Arterial Blood Gases
The arterial blood gas. It gives us so much information, and can be the most useful of tests. But the interpretation of the ABG causes a great deal of angst, confusion and consternation to not only medical students, but junior doctors alike. There are numerous ways of teaching ABG interpretation out there: starting from basic biochemical principles; calculation of base deficits, bicarbonate deficits and “delta gaps”; but I think it’s more about pattern recognition, and I can show you the pattern.
First thing is a matter of nomenclature. Acidosis is the underlying process of acid production in the body. Acidaemia is the measurement of an acidic pH within the blood. It is possible for acidosis to be present without acidaemia – this is a fully compensated state.
Before starting to interpret an ABG we must appreciate what effect changes in CO2 and HCO3 have on blood pH:
|Parameter||High Level||Low Level|
|Parameter||High Translation||Low Translation|
|pCO2||2.5||Alkali||Not the primary change.
|HCO3||26||Normal||Not the primary change.
|HCO3||35||Alkali||Not primary change
|HCO3||34||Alkali||Not primary change
The observant out there will ask how to tell the difference between
1. Full respiratory compensation for a metabolic acidosis, and
2. Full metabolic compensation for a respiratory alkalosis
On paper, not easy. But, respiratory compensation for a metabolic acidosis is far more common than the alternative – can you think of any cause of metabolic compensation for respiratory alkalosis? Metabolic compensation takes days to weeks, so in what situation would the patient hyperventilate for such a protracted period of time? But we know that respiratory compensation nearly always occurs in the event of any metabolic acidosis.
Likewise, metabolic compensation for a respiratory acidosis (Chronic type 2 respiratory failure), is far more common that respiratory compensation for a metabolic alkalosis (apart from in the milk-alkali syndrome).
I find application of the value-translate-interpret method pretty useful, and I teach it to the 4th years in Medicine 1 every month.