Page 346 - Small Animal Internal Medicine, 6th Edition

318 PART II Respiratory System Disorders

The premise of the A-a gradient is that PaO 2 (a) is nearly by 3). The oxygen saturation of hemoglobin (SaO 2 ) is depen­
equal (within 10 mm Hg in room air) to the partial pressure dent on the PaO 2 , as depicted by the sigmoid shape of the
VetBooks.ir of oxygen in the alveoli, PAO 2 (A), in the absence of a diffu­ oxyhemoglobin dissociation curve (see Fig. 20.30). However,
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sion abnormality or V/Q mismatch. In the presence of a
the SaO 2 is also influenced by other variables that can shift
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diffusion abnormality or a V/Q mismatch, the difference
widens (greater than 15 mm Hg in room air). Examination the oxyhemoglobin dissociation curve to the left or right
(e.g., pH, temperature, 2,3­diphosphoglycerate concentra­
of the equation reveals that hyperventilation, resulting in a tions) or interfere with oxygen binding with hemoglobin
lower PaCO 2 , leads to a higher PAO 2 . Conversely, hypoventila­ (e.g., carbon monoxide toxicity, methemoglobinemia). Some
tion, resulting in a higher PaCO 2 , leads to a lower PAO 2 . Physi­ laboratories measure SaO 2 .
ologically the PaO 2 can never exceed the PAO 2 , however, and Oxygen must be successfully delivered to the tissues, and
the finding of a negative value indicates an error. The error this depends on cardiac output and local circulation. Ulti­
may be found in one of the measured values or in the mately, the tissues must be able to effectively use the
assumed R value (see Table 20.6). oxygen—a process interfered with in the presence of toxici­
Clinical examples of the calculation and interpretation of ties such as carbon monoxide or cyanide poisoning. Each of
the A-a gradient are provided in Box 20.8. these processes must be considered when the blood gas
values in an individual animal are interpreted.
Oxygen Content, Delivery, and Utilization
The commonly reported blood gas value PaO 2 reflects the Acid-Base Status
pressure of oxygen dissolved in arterial blood. This value is The acid­base status of an animal can be assessed using the
critical for assessing lung function. However, the clinician same blood sample that is used to measure blood gases.
must remember that other variables are involved in oxygen Acid­base status is influenced by the respiratory system (see
delivery to the tissues besides PaO 2 , and that tissue hypoxia Table 20.6). Respiratory acidosis results if carbon dioxide is
can occur in spite of a normal PaO 2 . The formula for calculat­ retained as a result of hypoventilation. If the problem persists
ing the total oxygen content of arterial blood (CaO 2 ) is pro­ for several days, compensatory retention of bicarbonate by
vided in Table 20.6. The greatest contribution to CaO 2 in the kidneys occurs. Excess removal of carbon dioxide by the
health is oxygenated hemoglobin. In a normal dog (PaO 2 , lungs caused by hyperventilation results in respiratory alka­
100 mm Hg; hemoglobin, 15 g/dL), oxygenated hemoglobin losis. Hyperventilation is usually an acute phenomenon,
accounts for 20 mL of O 2 /dL, whereas dissolved oxygen potentially caused by shock, sepsis, severe anemia, anxiety,
accounts for only about 0.3 mL of O 2 /dL. or pain; therefore compensatory changes in the bicarbonate
The quantity of hemoglobin is routinely appraised by the concentration are rarely seen.
complete blood count. It can also be estimated on the basis The respiratory system partially compensates for primary
of the packed cell volume (by dividing the packed cell volume metabolic acid­base disorders, and this can occur quickly.
Hyperventilation and a decreased PaCO 2 occur in response
to metabolic acidosis. Hypoventilation and an increased
BOX 20.8 PaCO 2 occur in response to metabolic alkalosis.
In most cases, acid­base disturbances can be identified as
Calculation and Interpretation of A-a Gradient: primarily respiratory or primarily metabolic in nature on the
Clinical Examples basis of the pH. The compensatory response will never be
excessive and alter the pH beyond normal limits. An animal
Example 1: A healthy dog breathing room air has a PaO 2 with acidosis (pH of less than 7.35) has a primary respiratory
of 95 mm Hg and a PaCO 2 of 40 mm Hg. His calculated acidosis if the PaCO 2 is increased and a compensatory respi­
PAO 2 is 100 mm Hg. (PAO 2 = FIO 2 [P B − PH 2O] − PaCO 2 /R ratory response if the PaCO 2 is decreased. An animal with
= 0.21 [765 mm Hg − 50 mm Hg] − [40 mm Hg/0.8].)
The A-a gradient is 100 mm Hg − 95 mm Hg = 5 mm Hg. alkalosis (pH of greater than 7.45) has a primary respiratory
This value is normal. alkalosis if the PaCO 2 is decreased and a compensatory respi­
Example 2: A dog with respiratory depression due to ratory response if the PaCO 2 is increased.
an anesthetic overdose has a PaO 2 of 72 mm Hg and a If both the PaCO 2 and the bicarbonate concentration are
PaCO 2 of 56 mm Hg in room air. His calculated PAO 2 is abnormal, such that both contribute to the same alteration
80 mm Hg. The A-a gradient is 8 mm Hg. His hypoxemia in pH, a mixed disturbance is present. For instance, an
can be explained by hypoventilation. animal with acidosis, an increased PaCO 2 , and a decreased
Later the same day, the dog develops crackles bilater- HCO 3 has a mixed metabolic and respiratory acidosis.
ally. Repeat blood gas analysis shows a PaO 2 of 60 mm
Hg and a PaCO 2 of 48 mm Hg. His calculated PAO 2 is
90 mm Hg. The A-a gradient is 30 mm Hg. Hypoventila- PULSE OXIMETRY
tion continues to contribute to the hypoxemia, but hypoven-
tilation has improved. The widened A-a gradient indicates Indications
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V/Q mismatch. This dog has aspirated gastric contents into
his lungs. Pulse oximetry is a method of monitoring the oxygen satura­
tion of blood. The saturation of hemoglobin with oxygen is