Page 104 - Basic Monitoring in Canine and Feline Emergency Patients

High V/Q High V/Q
VetBooks.ir P O 2 = 100 P O 2 = 100

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P CO 2 = 40 P CO 2 = 40
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P N 2 = 573 P N 2 = 573
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A

P O 2 = 100 P O 2 = 100
A
A
P CO 2 = 40 P CO 2 = 40
A
A
P N 2 = 573 P N 2 = 573
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A

Normal alveolus Low V/Q
Fig. 5.9. High V/Q mismatch as a cause of hypoxemia. In high V/Q units, the alveolus is normally ventilated,
but blood is blocked from flowing past that alveolus to perform gas exchange; a classic example is a pulmonary
thromboembolism (PTE). However, this blood will find another route through the lungs and over-circulate adjacent
alveoli as shown by diagram on the right. This alveolus can oxygenate some of this blood, but cannot keep up to
oxygenate all of the ‘diverted’ blood. This essentially turns this previously normal unit into a low V/Q unit (ventilation is
unchanged, but flow has increased). This results in hypoxemia as shown in Fig. 5.8. The response of these low V/Q
units to oxygen will be similarly variable as in Fig. 5.8 depending on the degree of venous admixture created.
line are ideal as they reflect venous return from at arterial blood gas syringes, careful avoidance of
least half of the body. Other venous sites can be bubbles within the sample, and sealing the syringe
used as long as there is not a specific disease (i.e. prior to transport for analysis is important. Even in
thromboembolism) affecting the tissue bed that a sealed syringe containing no air bubbles, gas (car-
would alter results compared to systemic blood. bon dioxide, oxygen) can still diffuse through plas-
Venous samples can be used for acid–base analysis, tic. Therefore, it is recommended to analyze samples
but not for assessment of oxygenation status. within 15 minutes of collection.
Arterial samples are more technically challenging
to obtain, and are usually obtained from the dorsal
pedal or femoral arteries, although coccygeal, Analyzer
auricular, and lingual arteries (in anesthetized Due to the need to analyze samples shortly after
patients) are also used. If repeated arterial sampling collection, blood gas analysis cannot be sent out
is needed, an arterial catheter should be placed. See and must be performed in-house. Various analyzers
Further reading section for techniques for arterial are available but generally fall into two categories:
puncture and arterial catheter placement tech- portable cartridge-based systems versus benchtop
niques. Arterial samples are needed for assessment analyzers (see Chapter 1, Figs 1.5 and 1.6). Cartridge-
of oxygenation. based systems use single disposable cartridges to
For accurate pH, ventilation, and oxygen analy- analyze each sample; the cartridge contains both
sis, it is important that the sample not be allowed the calibration solutions and the electrochemical
to equilibrate with air. Therefore, use of vented sensors used for sample analysis. These analyzers

96 A.C. Brooks