The Rule of 15

I tweeted about this yesterday but wanted to do a whole post on it, because I think it’s a great tip! ABGs and acid-base disturbances are difficult topics to grasp. There are lots of great ways to remember how to interpret these, and we’ll cover them in time. But one of the hardest parts, especially for learners, seems to be the concept of proper compensation. How do I know if this primary problem is all that’s going on or if there is more? With anion gap metabolic acidoses, we teach students to use Winter’s Formula to calculate the expected pCO2 and thereby determine if you’re dealing with a purely metabolic problem, or if there is a concomitant respiratory derangement as well.

Of course, you can always use your handy ABG analyzer app (ABG Eval is my personal favorite), but what if you just want to know quickly? You can use the Rule of 15. Dr Jeremy Faust (@jeremyfaust on Twitter) mentioned this in a thread I was reading yesterday and it sparked my interest. I retweeted his tweet, but wanted to know more. I found a very quick video that he does along with Dr Corey Slovis on Academic Life in Emergency Medicine (ALiEM, another great resource for critical care education!). Basically, take the bicarb and add 15. That should give you roughly the expected pCO2 and if you throw a “7.” in front of it, roughly the expected pH. Watch the video here for more!

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