UCONN Emergency Medicine Interest Group

Posts Tagged ‘medicine’

Off Service #3: Critical Care

In OFF SERVICE on February 4, 2012 at 5:19 PM

I’m in the ICU this month, where acid-base disorders are more the rule than the exception. One issue that frequently comes up is metabolic acidoses and the specter of mixed acid-base disorders looming its ugly head. I feel this topic is frequently made overly complex, and thus ends up being ignored, when figuring it out is really not that complicated. I think this is because of the delta gap, or in Dr. Henry’s words, the “delta-delta”:

Delta-delta = (measured anion gap from labs – normal anion gap)/(normal HCO3 – measured HCO3 from labs) = (measured anion gap from labs – 12)/(24 – measured HCO3 from labs)

I think this equation gets easily confused because of the fact that the calculation involves subtraction in the denominator. Since I am not particularly facile at math at the drop of a hat (or at least, not with an ICU team as my audience), I will outline the very simple way that I have reasoned how to determine if a mixed acid-base disorder is present, in a way that should be easy to remember:

In the setting of an INCREASED anion gap metabolic acidosis (AKA an anion gap metabolic acidosis):

If there is an additional METABOLIC ALKALOSIS…

HCO3 is HIGHER than it should be, therefore…
The denominator is SMALLER, therefore…
The delta-delta is BIGGER, and…
If the delta-delta > 2, there is an increased anion gap metabolic acidosis AND a metabolic alkalosis.

If there is an additional NON-ANION GAP METABOLIC ACIDOSIS…

HCO3 is LOWER than it should be, therefore…
The denominator is BIGGER, therefore…
The delta-delta is SMALLER, and…
If the delta-delta < 1, there is an increased anion gap metabolic acidosis AND a non-anion gap metabolic acidosis.

For a more comprehensive discussion, see this selective project created by a UCONN student here.

Why should you care? Well, particularly if you are resuscitating with large volumes of normal saline (which contains a lot of chloride), the pH may not give a full indication of what is going on with the patient, and you may make changes in your management accordingly. For instance, you might want to switch to Lactated Ringers as your IV fluid.

Click below to see a BONUS section on patients with hypoalbuminemia, and how not to miss metabolic disturbances in this population!

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