Page 101 - Basic Monitoring in Canine and Feline Emergency Patients

Oxygen levels in arterial blood can never exceed Table 5.6. Physiological differentials for hypoxemia
levels in the atmosphere unless the patient is breath- (low PaO ) and their expected degree of oxygen
2
VetBooks.ir ing supplemental oxygen. Hypoxemia (low level of Cause of Disease
responsiveness.
oxygen in arterial blood, i.e. low PaO ) is the com-
2
mon clinical oxygen abnormality, but should be
distinguished from tissue hypoxia (oxygen debt at hypoxemia example Oxygen responsive?
the level of the tissues). For example, severe anemia Low inspired FiO / Altitude Yes
2
(low Hb) will not affect the PaO that dissolves into low barometric Empty oxygen
2
the blood from the lungs, and therefore the patient pressure cylinder
will not be hypoxemic as measured on an arterial Hypoventilation See Table 5.1 Yes (still need to fix
blood gas. However, without enough Hb to hold primary ventilation
and carry O in the blood to tissues, there will still disorder)
2
be significant tissue hypoxia. Diffusion Pulmonary Yes
Increasing the amount of oxygen in the alveolus impairment fibrosis
will improve diffusion into the blood, as will V/Q mismatch Pneumonia, Variable
increasing the surface area available for diffusion. pulmonary
Conversely, increasing the thickness of the tissue edema
(e.g. fibrosis) through which the oxygen must pass Right to left shunt Reverse PDA No
will worsen diffusion. For example, providing sup-
plemental oxygen means the ‘space’ in the alveolus Further information can also be found in Figs 5.4 to 5.9.
that was previously composed of mostly nitrogen PDA, patent ductus arteriosus in the heart.
Adapted from West’s Respiratory Physiology: The essentials,
and a little bit of oxygen will now be replaced with 10th edn.
all oxygen. This will greatly increase the partial
pressure difference between the alveolus and blood- recommended volumes. The dried ‘balanced’ hepa-
stream, and drive more oxygen into the blood (Fig. 5.5). rin that comes in designated blood gas syringes
Conversely, a disease such as pulmonary fibrosis contains physiologic amounts of electrolytes to
that thickens the membrane and increases the dis- compensate for heparin’s known ability to bind
tance across which oxygen must diffuse (i.e. a dif- cations, especially calcium, so that the heparin itself
fusion impairment) will cause less oxygen to pass does not distort the patient’s measured electrolyte
from the alveolus into the blood, resulting in values. If the dilution of the sample with heparin
hypoxemia (Fig. 5.6). Because increasing the oxy- exceeds 19%, then significant changes in PaO ,
2
gen gradient between the alveolus and blood will PaCO , and pH as well as dilution of electrolyte
2
improve diffusion, hypoxemia from a diffusion values may be seen (see Table 5.7). For accurate
impairment is still very oxygen responsive. measurement of ionized calcium (the variable most
Causes of hypoxemia are commonly divided into affected by heparin dilution), the final concentra-
five physiological categories. These categories, dis- tion should be <15 U heparin per mL of blood.
ease examples of each, and supplemental oxygen If dedicated blood gas syringes are not available,
responsiveness of each category, are outlined in an evacuated syringe technique using liquid heparin
Table 5.6 and Figs 5.4 to 5.9. For more detail, (1,000 U/mL), and 22 g needle on a 3 mL syringe
please see recommended supplemental texts. has been described by Hopper et al. (2005). After
drawing 0.5 mL heparin into the syringe, it is evacu-
ated completely, then 3 mL of air drawn in and
5.2 How the Monitor Works forcibly expelled three times. This technique resulted
in 0.04 mL of heparin remaining, which, when
Sample collection and handling
1 mL of blood was added to this syringe, created a
Whole blood anticoagulated with lithium heparin 4% dilution of the blood sample with heparin. This
is used for blood gas analysis. An appropriate technique produced acceptable clinical readings for
blood to heparin ratio is important as over- all variables, with the exception of low readings for
heparinization can lead to inaccurate results (see ionized calcium and minor decreases in chloride.
Table 5.7). Therefore, it is strongly recommended Venous samples can be obtained from any vessel,
to use blood gas syringes that contain lyophilized but will reflect the status of the tissue bed they are
heparin and to fill syringes to manufacturer’s draining. As such, samples obtained from a central

Venous and Arterial Blood Gas Analysis 93