CAPNOMETER/CAPNOGRAPH

Typical Number in Hospital: 2

Cost Bands: 4

References:

This measures carbon dioxide concentration in expired gases. It is used during anaesthesia and intensive care, and in lung function studies. In intensive care it may be used as a substitute for blood gas determinations or to monitor the performance of assisted ventilation.

The commonest type of capnometer uses changes in the infrared light transmission properties in gas mixtures containing carbon dioxide (see Infrared gas analyser). Since the concentration of oxygen, nitrous oxide, and carbon monoxide interferes with the transmission of infrared in mixtures containing carbon dioxide, corrections must be made if these are present. In some devices rapid responses are possible to allow detection of breath by breath carbon dioxide variation, and indication of the concentration occurring at the end of expiration.

A typical instrument contains the infrared transducer, a small pump to draw the gases through a narrow tube connected to the breathing circuit, a water trap, controls and a display unit. A pen recorder is sometimes added which can produce a plot of carbon dioxide percentage throughout the breathing cycle. The instrument may be combined with an oxygen analyser for some applications.