Page 121 - Basic Monitoring in Canine and Feline Emergency Patients

will not be accurate. Therefore, in most capnographs be. This occurs because many infrared waveforms
the infrared source is designed to emit wavelengths reaching the detector mean that few infrared wave-
VetBooks.ir within very narrow limits. A graphic representation forms have been absorbed by the passing CO
2
(therefore the concentration of CO is low). For
of the main components is found in Fig. 6.2.
2
example, if the patient exhales a high concentration
of CO this will cause an increase in infrared
Sample chamber 2,
absorption in the sampling chamber; therefore, a
This chamber is made of sapphire, which allows the low number of infrared waves will reach the detec-
infrared waves to pass through. When the patient tor, indicating a high ETCO .
2
expires, the gas expired also passes through this There is a proprietary algorithm that implements
chamber. The infrared waves will be absorbed by the information received by the detector to deter-
the expired CO based on its concentration (i.e. the mine the exact ETCO value displayed on the
2
2
higher the expired CO concentration, the higher screen of the capnograph.
2
the absorption of the infrared light). See Fig. 6.2. Quantitative capnographs are also classified based
on the location of the infrared detector: (i) main-
stream capnographs; or (ii) sidestream capnographs.
Infrared detector
An infrared detector is located opposite to the mainstream capnography In the mainstream cap-
infrared source (on the other side of the sample nograph, an airway adapter is inserted directly
chamber). When infrared waves reach this detector, between the breathing circuit and the endotracheal
a signal will be transmitted to the monitor. The tube. An infrared source and detector are attached
higher the number of infrared waveforms that to the airway adapter (sometimes it can be found
reach the detector, the lower the CO reading will as a single unit or as separate parts, see Figs 6.3 to 6.5).
2

Infrared
CO detector
2
CO
2
CO 2
Sample chamber containing expired gases. Expired CO 2
Infrared molecules absorb infrared (IR) waves and a small number of IR
source waves reach the infrared detector.
Endotracheal
tube

Patient

Fig. 6.2. Schematic representation of a capnograph using infrared spectrography with its three main components:
infrared source, sample chamber and infrared detector. Note the infrared source is emitting infrared waves toward
the sample chamber. The patient expires anesthesia gases, including CO . The infrared waves will be absorbed by
2
the expired CO molecules based on its concentration (i.e. the higher the expired CO concentration, the higher the
2
2
absorption of the infrared light). The infrared waves not absorbed by the CO molecules will reach the detector and
2
a signal will be transmitted to the monitor. The lower the number of infrared waveforms that reach the detector, the
higher the CO reading will be. CO , carbon dioxide.
2 2
Capnography 113