Capnographs - Basics
| Abbreviation / other names What does |
End Tidal CO2 (ETCO2), Expired CO2 Monitor
Capno = Carbon dioxide |
| What does it do? |
A Capnograph measures the amount of CO2 in the
expired air from the lungs of a patient. |
| Physiology |
Cells of our body require oxygen, which is given to them via
the blood. O2 is given to cells, and when used the
waste gas, CO2, produced within the cells is exhausted
into the blood. The Blood then transports it to the capillaries
in the lungs and the CO2 is then transferred into
the air in the lung. As the lungs exhale the CO2
is exhausted to atmosphere. |
| How it works |
There are two types of Capnographs, Main stream or Side stream.
Main stream. Capnographs have a CO2 sensor
that attaches to a disposable airway adaptor inserted between
the endotracheal tube in the patient and the ventilator circuit
(after the wye piece). The sensor is made up of a light source
that sends infra red light through the adaptor, the sample of
gas and a filter chopper wheel to the photo detector. Infra
red is absorbed by CO2 in the exhaled gas. The more
CO2 gas the more the infra red is absorbed and less
reaches the photo detector. This signal is sent to the Capnograph and processed to give a graphic reading on the screen. The Side stream Capnograph takes a sample of gas from the airway (from the same place as the Main stream sensor) through a small tube into the Capnograph and all of the measurement and signal processing takes place inside the Side stream Capnograph. (same method as Main stream Capnograph) |
| Units of measurement |
%, mmHg partial pressure |
| Typical values |
4% CO2, 35 mmHg partial pressure of CO2 |
| Picture of equipment |
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