Page 102 - Basic Monitoring in Canine and Feline Emergency Patients

Sea level = 760 mmHg Peak Everest = 253 mmHg Peak Everest = 253 mmHg
+100% FiO 2
VetBooks.ir

P O = 100 P O = 23 P O = 166
A
A
2
2
A
2
2
2
2
P A CO = 40 P A CO = 20 P A CO = 40
2
P A N = 573 P A N = 163 P A N = 0
2
2
PaO = 100 PaO = 23 PaO = 166
2
2
2
Fig. 5.5. Low barometric pressure (PB) or low fraction of inspired oxygen (FiO ) as a cause for hypoxemia. A normal
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alveolus is shown at the left with values in the alveolus representing humidified air after 47 mmHg water vapor
pressure has been added (see Fig. 5.3). When either the PB or FiO decreases (center), the driving force (diffusion
2
gradient) moving the oxygen across from the alveolus into the blood will decrease, resulting in less O moving into the
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capillary blood and a low PaO . Despite compensatory hyperventilation to drop the P CO , the small oxygen gradient
A
2
2
across the alveolus to the blood will result in hypoxemia. Addition of oxygen displaces nitrogen in the alveolus, which
greatly increases the diffusion gradient for oxygen. Therefore, low PB/FiO as a cause of hypoxemia is expected to
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be very oxygen responsive.
Normal alveolus Diffusion impairment Diffusion impairment
+ 100% FiO 2

P O = 100 P O = 100 P O = 673
A
2
2
2
A
A
P CO = 40 P CO = 40 P CO = 40
2
2
A
A
A
2
P A N = 573 P A N = 573 P A N = 0
2
2
2
PaO = 100 PaO = 50 PaO = 337
2
2
2
Fig. 5.6. Diffusion impairment as a cause of hypoxemia. While the amount of oxygen that enters the center alveolus
is normal, there are barriers (increased tissue thickness/distance, decreased surface area) that result in decreased
oxygen diffusion into the capillary blood. However, when the patient is given additional oxygen to breathe, the nitrogen
in the alveolus is replaced by O and the pressure differential between the alveolus and capillary blood will greatly
2
increase. This increased gradient will drive more O across into the blood despite the impaired diffusion barrier.
2
Diffusion impairment as a cause of hypoxemia should therefore be oxygen responsive.
94 A.C. Brooks