Acid-Base – the answer is 42! Now what was the question???

If you are, like me, 10+ years out of Med School – your eyes probably glaze over when you go to a lecture on the Henderson-Hasselbach interpretation of blood gas and acid-base status.  For a long time I have been using ‘experience’ and the help of some smart colleagues to get by when it comes to tricky ABGs etc.  But thanks to this new approach – the Fencl-Stewart (quantitative approach) – I am now understanding what it is all about and can’t wait for my next tricky case to try it out for real…

If you are keen -check out the latest series on Acid-base from Scott at Emcrit, this series goes through all the science in a clear and rational way and goes over a few cases to demonstrate the technique.  At the heart of the Fencl-Stewart approach is the concept of the SIG (strong ion gap) which can be thought of as the ‘anion gap’ we all know and love with a nice set of extras – kinda like the souped up, ultra modern sports version of the old one – a lot sexier!

The Strong Ion Gap (SIG) is similar to the older concept of the “anion gap” which includes all the ‘unmeasured’ components of the acid-base balance in the body, but better.  So how do you calculate it?  It is pretty easy and gives a more accurate idea of what is going on in the tricky metabolic derangements.

Here is the basic formula, work out each component first, then it is simple math to the result:

SIG =   Base deficit (the opposite of the base excess – you can read this off the gas result)

+   Na/Cl difference ( this is easy, it is:   Na – Cl  – 38)  OR   (Na + K – Cl – 42) if you want to include K+

+ Albumin correction  = (42  – [Albumin] ) / 4

–  Lactate (read of the gas result)

This will give you a number, in a normal patient it should be < 2, close to zero.   If it is raised, you are dealing with an acidosis due to unmeasured anions / acids – such as ketoacids, uremia, alcoholic KA,

Occasionally you might get a negative SIG result – this is due to excessive amounts of +ions such as K+, Ca++, Mg++, Li+, immunoglobulins,

Anyway if you want to really know what is going on with your sick patient give this technique a try.  It seems to make sense and actually explains a few of those phenomena we see all the time but just shrug off in the handover to the smart guys in ICU!!

Casey

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