Page 124 - Basic Monitoring in Canine and Feline Emergency Patients

atmospheric gases in the mask in addition to the
fresh gas flow into the mask. This dilutes the con-
VetBooks.ir centration of CO and leads to a falsely low
2
ETCO as compared to an intubated patient.
2
However, knowing that there is CO dilution, face
2
mask–derived ETCO readings can still be trended
2
over time within a single patient and provide clini-
cal information. It is noteworthy to mention that
some capnography systems (like the Oridion sys-
tems) have a dual nasal and mouth sampling device
that may give a better idea of real ETCO in
2
patients who are panting/breathing through their
mouth.
Other advantages of sidestream capnographs
include faster warm-up time than mainstream
units, low dead space added to the circuit, and the
capability to measure several respiratory gases at
once including anesthetic gases. The line and adap-
tor that attach to the ETT are also lightweight
which reduces the chance of kinking or disconnec-
tion of the system.
One other aspect of the sidestream capnograph
that needs to be mentioned is that condensation
from humidified gas and patient’s secretions may
develop and it can accumulate in the sampling line.
This can lead to inaccurate ETCO readings as it
2
can affect the sampling cell function or sometimes
cause occlusion of the sampling line. Frequent
replacement of the sampling line may be required.
To avoid this or to at least decrease the frequency
of sampling line replacement, most units come with
a water trap (see Fig. 6.7) located at the end of the
sampling tube. Some units also have water perme-
®
able tubing such as Nafion tubing. This tubing has Fig. 6.7. Water trap sidestream capnograph. This is a
a unique property to transfer moisture from one sidestream capnograph and the water trap can be seen
side of the membrane to the other. This is done on the side of the machine.
chemically via differential vapor pressure (the
water molecules move from a higher pressure to a period between the mouth and the detection cham-
lower pressure) and is therefore very selective for ber than traditional sidestream technology and thus
water molecules only. If there is high moisture pres- a capnograph curve that is almost synchronized
sure in the tubing, the water molecules will be with the passage of air at the mouth. In addition,
transferred outside of the tubing and the gas that the microstream system has less dead space than
reaches the analysis chamber will be moisture-free traditional units (less than 0.5 mL of dead space),
and will not interfere with the CO reading. which makes this a useful ETCO monitor for the
2
2
A microstream system has also been developed small-sized patient. Other benefits of the micros-
which is a variation of the traditional sidestream tream technology include a low-flow 50 mL/min
units. In the microstream technology there is no sen- sample rate, no dilution with supplementary oxy-
sor at the airway. It uses laser-based molecular gen, and no cross reaction with other gases. Using a
spectroscopy as the infrared emission source. The low-flow sample rate is especially beneficial for our
expired CO travels along a thin tube before reach- small-sized patients which tend to have a low tidal
2
ing the chamber where the detector is located. The volume and a fast respiratory rate. Using a high-
microstream system allows for a smaller transition flow sampling rate (i.e. 150 mL/min used with

116 L.A.M. Ilie