ABG Interpretation: Alternative Approaches to Acid Base Analysis (Lesson 15)

A lecture on the base excess and Stewart approaches to acid-base balance, including qualitative and quantitative comparisons to the traditional approach.

[@crit-ic]

This video is an absolute perfect overview of the Stewart approach! Best one out there in my opinion! Thank you so much for creating this!

[@MikeBirkhead]

Recently (yesterday), I came across this idea of strong ion difference. It seems that the underlying argument is that pH is the result of the contribution of many acids, bases, and buffers. However, knowing all of the contributors is only important when trying to predict the future pH given a specific intervention (prevent overcorrection, for example). At equilibrium, however, the isohydric principle allows calculation of pH given the knowledge of the concentration of any weak acid, conjugate base, and equilibrium constant (i. e. Henderson-Hasselbach). Furthermore, I read a paper that mathematically proved that strong ion difference had no relavence on pH rather than simplifying the calculations used for prediction (otherwise a fourth order polynomial seems to be required). I had little understanding of how that discussion or criticism fit into the history or acid/base disorder, so this video proved to be timely! Thanks!

[@DearPronta]

Thank you for this lecture! Clear, comprehensive and extremely useful!

[@carltonmiller6701]

I must confess. Never thought I'd find humor in an abg video. Ur analogy about billiard balls and also the summation of the numerator in the Stewart equation approximating bicarbonate concentration had me dying. Refuted that guys life's work in a few seconds lol

[@lk09tst]

Hi Eric. I think Stewards method gives us a theurapethic understanding why for example normal saline makes a patient acidotic, the other methods does not. There are other simplified equations of the steward approch which have been evaluated in clinical practice. For example the steward approach can help the clinican to determine if a for example BE of -10 is just caused by a lowered SID and/or high lactate or if other acids are also present. For me personally and for others in my class the steward method has added an extra understanding for acid-base analysis. Emcrit.org presents an bedside adapted algoritm for the steward approach which is very usefull. Yours M from Sweden

[@StrongMed]

ELF: Good question. Obviously, I didn't describe this well. A better way for me to have phrased this would have been "In the absence of pathologic unmeasured anions, Atot should equal the traditional anion gap". For example, phosphate and albumin are traditionally referred to as "unmeasured anions" (even though we can obviously measure them), but are non-pathologic. It's only pathologic anions like betahydroxybutyrate and lactate that make Atot not equal the AG. Hope that clarifies!

[@plpoirot]

That was a very comprehensive lecture. Thank you

[@johnolive9477]

Great video. Thank you so much!

[@gihansaleh3721]

Many thanks for your clear illustration

[@sunving]

Thank you Dr Eric Strong ! I never heard of those Methods at all until today. Let’s alone I barely get with traditional approach. Anyhow this is a great lecture and comprehensive. I thank you that put much effort into this.

[@HAIDERSAEED90]

Thanks dr. Eric

[@drandrewtan]

Best video on Stewart approach!

[@StrongMed]

Thank you!

[@Henipah]

Great lecture

[@captc502]

I got excited when I saw a method other than base excess and the traditional approach, and then got sad to see it taken away :( How dare you play with my emotions? --> but I agree with Dr. Stock - this approach would have been very satisfying as a medical student when I was wondering how the hell chemistry actually plays into medicine and why it just always seemed like ions like sodium and potassium seemed to have some suspicious, elusive effect on seemingly unrelated pH.

[@sagacup709]

Thank you

[@ExtremelyLoudFart]

Thank you :)

[@noeliadiaz9135]

love it

[@dylannorton578]

I agree with the previous comment - the Stewart approach gives us a better way of understanding what is happening physiologically, especially when we give fluids. For a good discussion of how a simplified Stewart approach can be used clinically, I suggest watching Scott Weingart's lecture series on his method on the Emcrit website. emcrit.org/podcasts/acid-base-i/ Although I don't think there is any evidence that using the Stewart approach results in better clinical outcomes, Weingart makes a strong argument that it can be used to quickly and accurately identify and understand the most important acid-base disorders we see in clinical practice.

[@StrongMed]

Sir, I absolutely appreciate your opinion about this. However, I'm just unconvinced that the Stewart approach is a superior description of acid-base physiology. On the other hand, I am convinced that the Stewart approach is more complicated and less intuitive. As such, it should only be adopted if there is some clinical advantage over the approach which most physicians currently use, and as you indicate, there isn't any evidence of this. The example of why the Stewart approach might be a superior description of physiology which all Stewart advocates always cite is why patients get a metabolic acidosis with saline. For a discussion, which is certainly not conclusive, of some empiric evidence supporting the opinion that this is due to simple dilution of HCO3-, see: PMID: 11046200. In general, I am a strong proponent of understanding normal physiology before trying to tackle pathophysiology. However, even if Stewart proves to be an accurate description of acid-base physiology, I still would not advocate its use in everyday clinical practice, because it's just too convoluted. I would compare it to using matrices to solve a simple 2x2 system of linear equations, or using Lagrangian mechanics to predict how far a thrown baseball will travel. Both of those may be 100% accurate methods to solve problems, and a mathematician and physicist respectively may find those methods more elegant than the traditional methods of basic algebra. However, the substantially increased complexity is not justified as the more common, basic approaches provide the right answer. And in the case of acid-base derangements, the "traditional" approach not only gets the "right answer", it also provide insight into the physiologic mechanisms in the overwhelming majority of situations. (the normal gap acidosis from saline infusion being the main, possible exception to this.) I suspect if I was doing acid-base research, or trying to develop a new formulation for crystalloid replacement fluid, I might favor Stewart. But at the bedside, it's just too cumbersome. I do not currently have the time to watch/listen to the lectures on the Emcrit website to which you refer, but will do so when I do have the time; if his explanation of how to employ Stewart at the bedside changes my opinion about this, I'll post an update here.

[@dylannorton578]

Thanks for your comments, Eric. I think you make some great points. I would definitely encourage you to check out Weingart's lectures when you have time. He makes a strong argument that the Stewart approach helps his practice in critical care. He uses a greatly simplified method which he believes to be accurate. Fundamentally, it really doesn't matter which approach you choose, as long as you get the information you need to provide the best patient care. Thanks for all these great lectures! They are a tremendous resource.

[@ExtremelyLoudFart]

What I know is that measured cations minus measured anions=unmeasured anions=AG. So what do you mean by 'in the absence of unmeasured anions, Atot should equal the traditional anion gap'?

[@velmurugans1269]

Is there any simple formula to calculate A total