Question 17 from the second paper of 2007 and Question 3 from the first paper of 2006 interrogated the candidate's understanding of oxygenation variables. Specific issues under review were the advantages and limitations of the A-a gradient, PaO2/FiO2 ratio, PaO2 and SpO2. The college have never asked about any of the other indices (eg. the A/a ratio, or the respiratory index), nor mentioned the shunt equation. Local digressions on these topics include the chapter on oxygen tension - based indices of oxygenation as well as the shunt equation and content - based indices of oxygenation. There is probably too much material in those.
In order to simplify revision, the indices are presented here in a tabulated format:
An alternative tabulated answer would resemble the following:
PaO2 |
SpO2 |
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Advantages |
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Disadvantages |
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Index |
Calculation |
Advantages |
Disadvantages |
SpO2 | Derived from the difference in absoption of two infra-red light wavelengths |
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PaO2 |
Measured directly from the arterial blood by the Clark electrode in the blood gas analyser. |
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A-a gradient |
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PaO2/FiO2 ratio |
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a/A ratio |
Arterial pO2 divided by alveolar pO2. |
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Respiratory index |
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Estimated shunt fraction (Fshunt) |
(using a CaO2-CVO2difference of around 30-50ml/L) |
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Measured intrapulmonary shunt |
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