ABG and ECG week!

​This week at fellowshipexam.com we are taking on two of the most feared props in the exam itself – the ECG and the ABG. In particular, the ABG seems to be a source of significant concern for most candidates. The bad news is that even with the advent of the new examination the ABG plays a starring role. In the 2015.1 exam 2 of the thirty questions features were centred on an ABG. That’s just about 7 % of the marks available for a single prop. 

"The good news is that with a systematic approach the ABG can be broken down easily..." 

​The good news is that with a systematic approach the ABG can be broken down easily and those marks collected. Let’s look at question 23, which features a HARD blood gas….

After being presented with this monstrosity we are asked four questions.

1. List three abnormalities and explain their significance (6 marks)
2. Calculate 2 values and explain how they contribute to your diagnosis (2 marks)
3. List four differential diagnoses for this patient’s condition (4 marks)
4. List three changes on the ECG that could be expected for this patient based on his blood results.

 
All in six minutes! We reiterate our opinion that this question is hard!
 
Right, so a systematic approach to the ABG is necessary. We teach the Own the ABG approach.
 
Looking at the Acid-Base balance we can note immediately:

• A normal pH
• A low bicarbonate and base excess– signifying a metabolic acidosis
• A low CO2 value – signifying a respiratory alkalosis
• With the normal pH it is difficult to say which of these is the primary disturbance
• The ANION GAP calculates as 143 – 20 (19.9) – 113 = 10 (normal)
• Incidentally while calculating the anion gap we note a critically low potassium (likely the cause of weakness and a serious risk for VF)
• Given this is a venous sample we can’t calculate an AA gradient
• Without a urea we can’t calculate either a U:C ratio

 
So, working through the ABG systematically we can answer the questions pretty easily. 

​Our answers and comments are below.

1. List three abnormalities and explain their significance (6 marks​
   1. Life threatening hypokalaemia (must be in there for context)
      1. Risk of VF and cause of weakness hypokalaemic paralysis
   2. Mildly low bicarbonate
      1. Signifies a metabolic acidosis
   3. Mildly low CO2
      1. Signifies resp acidosis
2. Calculate 2 values and explain how they contribute to your diagnosis (2 marks)
   1. ANION GAP (as above) = 10
      1. Therefore this a non anion gap acidosis
   2. At this stage we should comment that it’s unclear to us what the second item we should calculate is...
      1. Without a pH disturbance and therefore identification of a primary disorder, calculating compensation is pointless
      2. We can’t do an AA gradient or a urea:creatinine ratio
   3. Therefore we would calculate a delta ratio and hope that this is what the examiners were looking for (being prepared to write off the marks if it wasn’t)
      1. Change in AG (-2)/Change in bicarbonate (5) = -0.4
      2. Comment: this confirms our NAGMA
3. List four differential diagnoses for this patient’s condition (4 marks)
   1. This is slightly harder given the age and lack of any medical history, but our list would be:
      1. Abuse of carbonic anhydrase inhibitors
      2. Excessive diarrhoea
      3. Renal tubular acidosis (undiagnosed)
      4. Undiagnosed hyperparathyroidism (but you could substitute any variety of other NAGMA cause here hoping for the last mark).
4. List three changes on the ECG that could be expected for this patient based on his blood results.
   1. We haven't this one on our blog. If you can't list the ECG features of hypokalaemia you shouldn't be taking the exam!


Although some of the questions in this examination stunner are a bit opaque for us, we remain convinced that a systematic approach to props like the ABG puts candidates in the best position to succeed. We've set our course candidates a number of ABGs to practice, and we're planning on discussing the issue in more depth at our FACE-TO-FACE meeting in Melbourne. As always we'll be preaching that a systematic approach and practice are what yield the best chance for success on the big day.