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

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

References:

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.