Capnometry is a method, which measures the volume of carbon dioxide (CO2) in the exhaled gas. The method is based on the CO2 molecules count, which can be inferred from the absorption of the sampling gas using a light with a wavelength of 426 nm.
The measurement is done either in real-time by the MAINSTREAM technique, where the CO2 sensor is inserted directly between the ETT (endotracheal tube) and the breathing circuit - therefore it extends the volume of the dead space, but without a time delay. The other option is the SIDESTREAM technique, in which the sampling gas is aspirated from the expiratory branch of the circuit. This causes a delay in recording, although the dead space is not extended.
Graphic visualization of capnometry is called capnography. The normal capnogram can be divided into several characteristic phases. The EtCO2 value and its changes inform us practically about the condition of the circulation.
Phase I - The initial gas exhaled is from the conducting airways Phase II - Alveolar gas containing CO2 mixes with gas from the anatomic
airways, and the CO2 concentration rises. Phase III - The curve plateaus as alveolar gas is exhaled (this phase is often referred to
as the alveolar plateau). The concentration of CO2 at the end of the
alveolar phase (just before inspiration begins) is referred to as the
end-tidal PCO2, or PetCO2.
