Capnography

Principle of operation

A capnograph is an instrument used to measure the CO₂ concentration of the expired air. Generally it is based on infrared (IR) analysis with a single beam emitter and non-dispersive infrared absorption measurement with a solid state mainstream sensor and ratiometric measurement of red/IR absorption, similar as with oximetry (see Pulse oximetry).

Another, still experimental approach (see ref. 1) is based on an acoustic impedance measurement of the air with an electro-acoustic sensor coupled to an acoustic resonator. The impedance characteristic is depending on the sound velocity within the gas mixture contained in the acoustic resonator. The relation between the acoustic impedance and the CO₂ concentration is approximately linear. Since the sound velocity (and so the acoustic impedance) is also is dependent on temperature and humidity, the outcome should be corrected for these parameters.

Applications

Capnography is widely used in clinical practice, such as pulmonology, anesthesiology and in the IC-unit.

Literature

1. http://www.mrtc.mdh.se/publications/1006.pdf