Page 103 - Basic Monitoring in Canine and Feline Emergency Patients

Normal alveolus Shunt Shunt + 100% FiO 2
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P O = 100 P A O = 100 P A O = 673
A
2
2
2
P CO = 40 P CO = 40 P CO = 40
2
A
A
2
A
2
P N = 573 P N = 573 P N = 0
A
2
A
2
2
A
PaO 2 = 100 PaO 2 = 673
PaO = 100 PaO 2 = 45 PaO = 54
2
2
= 40 PaO = 40
PaO 2 2
Anatomic RL shunt Anatomic RL shunt
Fig. 5.7. Anatomic shunt as a cause of hypoxemia. Venous blood traveling through an aberrant vessel or opening
between the right and left sides of the circulation will allow large amounts of venous blood to bypass oxygenation by
the lungs. The degree of hypoxemia created will depend on how much venous admixture the shunt contributes to the
total amount blood flow out of the left side of the heart. While the addition of oxygen can increase the oxygen content
of the blood that does pass an alveolus (shown as increased ‘redness’ below the right alveolus), this is not enough to
overcome the effect of the shunt. This is because hemoglobin cannot be saturated >100% and the majority of oxygen
in blood is carried bound to hemoglobin. The small amount of additional ‘red’ oxygenation achieved as increased PaO
2
cannot overcome the downward ‘drag’ or dilution that the blue desaturated venous blood has on total oxygen content.
Therefore, shunt as a cause of hypoxemia should have minimal to no response to oxygen supplementation.
Normal alveoli
Low V/Q
mismatch +
Low V/Q mismatch 100% O 2
P A O 2 = 100 P A O 2 = 673
P A CO 2 = 40 P A CO 2 = 40
P A N 2 = 573 P A N 2 = 0
PaO 2 = 100 PaO 2 = PaO 2 = 673 PaO 2 =
60 101
PvO 2 = 40 PvO 2 = 40

Zero V/Q mismatch
(physiologic shunt)
Fig. 5.8. Low ventilation/perfusion (V/Q) mismatch as a cause of hypoxemia. Alveoli that are not ventilated (V/Q = 0)
as shown in the lower left essentially act as physiological shunts and can be considered similar to micro versions of the
anatomical shunts discussed in Fig. 5.7. They will allow deoxygenated venous blood to bypass the lungs and mix with
oxygenated blood on the left side of the heart, reducing overall oxygenation of the blood (e.g. increased venous admixture).
Just as in Fig. 5.7 there is a limit to how much venous admixture can be overcome by addition of supplemental oxygen.
Therefore, V/Q mismatch as a cause of hypoxemia is variably responsive to oxygen supplementation depending on the
amount of venous admixture created, and becomes less response as V/Q mismatch worsens.

Venous and Arterial Blood Gas Analysis 95