Page 300 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Respiratory Acid-Base Disorders 291

If breathing room air, the alveolar gas equation dictates These patients are likely to have primary pulmonary dis-
that, at steady state, arterial or alveolar (PAO 2 ) oxygen ten- ease, but extrapulmonary disorders cannot be completely
sion will decrease with an increase in PCO 2 : ruled out.

PACO 2 HYPOXEMIA
PAO 2 ¼ PIO 2 ð4Þ
R
Arterial blood gas analysis is not only essential for deter-
where R is the respiratory exchange ratio that accounts mining PaCO 2 levels and acid-base condition of a patient;
for the difference between CO 2 production and O 2 con- it also provides information pertaining to a patient’s
sumption at steady state, PIO 2 is the inspired oxygen ten- oxygenation status. There are five main reasons for hyp-
sion, and PaCO 2 is the alveolar PCO 2. In normal animals, R oxemia, including low fraction of inspired oxygen,
is approximately 0.8. Because of the high solubility of hypoventilation, diffusion impairment, ventilation-perfu-
CO 2 ,PaCO 2 can be substituted for PaCO 2 in equation sion mismatching, and shunt (Box 11-1).
(4) under the assumption that PaCO 2 will equal PaCO 2 .
LOW PARTIAL PRESSURE OF
INSPIRED O (PIO )
PaCO 2 2 2
PAO 2 ¼ PIO 2 ð5Þ
R Low levels of inspired oxygen produce patient hypoxemia
by reductions in mean alveolar oxygen levels (PAO 2 ), sub-
Thus the difference between PAO 2 and PaO 2 can be sequently reducing PaO 2 . Although relatively uncommon
calculated as: in veterinary medicine, this type of hypoxemia can result
from a decrease in barometric pressure (i.e., residence at
ðA aÞO 2 gradient ¼ PAO 2 PaO 2 high altitudes or nonpressurized airline flights) or an

PaCO 2 improper inhalant anesthetic technique (e.g., administra-
¼ PIO 2 PaO 2 ð6Þ
R tion of N 2 O without O 2 ). In these cases, there is a
subsequent increase in alveolar ventilation secondary to
Considering R ¼ 0. 8, and 1/0.8 ¼ 1.25: hypoxemia, which in turn decreases PaCO 2 . The (A a)
O 2 difference remains within normal limits because of
ðA aÞO 2 gradient ¼ðPIO 2 1:25PaCO 2 Þ PaO 2 ð7Þ the concomitant decrease in PIO 2 .
HYPOVENTILATION
At sea level in a patient breathing room air, PIO 2 is
approximately 150 mm Hg. This can be substituted in As previously discussed, the prevailing PAO 2 is determined
equation 7: by the balance between the removal of oxygen by the
blood and replenishment of oxygen by alveolar ventila-
ðA aÞO 2 gradient ¼ð150 1:25PaCO 2 Þ PaO 2 ð8Þ tion. According to equations (4) and (5) above, as alveo-
lar ventilation decreases, PAO 2 and PaO 2 decrease while
Values below 15 mm Hg are generally considered nor- PaCO 2 and PaCO 2 must increase. As a result, the (A a)
mal. 16 If the (A a) O 2 ratio is widened, a component O 2 gradient does not change. If the (A a) difference
of the hypoxemia results from ventilation-perfusion is widened, there may be a component of the hypoxemia
mismatching. It should be remembered that FIO 2 is attributable to primary lung disease, such as ventilation-
dependent on barometric pressure and will be lower at alveolar perfusion mismatching or right-to-left shunting.
higher altitudes. At an altitude of 500 m (approximately In addition, the alveolar gas equation also predicts that
1640 feet), PIO 2 ¼ 140 mm Hg, whereas at 1000 m although increases in alveolar ventilation can change
(3280 feet), PIO 2 ¼ 130 mm Hg. Although it has long PAO 2 considerably, they can only moderately increase
been thought that in the hypercapnic patient the alveo- PaO 2 . Due to the sigmoid shape of the oxygen-
lar-arterial oxygen difference differentiates hypoxemia
caused by pure hypoventilation from hypoxemia in which
other factors play a role, this idea has been seriously BOX 11-1 Mechanisms of
challenged, 26,33 because the (A a) O 2 gradient may Derangement in
be increased in some patients with extrapulmonary Arterial Oxygenation
disorders. Clinically, a normal gradient excludes pulmo-
nary disease and suggests some form of central alveolar Low fraction of inspired oxygen (FIO 2 )
hypoventilation or an abnormality of the chest wall or Diffusion barrier
inspiratory muscles. 67 To increase the specificity of the Hypoventilation
test to diagnose the ventilation/perfusion mismatch, Ventilation-perfusion mismatch (V-Q mismatch)
only patients with (A a) O 2 gradient values greater Right-to-left shunt
than 25 mm Hg should be considered abnormal. 16