Page 123 - Basic Monitoring in Canine and Feline Emergency Patients

The detector is attached to the monitor via an stream capnograph. In addition, since mainstream
electrical wire. units do not remove gas from the anesthetic circuit,
VetBooks.ir sidestream capnography In sidestream capnog- there is no need to separately scavenge anesthetic
gases. Mainstream units are more durable than
raphy, the detector is in the main unit itself (away
nents, do not require additional sample tubing, and
from the airway) and a very small pump aspirates sidestream units, have fewer disposable compo-
gas samples (usually 50–150 mL/min, depending on tend to be reasonable in cost. Mainstream analyz-
whether the machine measures other gases) from ers also provide instantaneous readings.
the patient's airway through a small tube into the In contrast, sidestream capnographs have the
main unit. This sampling tube is connected to a unique advantage of allowing monitoring of non-
T-piece inserted at the endotracheal tube or anes- intubated subjects by connecting the sidestream
thesia mask connector (Fig. 6.6A). The gas that is micro tube to a face mask or a nasal catheter line.
withdrawn from the patient often contains anes- When using nasal ETCO , it is unlikely that dilu-
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thetic gases; the exhausted gas from the capnograph tion of CO will occur and it has been demon-
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must be routed to a gas scavenger or returned to the strated that the general waveform is comparable to
patient’s breathing system (Fig. 6.6B). and representative of the ETCO obtained from an
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Each method of sampling comes with its own endotracheal tube (as long as the patient breathes
unique set of advantages and disadvantages. through its nose). When using a face mask, signifi-
Mainstream technology generally produces more cant dilution of CO is expected. This is because
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sharply delineated waveforms compared to a side- the samples of expired gas are mixed with the

(A) (B)

Pump aspirating gas samples into the T piece connection to Exhausted gas Gas scavenger
main unit. endotracheal tube
The larger size plastic container below
the aspirating pump is a water trap.
Note that the infrared sensor is inside
the main unit thus not visible.

Fig. 6.6. Sidestream capnograph. (A) A sidestream capnographs with a small aspiration pump that takes a sample
of expired gas directly from the patient’s expired gas (which includes CO and anesthetic gas) and brings it to the
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infrared sensor located inside the main unit. (B) After the gas sample is analyzed, the exhausted gas that has been
sampled from the patient by the capnograph is routed to a gas scavenger to remove CO and any waste anesthesia
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gases (such as isoflurane). CO , carbon dioxide.
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Capnography 115