Page 347 - Small Animal Internal Medicine, 6th Edition

CHAPTER 20 Diagnostic Tests for the Lower Respiratory Tract 319

related to the PaO 2 by the sigmoid oxyhemoglobin dissocia­ the probe, animal motion (e.g., respirations, shivering), and
tion curve (see Fig. 20.30). Pulse oximetry is noninvasive, weak or irregular pulse pressures (e.g., tachycardia, hypovo­
VetBooks.ir can be used to continuously monitor a dog or cat, provides lemia, hypothermia, arrhythmias).
The value measured indicates the saturation of hemoglo­
immediate results, and is affordable for most practices. It is
a particularly useful device for monitoring animals with
affected by factors other than pulmonary function, such as
respiratory disease that must undergo procedures requiring bin in the local circulation. However, this value can be
anesthesia. It can also be used in some cases to monitor the vasoconstriction, low cardiac output, and local stasis of
progression of disease or the response to therapy. More and blood. Other intrinsic factors that can affect oximetry read­
more clinicians are using these devices for routine monitor­ ings include anemia, hyperbilirubinemia, carboxyhemoglo­
ing of animals under general anesthesia. binemia, and methemoglobinemia. External lights and the
location of the probe can also influence results. Pulse oxim­
METHOD etry readings of oxygen saturation are less accurate when
Most pulse oximeters have a probe attached to a fold of tissue, values are below 80%.
such as the tongue, lip, ear flap, inguinal skin fold, toe, or tail These sources for error should not discourage the clini­
(Fig. 20.31). This probe measures light absorption through cian from using this technology, however, because changes
the tissues. Other models measure reflected light and can in saturation in an individual animal provide valuable infor­
be placed on mucous membranes or within the esophagus mation. Rather, results must be interpreted critically.
or rectum. Artifacts resulting from external light sources are Examination of the oxyhemoglobin dissociation curve
minimized in the latter sites. Arterial blood is identified by (see Fig. 20.30) in normal dogs and cats shows that animals
the oximeter as that component which changes in pulses. with PaO 2 values exceeding 85 mm Hg will have a hemoglo­
Nonpulsatile absorption is considered background. bin saturation greater than 95%. If PaO 2 values decrease to
60 mm Hg, the hemoglobin saturation will be approximately
INTERPRETATION 90%. Any further decrease in PaO 2 results in a precipitous
Values provided by the pulse oximeter must be interpreted decrease in hemoglobin saturation, as illustrated by the steep
with care. The instrument must record a pulse that matches portion of the oxyhemoglobin dissociation curve. Ideally,
the palpable pulse of the animal. Any discrepancy between then, hemoglobin saturation should be maintained at greater
actual pulse and the pulse received by the oximeter indicates than 90% by means of oxygen supplementation or ventila­
an inaccurate reading. Common problems that can interfere tory support (see Chapter 25) or specific treatment of the
with the accurate detection of pulses include the position of underlying disease. However, because of the many variables
associated with pulse oximetry, such strict guidelines are not
always valid. In practice, a baseline hemoglobin saturation
value is measured, and subsequent changes in that value are
then used to assess improvement or deterioration in oxygen­
ation. Ideally, the baseline value is compared with the PaO 2
obtained from an arterial blood sample collected concur­
rently to ensure the accuracy of the readings.

Suggested Readings
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T
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Dillon AR, et al. Lung histopathology, radiography, high­resolution
FIG 20.31 computed tomography, and bronchio­alveolar lavage cytology
Monitoring oxygen saturation in a cat under general are altered by Toxocara cati infection in cats and is independent
anesthesia using a pulse oximeter with a probe (P) clamped of development of adult intestinal parasites. Vet Parasitol.
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