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  1. We gave you two exercises this week, detailed below. The take home message is the importance set by the context of the clinical information you are given along with your ABG.

EXERCISE 1.
We asked you to write down the major abnormalities you would expect to find in an ABG for the following clinical scenarios.

a. A 28 year old insulin dependent diabetic presents to the emergency department with an altered level of consciousness. His vital signs are:
  • HR          128 /min
  • BP           105/66 mmHg
  • RR           35 /min
  • sats         98% RA
  • T             37.6 degrees celcius

Hopefully you picked up that this patient is likely to have DKA. Thus the major abnormalities you would expect are:
  1. a RAGMA 
  2. a delta ratio of roughly 1
  3. a compensatory respiratory alkalosis
  4. hyperglycaemia
  5. spuriously low Na+ which will need to be corrected

b. A 65 year old lady with a history of diverticular disease presents to the emergency department with a history of abdominal pain, bloody diarrhoea, fevers and lassitude. Her vital signs are:
  • HR          128 /min
  • BP           105/66 mmHg
  • RR           35 /min
  • sats         98% RA
  • T             38.6 degrees celcius

Again, hopefully you picked up that this lady is developing intra-abdominal sepsis, most likely due to diverticulitis. The major abnormalities you would expect are:
  1. A lactic acidosis, marked by a RAGMA and a high lactate
  2. A delta ratio of approximately 1
  3. A compensatory respiratory alkalosis.

The point the exercise is trying to make is that the clinical information not only provides valuable context, but valuable clues. When you're given an ABG in the exam, don't do what a lot of candidates do and run straight to the numbers and get turned around with adding, subtracting and dividing. Take 15 seconds to think about what the clinical context is, and what abnormalities you would expect to find. Both of the questions above are based on real examination questions and if you had written down what we did above you would have got the majority of the questions correct without even looking at the numbers in the ABG. Knowing what you're likely to encounter before you embark on interpreting the ABG is a valuable technique to use in the exam.

So, don't forget  the context!
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EXERCISE 2.

Write down the major abnormalities and the unifying diagnosis for the ABG below....

FiO2     21%

pH        7.10
pO2      105  mmHg
pCO2    20  mmHg
HCO3-  8    mmol/L
BE         -14

Na+     140 mmol/L
Cl-        104 mmol/L

Again, hopefully using a stepwise method for interpreting the ABG you would have discovered the following.

1. A severe acidaemia
2. A metabolic acidosis and a respiratory alkalosis
3. An anion gap of 140 - 104 - 8 = 28
4. A delta ratio of (28-12)/(24-8) = 16/16 = 1.0
5. Expected CO2 values for HCO3 of 8 as = 8 + 1.5 x 8 = 20
5. And put all that together to denote a primary RAGMA, with a compensatory respiratory alkalosis.

BUT! If you specified a unifying diagnosis (such as sepsis, DKA, renal failure etc) you are wrong. There is absolutely no diagnostic information the ABG tells besides a simple description of the metabolic processes occurring. In fact, its impossible to even say whether the acidosis is due to ketones, lactate, uraemia (or even salicylate poisoning!). So, again, this exercise is designed to reinforce the idea that it while it's important to be proficient at calculations it's critical to remember the context you are given.

Finally, recently in the Pearl of the Week blog we dissected an ABG from the 2015.1 exam. You can check out the blog post here.