Just returned from Castlefest 2013--best ultrasound conference ever!

So last podcast, I bashed on sodium bicarbonate or as John Kellum and David Story call it: chloride-free sodium. This episode I talk about all the good reasons to use NaBicarb. This is part of a series

* Part I lays out the background of the quantitative approach

* Part II puts it in mathematical terms to allow calculation of acid base status

* Part III takes you through some real world examples

* Part IV discusses the Acid-Base Effects of IV Fluids

* Part V down with the Bicarb

* Part VI is this one: ok, bicarb is not all bad

The Acid Base Series

EMCrit Podcast – Acid Base Ep. 7 – Bicarb Updates, Quantitative Approach, and Prof. David StoryPodcast 97 – Acid-Base VI – Chloride-Free SodiumPodcast 96 – Acid Base in the Critically Ill – Part V – Enough with the Bicarb AlreadyEMCrit Podcast 50 – Acid Base Part IV – Choose the Solution Based on the ProblemEMCrit Podcast 46 – Acid Base: Part IIIEMCrit Podcast 45 – Acid Base: Part IIEMCrit Podcast 44 – Acid Base: Part I

A physiology quandary

Owen, an anaesthesia registrar, wrote with this comment:

[...On increasing minute ventilation on vented patients with any bicarb given: Great idea and probably what most of us do, but even if you don't then with each breath the patient will be getting rid of more CO2 than previously so there should be more weak acid loss.]

This is one of those situations where I was gobsmacked for a second. When I started to think about this, it seemed intuitively wrong and yet conceptually right. I knew I needed to find someone far smarter than me. I reached out to Mel Herbert, who recommended David Story. Dr Story is Chair of Anaesthesia at the Melbourne Medical School and a physiology god. Here is his response:

Dr. Story, Here is the quandary. As you saw, I did that acid-base show with Dr. Kellum discussing NaBicarb use for the critically ill. Both Dr. Kellum and I believe and the evidence bares out that in a patient who can't get rid of the excess CO2, there will be negligible changes in pH from the bicarb administration.Now in an apneic patient, I think this is inarguable. However, in a mech. ventilated patient with no resp drive (let's say a pt we gave NMBs to), I perpetrated the situation would be the same. In response of my listeners brought up this question: If the minute ventilation is kept the same, but the ETCO2 rises (and by extension, the return of CO2 to the alveoli),