Page 343 - Small Animal Internal Medicine, 6th Edition

CHAPTER 20 Diagnostic Tests for the Lower Respiratory Tract 315

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FIG 20.28
Position for obtaining an arterial blood specimen from the
femoral artery. The dog is in left lateral recumbency. The
right rear limb is being held perpendicular to the table to
expose the left inguinal area. The pulse is palpated in the
femoral triangle between two fingers to accurately locate
the artery. The needle is laid directly on top of the artery,
then is stabbed into it with a short, jabbing motion.

TABLE 20.5
Approximate Ranges of Arterial Blood Gas Values for
Normal Dogs and Cats Breathing Room Air
MEASUREMENT ARTERIAL BLOOD

PaO 2 (mm Hg) 85-100
PaCO 2 (mm Hg) 35-45 FIG 20.29
HCO 3 (mmol/L) 21-27 Position for obtaining an arterial blood specimen from the
pH 7.35-7.45 dorsal pedal artery. The dog is in left lateral recumbency,
with the medial surface of the left leg exposed. A pulse is
palpated just below the tarsus on the dorsal surface of the
metatarsus between the midline and the medial aspect of
the distal limb.
INTERPRETATION OF RESULTS
Approximate arterial blood gas values for normal dogs and
cats are provided in Table 20.5. More exact values should be with oxygen when the PaO 2 is greater than 80 to 90 mm Hg,
obtained for normal dogs and cats using the actual analyzer. and clinical signs are unlikely in animals with such values.
The curve begins to decrease more quickly at lower PaO 2
PaO 2 and PaCO 2 values. A value of less than 60 mm Hg corresponds to a
Abnormal PaO 2 and PaCO 2 values can result from technical hemoglobin saturation that is considered dangerous, and
error. The animal’s condition and the collection technique treatment for hypoxemia is indicated. (See the section on
are considered in the interpretation of blood gas values. For oxygen content, delivery, and utilization later in this chapter.)
example, an animal in stable condition with normal mucous In general, animals become cyanotic when the PaO 2
membrane characteristics evaluated for exercise intolerance reaches 50 mm Hg or less, which results in a concentration
is unlikely to have a resting PaO 2 of 45 mm Hg. The collection of nonoxygenated (unsaturated) hemoglobin of 5 g/dL or
of venous blood is a more likely explanation for this abnor­ more. Cyanosis occurs as a result of the increased concentra­
mal value. tion of nonoxygenated hemoglobin in the blood and is not
Hypoxemia is present if the PaO 2 is below the normal a direct reflection of the PaO 2 . The development of cyanosis
range. The oxyhemoglobin dissociation curve describing the depends on the total concentration of hemoglobin, as well
relationship between the saturated hemoglobin level and as on the oxygen pressure; cyanosis develops more quickly
PaO 2 is sigmoid in shape, with a plateau at higher PaO 2 values in animals with polycythemia than in animals with anemia.
(Fig. 20.30). Normal hemoglobin is almost totally saturated Acute hypoxemia resulting from lung disease more often