Larsen & Toubro Star 55, Operating Manual

Page: 52 / 168

CAPNOGRAPHY
Theory of operation
The principle of Capnography is based on the absorption of infrared radiation
by CO
2
. The technique is known as non-dispersive infrared absorption technique.
The spectral region is particularly appropriate for measuring carbon dioxide
because it has a strong absorption band in the near Infrared wavelength. CO
2
selectively absorbs specific wavelengths of Infrared light. The main part of the
system is CO
2
bench, which consist of one IR source and detector. A sample
of patient's expired gas that consists of CO
2
, Water vapors, Nitrogen, Anaesthetic
Agents, etc. is drawn from a lightweight T-piece through a sample line into the
CO
2
bench. Expired air is aspirated into measuring chamber (CO
2
bench) by a
small pump. The amount of light passing through a sample cell varies according
to the concentration of CO
2
in the sample cell. When concentration of CO
2
in
sample cell is high, more light is absorbed by the sample and therefore a small
amount of light reaches the detector as compared to low concentration of CO
2
.
The amount of light absorbed is proportional to the concentration of CO
2
. The
CO
2
concentration measured by the monitor is usually expressed as end tidal
concentration of CO
2
(EtCO
2
), expressed in terms of mmHg or percentage (%)
or Kilopascal (Kpa).
Capnogram - Nature of waveform
CO
2
waveform (Refer Figure 2.16) reflects various stages in breathing. Capnograph
is an important diagnostic tool because its shape is virtually identical in all
basically healthy people.
Figure 2.16
2.16