REVISITING THE ABG.

We really enjoyed our second face to face in Melbourne a week or so ago.

A big focus was on understanding the exam, and unlocking the questions. More on this in the next blog. We also took the chance to revisit the ABG, and were struck again (as we always are) by the importance of context.

Take the following ABG for example....

"a different list of differentials would need to be presented, even thought the acid-base findings might be the same"

A question that seems to come up regularly is: in an ABG like the one below how do we determine what the primary Acid-Base disorder is?

pH                7.1
HCO3             9  mmol/L
PCO2            58  mmHg

So, looking at the ABG it's obvious that there is both a metabolic acidosis and a respiratory acidosis.

How do we know which is the primary disturbance?

You can answer two ways:
1. We can't!
2. They are both primary, or independent, processes as each disturbance moves in the direction of the pH derangement.

So, there is no compensation, and candidates addressing the question will need to acknowledge that 2 processes are occurring, and specify a list of differentials for each.

How do we know what's actually going on?

Don't forget to take in the clinical scenario given with the ABG, which gives it context.

The ABG might above might feature in a question something like the following:

"A 45 year old haematology patient presents with a fever and a productive cough."

The metabolic acidosis (assuming it's a RAGMA) probably relates to sepsis or renal failure. The respiratory acidosis is probably due to pneumonia induced respiratory failure. If a different stem was given (for example a head injured trauma patient) a different list of differentials would need to be presented, even thought the acid-base findings might be the same.

Obviously that is a fairly brief discussion of the point. There are a large number of clinical situations where the ABG above could feature.

Just remember:

1. A metabolic and respiratory disturbance in the same direction imply that two independent processes are occurring. In this context the notion of primacy is irrelevant.

2. Make sure that when this happens you relate a differential for both back to the actual patient you have been given.

Above all, when you are interpreting answers for the exam, make sure you answer the case that is given, rather than a generic list of differentials. ED consultants are specific.