Page 100 - Basic Monitoring in Canine and Feline Emergency Patients

Normal alveolus Hypoventilation Hypoventilation + 100% FiO 2
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P O = 100 P O = 70 P O = 643
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P CO = 40 P CO = 70 P CO = 70
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P N = 573 P N = 573 P N = 0
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PaO = 100 PaO = 70 PaO = 643
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Fig. 5.4. Hypoventilation as a cause of hypoxemia. In the center alveolus, the gas is not being refreshed often
enough with a new breath, resulting in a dropping P O and rising P CO as compared to the normally ventilated
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alveolus on the left. As seen on the right, administration of oxygen will displace the inert nitrogen in the alveolus,
which greatly increases the diffusion gradient for oxygen and improves movement of oxygen into the blood. However,
note that P CO is still elevated and this patient still needs ventilation support to correct this. Hypoventilation as a
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cause of hypoxemia should be oxygen responsive as long as the patient is not completely apneic.
gasses. As air is warmed and humidified by passing the idea that venous (desaturated) blood is mixing
through the airways, some partial pressure ‘space’ with the arterial (saturated) circulation without pass-
is taken up by water vapor, decreasing the partial ing an alveolus to become oxygenated. This venous
pressure of oxygen entering the alveolus to approx- blood drops the overall oxygen content on the arte-
imately 150 mmHg. Once within the alveolus, the rial side. In health, this venous admixture comes
presence of a new gas (CO ) means that the partial from the bronchial circulation and some of the car-
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pressure of alveolar oxygen (P O ) drops further to diac venous return. This difference between the pul-
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around 100–110 mmHg. (Fig. 5.4). This oxygen monary arterial blood and pulmonary capillaries is
will then dissolve down its concentration gradient the normal A–a gradient which should be < 20 mmHg
into the plasma of the pulmonary capillaries. This (see Box 5.6 and Fig. 5.4). In diseased lungs or patho-
is now the partial pressure of oxygen in arterial logical shunts, an increased pathological amount of
blood (PaO ). A common real-world analogy is to venous admixture (see Figs 5.6 to 5.9) dilutes the
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think of PaO in plasma as similar to the CO ‘fizz’ arterial oxygen content, resulting in hypoxemia.
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in soda pop. The total amount of gas in the liquid Since oxygen is not a very soluble gas, not
is the main determinant of the pressure, but other enough O can be carried as PaO alone to supply
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factors such as temperature (see Section 5.4) can the tissues’ oxygen needs. Therefore, the body has
affect solubility and hence the partial pressure of evolved hemoglobin (Hb) as a transport vehicle to
the gas. This is important to remember as it is the carry more oxygen in the bloodstream; in fact, the
pressure, not the total amount, of the gas that is majority (>98%) of the oxygen in blood is carried
reported by the analyzer. bound to Hb. Each Hb molecule can carry up to
A normal PaO in a healthy patient breathing four oxygen molecules and each red blood cell
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room air is generally between 80–100 mmHg. This is contains approximately 270 × 10 molecules of Hb.
slightly lower than the 100–110 mmHg present in The collective saturation of all the Hb in the blood-
the pulmonary capillary blood. This is because as stream is referred to as the SaO and is reported as
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fully oxygenated pulmonary capillary blood makes a percent. The relationship between PaO and SaO
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its way through the left side of the heart into systemic is depicted graphically as the oxyhemoglobin disas-
circulation, a small amount of normal venous admix- sociation curve; more information about this curve/
ture occurs. The term ‘venous admixture’ refers to relationship can be found in Chapter 4.
92 A.C. Brooks