Physiology of capnography

Bhavani Shankar Kodali MD

How is a capnogram related to a tidal volume?

* Area under the CO2 curve = Effective Alveolar ventilation

* Area above the curve and below the PaCO2 = Physiological dead space


Determination of components of tidal volume using a capnogram

Physiological dead space sub-divided into anatomical dead space and alveolar dead space

A volume capnogram can be used to understand how a capnogram can be related to a tidal volume and its components.

A horizontal line (red line in the figure) representing PaC02 (arterial blood sampled during the PETCO2 recordings) is drawn on the CO2 trace. The area under the curve, green area, is the volume of C02 in the breath and represents effective alveolar ventilation. The remaining area below the horizontal red line represents wasted ventilation (physiological dead space). A vertical line is constructed through phase II so that the two areas p and q are equal. Area orange represents anatomical dead space and area brown represents alveolar dead space. Therefore, physiological dead space is represented by area brown plus area orange.1-4


1. Fletcher R. The single breath test for carbon dioxide. Thesis. Lund. Sweden 1980.

2. Fletcher R, Jonson B, Cumming G, Brew J. The concept of dead space with special reference to the single breath test for CO2. Br J Anaesth 1981;53:77-88.

3. Bhavani-Shankar K, Moseley H, Kumar AY. Delph Y. Capnometry and anaesthesia. Can J Anaesth 1992;39:6:617-32.

4. Bhavani-Shankar K, Kumar AY, Moseley HSL, Hallsworth RA. Terminology and the current limitations of time capnography: A brief review. J Clin Monit 1995;11:175-82.


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