Chapter 18
Troubleshooting Hypoxemia
Blue is bad!
Rebecca A. Johnson
Department of Surgical Sciences, School of Veterinary Medicine, University of Wisconsin, USA
- Q. How is adequacy of oxygenation determined and quantified?
- A. Oxygenation can be determined in three ways, depending on the specific information required: (i) the partial pressure of oxygen dissolved in arterial plasma (PaO2; mmHg), (ii) the percent of arterial hemoglobin (Hb) saturated (bound) with oxygen (SaO2; %), and (iii) the arterial blood oxygen content (CaO2), calculated by:
Although CaO2 depends on both Hb and PaO2, Hb concentration is quantitatively the most important contributor to oxygen content. It follows that overall tissue oxygen delivery is therefore the product of oxygen content and cardiac output [1].
- Q. How do anemia and hypoxemia affect O2 content?
- A. Table 18.1 can be used as an example to determine the relative effects of Hb, PO2, and SO2.
Table 18.1 Examples of the effects of anemia and hypoxemia on total oxygen content.
FIO2
PaO2 (mm Hg)
SaO2 (%)
Hb (g/dl)
CaO2 (ml/dl)
Normal
0.21
100
98
15
19.9
Anemia
0.21
100
98
5
6.9
Hypoxemia
0.21
50
85
15
17.3
Hyperoxemia
1.0
500
99
15
21.6
Anemia + Hyperoxemia
1.0
500
99
5
8.2
Anemia + Hypoxemia
0.21
50
85
5
5.8
Anemia (i.e., low Hb levels) has a significantly greater effect on CaO2 and O2 delivery than does hypoxemia, manifest as low PO2 levels. O2 therapy (inspired fraction of O2 [FIO2 = 1.0]) does little to improve CaO2 in the anemic patient. The combination of anemia and hypoxemia results in the lowest levels of CaO2 and should be treated aggressively in the clinical patient [2,3].
- Q. How is hypoxemia detected in anesthetized animals?
- A. Hypoxemia, defined as an abnormally low level of O2 in the blood, is typically detected by a combination of the patient’s clinical signs as well as the use of specific monitoring equipment. Physiologic abnormalities of mild hypoxemia include tachycardia and hypertension due to reflex sympathetic stimulation; however, these reflexes may be blunted in patients under general anesthesia. Patients with severe hypoxemia (<5 g/dl deoxygenated Hb) will be cyanotic and may have bradycardia (or continued tachycardia), cardiac depression, and circulatory collapse. However, to definitively diagnose hypoxemia, arterial blood gas analysis should be performed. Although pulse oximetry easily and non-invasively measures the percent saturation of hemoglobin with oxygen (SpO2), oximetry only provides an estimate of PaO2, it assumes the oxyhemoglobin curve is not shifted, and it assumes abnormal forms of hemoglobin (i.e., carboxyhemoglobin) are not present. Thus, although clinical signs and SpO2 may aid in the detection of hypoxemia, PaO2 values are essential in determining the presence and degree of clinical hypoxemia [4].
- Q. What are the major mechanisms underlying hypoxemia?
- A. There are five main contributors to patient hypoxemia: (i) hypoventilation, (ii) ventilation-perfusion inequalities, (iii) right-to-left pulmonary or cardiac shunting of blood, (iv) impairments to gas diffusion, and (v) decreased inspired oxygen levels [5].
Hypoventilation is defined as an elevation of arterial carbon dioxide levels (PaCO2; see Chapter 19) relative to CO2 production. When alveolar CO2 levels rise (PACO2), the alveolar (and arterial) PO2 must be reduced based on the alveolar gas equation, especially when breathing room air [6]:
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