A 49-year-old female, with a history of pulmonary vasculitis is found collapsed in the ward with shallow breathing and a GCS of 6.
An initial arterial blood gas on room air (FiO2 0.21) reveals:
|
Parameter |
Result |
Normal Range |
|
|
Barometric pressure |
760 mmHg (100 kPa) |
||
|
pH |
7.13* |
||
|
PCO2 |
80 mmHg (10.5 kPa)* |
35– 45 (4.6 – 6.0) |
|
|
PO2 |
38 mmHg (5.0 kPa) |
||
|
Bicarbonate |
26 mmol/L |
22 – 27 |
|
|
Base Excess |
+2 mmol/L |
-2 – +2 | |
What is the cause of the hypoxia?
Give the reason for your answer
Hypoventilation.
No reason to believe there is parenchymal disease / vasculitis as the A-a gradient is 13 mmHg. This fits with the clinical picture of coma, shallow breathing and hypercapnia
This question, though it is a blood gas interprestation question, has been placed into the category of respiratory failure SAQs because there is no complex acid-base disorder to diagnose.
However, it relies on the candidate knowing the alveolar gas equation. Here it is:
PAO2 = (0.21 x (760 - 47)) - (PaCO2 x 1.25)
Essentially, at 760mmHg barometric pressure, there should be about 150mmHg occupied by oxygen and carbon dioxide in the alveolus- given that in the alveolus the water vapour pressure is 47mmHg, and FiO2 remains 21%.
Thus, PAO2 can be expressed as 150 - (PaCO2 x 1.25)
Thus, if the PaCO2 is 80, the O2 should be around 50.
Thus, if the PaO2 is 38, one might surmise that the A-a gradient is 12.
With a normal A-a radient, the only possible explanation for the hypoxia is hypoventilation. Identifying such situations is is almost the only effective use of the A-a gradient in critical care. As with other tension-based indices of oxygenation, this value gives little information about the oxygen content of the blood.
Here is an intersting digression about the alveolar gas equation:
Cruickshank, Steven, and Nicola Hirschauer. "The alveolar gas equation."Continuing Education in Anaesthesia, Critical Care & Pain 4.1 (2004): 24-27.