Make sure you own the abg.

For the second fellowshipexam.com microblog we thought we’d have a look at a recent question published by the college from the first new exam paper.

A 55 year old female patient is brought by ambulance to the emergency department after being involved in a high speed MVA. The pelvic x-ray from her trauma series is reproduced (in the previous question, showing an open pelvis).

Massive transfusion has been administered while waiting for the retrieval team to provide transfer to the local level 1 trauma centre. An arterial blood gas has been performed.

pH              7.20
pCO2          30
PO2            58
HCO3         14
Na+           140
K+              6.0
Cl-             100

In the context of the case we are asked two specific questions:

"The most important thing is to have a consistent, well drilled approach to questions such as these, so that the number of marks garnered (and therefore the candidate’s chance of passing) is maximized."

1) To provide 2 calculations to help interpret the gas result.

2) Using the scenario and the derived values to list three potential causes for the abnormal results.

So, questions like this can turn into a bit of a game of  “guess what the examiner is thinking”. But, as we teach in the fellowship.com course, having a well drilled and practiced approach is the key to maximizing your chances of receiving a high mark.

Using three of our six minutes allotted to the question let’s start breaking down the arterial blood gas with the first two steps of the OWN the ABG method (check out http://owntheabg.weebly.com/).

Step 1: The acid base balance.

Straight away we can make quite a few observations:

1. There is an acidaemia.
2. The bicarbonate is low, which gives us a metabolic acidosis. (We could also correct the CO2 expected for the bicarb, but that’s a story for another day!)
3. We can move on to define our metabolic acidosis by calculating an anion gap, which is 26. We therefore have a raised anion gap metabolic acidosis (RAGMA), which has a specific list of differentials, especially hyperlactataemia in the context of the shocked patient).

This is almost certainly the first variable we are expected to derive, and having a sound knowledge of the causes of a RAGMA will allow a candidate to quickly put together half of the answer for question one, and a third of the answer for question 2.

Step 2: The alveolar-arterial oxygen gradient

Moving on to the second step of the OWN the ABG method, we can calculate an Aa gradient.

With rough working, assuming an FiO2 of 50% (given in the question) at sea level, we can work out an inhaled pO2 of roughly 375mmHg, which gives us an Aa gradient of about 279.5mmHg. ABGs and detailed interpretation by the OWN the ABG method form an integral part of the fellowshipexam.com course syllabus.

We are only halfway through our blood gas, and we have found our second derived value we are expected to calculate, which again suggests a list of differentials which can be applied to the question to complete our answers.

So, based on a systematic approach to the blood gas we can answer….

 Question 1). To provide 2 calculations to help interpret the gas result.

1.     A Raised Anion Gap metabolic acidosis
2.     A grossly raised Alveolar-arterial oxygen gradient

Question 2) Using the scenario and the derived values to list three potential causes for the abnormal results. In the context of a trauma patient receiving a massive transfusion (don’t     forget the stem!):

1. RAGMA – a type A lactic acidosis, due to tissue hypoperfusion from hypoxia and shock
2. Aa gradient (V/Q mismatch) caused by either

                        a) ARDS (in this case TRALI) from the massive transfusion
                        b) Fat embolism from multiple fractures

A thorough knowledge of the Anion Gap and Aa gradient will assist candidates in formulating a strong answer like this. Arguably ii(b) – our third cause – could be substituted with any number of other causes based on the breakdown of the ABG. We’d love to hear your thoughts below.

Although there are a lot of cognitive steps in this process, there is very little writing. The most important thing is to have a consistent, well drilled approach to questions such as these, so that the number of marks garnered (and therefore the candidate’s chance of passing) is maximized.