Strong Ions!

[Geri_Gal]

Just wondering if anyone has any info or practical experience re: the "strong ion" calculations and clinical acid-base disorders. I must admit I'm a bit skeptical. This seems to be used more in surgical critical care. Can someone enlighten me? I won't give normal saline in large volume resuscitations -- just don't make me calculate the strong ion gap!

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[southerndoc]

It's called Stewartian or quantitative acid-base medicine, named after Peter Stewart from Dartmouth. We use it in our SICU. It's actually pretty useful to learn. I would recommend buying the PDF and reading it. You can get information on it at www.acidbase.org

 

[Geri_Gal]

Thank you, Southerndoc. I went to the website, but was not able to learn much without buying their material. Can you explain a bit of quantitative acid-base medicine? Can't afford much more reading material on my stipend.

 

[KGUNNER1]

Geri,
Read anything John Kellum from Pittsburgh has published in the past 10 yrs or so. He has many nice reviews on the topic. Just do a pub med or Ovid search and you'll find all kinds of stuff.

kg

 

[LukeWhite]

John Kellum is AWESOME.

If you want info on strong ion physiology, I have spent the last year collecting every article on strong ion difference I can find. I have a few dozen that I can send to anyone who's interested.

 

[jdh71]

Just wondering if anyone has any info or practical experience re: the "strong ion" calculations and clinical acid-base disorders. I must admit I'm a bit skeptical. This seems to be used more in surgical critical care. Can someone enlighten me? I won't give normal saline in large volume resuscitations -- just don't make me calculate the strong ion gap!

What's to be skeptical about? Look at Stewart's paper - the physical chemistry dictates and clearly demonstrates that the ions dictate the pH, specifically the strong anions. It explains all manner of difficult mechanistic questions, like, "why do patients get an acidosis with large amounts of normal saline". Because the Cl- dictates a maintence of charge and since the body systems exist in an aquous environment, H+ is the natural, and easiest to allow for this simple natural principle to be maintained. It also explains why giving an acidotic patient Bicarb is useless - if you look at the calculations the Bicarbonate ion, in the aqueous physiological milleu does NOT change pH. It goes against some of the pathophys dogma were given in school, but you know what they say about bad or flase ideas in medicine.

Calculating the strong ion gap is tricky, but there are many methods for estimating the gap close enough for so that it doesn't make a difference - I suggest google.

Here's the take home point, IMHO, the importance of Stewart's physical chemical basis for acid-base and it's implication for treating patients is still be looked at - in other words management of the acidotic patient doesn't really change.

As pointed out above, read anything by Kellum on the topic.

 

[LukeWhite]

Woah, but let's not get too carried away! Infusing bicarb in acidotic patients IS effective. The Stewart take-home point is that it's not because of the bicarb: we're not so much infusing bicarb as sodium cation unpaired from a strong anion. The SID in the body is 40ish...in NaCl it's 0, and in NaHCO3, it's infinite.

 

[jdh71]

Woah, but let's not get too carried away! Infusing bicarb in acidotic patients IS effective. The Stewart take-home point is that it's not because of the bicarb: we're not so much infusing bicarb as sodium cation unpaired from a strong anion. The SID in the body is 40ish...in NaCl it's 0, and in NaHCO3, it's infinite.

Right, but it's not the "bicarb" is it? Like I said, knowing this doesn't actually change management . . .

 

[LukeWhite]

It's not bicarb, but try giving bicarb without a strong cation: not really possible! Giving bicarb isn't useless. It's just not the bicarb part that's the driving force.

 

[jdh71]

It's not bicarb, but try giving bicarb without a strong cation: not really possible! Giving bicarb isn't useless. It's just not the bicarb part that's the driving force.

The bicarb is useless, in and of itself, for tretaing acidosis. The sodium is the ion the drives up the pH! The proper response should be, "try giving a strong cation without bicarb" . . . you see any other anion, but bicarb, is going to either keep the system neutral or push it more towards acuidosis. Bicarb has this cute little trick of becoming CO2 and leaving the physiologic system through the lungs, therby leaving a strong cation, while removing an anion - and it's the anions that are the problem in acidosis.

 

[BADMD]

It's not bicarb, but try giving bicarb without a strong cation: not really possible! Giving bicarb isn't useless. It's just not the bicarb part that's the driving force.

I'd go one step further. There is evidence that the bicarb my be harmful. I actually looked into getting sodium acetate as a resuscitation fluid, however in its current form it is way to expensive. I have basically switched to using LR as my crystaloid of choice just to drop the chloride content. Sure, it has some lactate in it, but that usually gets metabolized.

 

[LukeWhite]

jdh71: let's put it another way. Since you can't give bicarb without a strong cation, giving bicarb is useful even if the bicarb component itself is not. When we talk about giving bicarb, we are implying "bicarbonate + strong cation." But this is where the science devolves into semantics.

BADMD: Agreed. Bicarb is probably not the best weak anion. There's some evidence that the lactate isomer in LR harmful and that by switching the racemate some improved effect can be obtained, but maybe this is angels dancing on pins? Until it's widespread, I'm on board the LR bandwagon too, especially for high-volume resuscitations.