
The anesthesia disaster early warning system.
in Anesthesia 1986;3:194-205.
Ward SA. The capnogram:
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Curley MAQ, Thompson JE. End-tidal CO2 monitoring in critically ill infants and
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Kalenda Z. Mastering
infrared capnography. Utrecht, The
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Bhavani-Shankar K, Philip JH. Defining
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13. Nunn's Appplied Respiratory Physiology. 5th edition. Boston: Butterworth-Heinemann, 2000;243
Fowler WS. Lung function studies V. Respiratory dead space in old age and
in pulmonary emphysema. J Clin
Fletcher R. The single breath test for carbon dioxide (Thesis). Lund,