Base deficit

[Harrie]

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I need some help understanding the concept of base deficit. Anyone want to give me a mini-lesson?

 

[supercut]

I know what you mean. I struggled with it for a long time. I've never really tried to explain it, but I've become much more comfortable with it's use.

Bascially, it is an indicator of perfusion. It's kind of an indirect way of measuring lactic acid (which builds up when perfusion is decreased) Serum lactate can also be used in the same way.

Usually it's reported as base excess (BE). If the lab reports a base excess as a negative number, (say BE is -5) you then say the pt has a base deficit of 5. Normal is -2 to +2.

Here's how I think of it....a deficit of base translates into an excess of acid. Which means that the pt is in lactic acidosis due to anaerobic metabolism which means the pt isn't perfusing organs well. That means I better be recussitaitng the pt.

Usually the trend is followed, eg if you have given the pt fluid/blood and the BE becomes less negative (eg from -5 to -3...or think of it as base deficit goes from 5 to 3), you are gaining. If it stays the same or becomes more negative, you are falling behind.

Serum lactate is used in the same way, but without the confusion of whether it's a postive or negative value. A larger lactate is worse than a smaller lactate. I'm seening lactate starting to replace base deficit because now labs can process it faster. Used to be the turnaround time on serum lactate wasn't good enough to make it useful. Base deficit can be obtained from a blood gas very rapidly.

Anyway, hope this helps.

 

[Docxter]

The base deficit (or excess) is the amount of base (or acid) needed to titrate a serum pH back to normal when the contribution of respiratory factors is taken out of the equation. That is, how much acid or base would be required to correct the patient's pH to 7.4 if the pCO2 were 40?

The BD/BE is a quick indication of the metabolic component of the patient's condition. If there is a large base deficit it indicates that even if the patient's respiratory problems were resolved, there is a significant metabolic acidosis.

The BD can be used to calculate the whole body base deficit in the same way you would calculate the amount of sodium to replete in hyponatremia and is used to calculate how much bicarbonate to give the patient. Bicarbonate should be given when adequate ventilation is assured. Generally, it is used to correct the pH up to 7.1 and take it from there. The base deficit/base excess can be used in conjunction with the other data provided on a blood gas to assess the patient's acid-base status. And you don't need an ABG for base excess calculation. Did you know that you can do a blood gas on venous blood too (VBG)?

P.S. Second thoughts. Why am I answering this question?? I'm a radiology resident. I only read two ABGs a year at most.

 

[Tenesma]

base deficits/excesses are a waste of time... all you need to know is the pH and the PCO2... if the patient is acidotic and it isn't due to respiratory issues then you know there is a metabolic acidosis.... pretty simple huh? you don't need any fancy numbers/calculation...

and using serum lactate is a waste of time as well --- its ONLY proven usefulness is as a prognostic marker for severity of disease (ie: if it is high, then there is a higher mortality rate)...
there are many reasons why lactate isn't useful, ESPECIALLY when there is hepatic dysfunction (regardless of whether patient has a metabolic acidosis or not).

and using BD as a guideline for bicarbonate titration?... there are very few indications for bicarbonate administration, and in fact there are more and more metabolic acidoses that SHOULD NOT be treated with bicarbonate as they will increase morbidity (DKA being a prime example of when NOT to give bicarbonate)

 

[Docxter]

Originally posted by Tenesma
base deficits/excesses are a waste of time... all you need to know is the pH and the PCO2... if the patient is acidotic and it isn't due to respiratory issues then you know there is a metabolic acidosis.... pretty simple huh? you don't need any fancy numbers/calculation...

I disagree. You need at least a HCO3 or BE/BD in addition to the pH and PCO2 to determine the severity (not presence) of a mixed acid-base disorder. You can even have triple acid-base disorders (resp. acidosis, metabolic acidosis and metabolic alkalosis) in the same patient. And you should know the amount of expected compensation for PCO2/HCO3 changes in the various acid/base settings. The coarse estimation of expected PCO2 by looking at the decimal points in pH is very inaccurate.

Originally posted by Tenesma
and using serum lactate is a waste of time as well --- its ONLY proven usefulness is as a prognostic marker for severity of disease (ie: if it is high, then there is a higher mortality rate)...
there are many reasons why lactate isn't useful, ESPECIALLY when there is hepatic dysfunction (regardless of whether patient has a metabolic acidosis or not).

Agreed.

QUOTE]Originally posted by Tenesma

and using BD as a guideline for bicarbonate titration?... there are very few indications for bicarbonate administration, and in fact there are more and more metabolic acidoses that SHOULD NOT be treated with bicarbonate as they will increase morbidity (DKA being a prime example of when NOT to give bicarbonate) [/QUOTE]

But THERE ARE indications for bicarb administration (few as they may be) and people working in intensive care settings should know them. It's like saying since disease X is not seen very often, I don't need to know anything about it. Calculating the whole body BE/BD will help you knowing how much bicarb to give. As simple as that. Of course some surgeons use BE/BD as a shortcut to see if a metabolic acidosis/alkalosis is present because they don't want to learn the PCO2/HCO3 compensation relationships for the various acid/base settings. In triple acid-base disorders (not as rare as one might think), you can't use BE/BD for determination of the metabolic component.

 

[Tenesma]

docxter:

you need to understand how base deficit and base excess is calculated... yes, that is right...
BE = (1 - 0.014 x Hgb)[(calculatedHCO3 - 24.8) + (1.43 x Hgb + 7.7)(pH - 7.40)]

if the HGB isn't measured the lab will assume that HGB=15 (which in most patients, especially ones needing ABGs, is very inaccurate)

HCO3 is calculated based on pH and PCO2

so now you can see how a calculated value is useless, especially in the setting of the inaccuracy regarding HGB as well as it is a calculation based on a calculation (HCO3)...

so do you need BE or HCO3 to know that somebody has a metabolic acidosis... no, you can figure everything out based on pH and PCO2

the point you make about accuracy is a well taken one, ABGs vary based on temperature of patient, temperature of measurement, time between drawing and measurement in the lab...

you are also right that there are indications for bicarbonate infusions... but those aren't dosed based on base deficits... but since you are adamant about your point, please tell me how much bicarb to give if the base deficit is 5 or if the base deficit is 10???

in fact most labs around this country don't even provide excess information when they report an ABG - all they report now is ph, PCO2 and PO2 ---- nothing is better than going back to the basics and keeping it simple

 

[Docxter]

Originally posted by Tenesma

you need to understand how base deficit and base excess is calculated... yes, that is right...
BE = (1 - 0.014 x Hgb)[(calculatedHCO3 - 24.8) + (1.43 x Hgb + 7.7)(pH - 7.40)]

if the HGB isn't measured the lab will assume that HGB=15 (which in most patients, especially ones needing ABGs, is very inaccurate)

See, this is where people get all confused by the formulas. Yes, if Hgb is not measured simultaneously, BE calculations will be inaccurate. But, the key question is "how inaccurate?" Even if the patient's Hgb is 7 and the machine operator assumes Hgb 15 (a gross overestimate), the margin of error is going to be about 10-14%, which is nothing. What, the "real" base excess is going to be 3.3 instead of 3? Who cares? The margin of error in the formula within wide variations of Hgb is minimal.


so do you need BE or HCO3 to know that somebody has a metabolic acidosis... no, you can figure everything out based on pH and PCO2

See this is what happens when you respond to someone without paying attention to exactly what they said in the first place. Read my sentence again. I never said you can't determine the "presence" of a metabolic disorder by pH and PCO2 only, but I said you can't determine the"severity" of the metabolic component. Also, you still can't determine even the "presence" of triple acid-base disorders anyway.


are also right that there are indications for bicarbonate infusions... but those aren't dosed based on base deficits... but since you are adamant about your point, please tell me how much bicarb to give if the base deficit is 5 or if the base deficit is 10???

It's use is similar to the one used for sodium replenishement in hyponatremia.

The total body base deficit is:
BD x 0.5 x kg = mEq of HCO3

However, you don't correct the entire base deficit. For replenishment, you only give BD x 0.3 x kg. Of course you don't HAVE to use base deficit for calculating how much to give. There are other ways too.

A good book for anyone interested in the topic is: Rose - Clinical Physiology of Acid-Base and Electrolyte Disorders. It's long ~900 pages, but I read the old edition way back in medical school. It's amazing. It gives a lot of real-life clinical examples. I got to wean from this discussion board for a few days or weeks. Too much time spent here.

 

[Tenesma]

i give up... docxter there is a lot more to critical care than calculated numbers or calculations... if you can't measure it directly it is useless... but i commend you for reading up on base excess/deficit considering your rare exposure to ABGs!!

 

[Docxter]

[Originally posted by Tenesma [/i]
docxter there is a lot more to critical care than calculated numbers or calculations...

Thanks for the important tip.

if you can't measure it directly it is useless...

Just like calculation of PA and heart chamber pressures and transvalvular gradients on echo, and just like so many other calculations done in medicine everyday, all very useless, right? Dude, do you really believe in what you just said?

but i commend you for reading up on base excess/deficit considering your rare exposure to ABGs!!

No reading about this stuff now. It's just what I remember from the mid-nineties. I once used to know a lot about acid-base, but hey, you forget these things. At least I did try to learn them at one point time in my life. The book that I mentioned in the prior message was something to consider in addition to the little chapter in your anesthesia textbook, in case you're interested in learning more.

 

[Tenesma]

docxter... no need to get snippy with me... 🙂

first of all, i don't calculate PA numbers - I measure them... transvalvular gradients - I measure them (yes I realize they are based on formulas) and I realize they are a rough estimation... like i said, if i can't measure it directly then it is useless... For me to rely on a calculated number based on another calculated number just adds extra numbers to confuse and misinterpret... So the things that I DON"T use: SVR, SVRI, CI, PVR, ABG base excess/HCO3, and the list goes on. I understand physiology well enough that I can make decisions based on real numbers...

and like i said, don't become condescending with me (I enjoy my interaction with radiologists in the ICU - i consider them to be great intellectuals, and based on your other posts I had and still have you lumped in with them)

I read Rose's older (paperback) version - currently at Beth Israel (my sister institution) he has a reputation for being a great nephrologist (plus we have to thank him for UptoDate), and I tried to memorize everything he said because he made it all seem so simple and straightforward.... I still disagree with him, and if you read more and more about ABGs you will find that in the nephrology/ICU community there is a growing trend towards understanding strong and weak anions which is felt to be a better reflection of the acid state... i digress, i like to keep it simple... and here at MGH that is how we like it:simple...

i hope you realize this conversation is about a difference in styles and not a condemnation of a radiologists views on critical care management

 

[Astroman]

Actually Tenesma, in my opinion it was you that got snippy and especially condescending first.

But thanks to you both, I just learned a lot.

 

[buggs]

That was three or four posts before Tenesma mentioned he's at MGH. Impressive.

 

[Docxter]

Originally posted by Tenesma

... and here at MGH that is how we like it:simple...

Please don't generalize it to the whole hospital. Maybe in the SICU things are kept simple as possible so as to not overwhelm their residents, but that's not how we did it in the MICU or SDU. We used all the info available to us.

 

[Tenesma]

docxter: very cute.... that's probably why we call the MICU: the "death star" 🙂

 

[Astroman]

So let's see... What's the score?

Dox: Gives good explanation.

Tenesma: Responds using "waste of time" twice and questions bicarb admin altogether.

Dox: While still friendly, the first to use the arguably annoying quote/response method.

Tenesma: Claims Dox doesn't know how to calculate a base deficit (which is clearly not the case). Asks for actual calculations based on base deficits.

Dox: Answers the calculation challenge posed by Tenesma. Begins to get frustrated, "See, this is what happens..."

Tenesma: Makes the bold statement about indirectly measured values being "useless." Gives up.

Dox: Calls Tenesma on the indirect measurement claim. Asked for confirmation on the statement. But then by suggesting a book for reading, implies that Tenesma needs to learn more.

Tenesma: (Decides not to give up). Clearly taken aback, claims Dox to be snippy and condescending. Then the gratuitous MGH name-drop.

Dox: Makes uncalled-for jab at surgery residents.

Tenesma: Makes uncalled-for jab at medicine residents.

Who wins???

 

[NaeBlis]

Originally posted by Astroman

Dox: Makes uncalled-for jab at surgery residents.

Tenesma: Makes uncalled-for jab at medicine residents.

Who wins???

LOL. Tenesma is an anesthesia resident. So we have and med resident and an anesthesia resident trying to one-up each other on the surgury forum. See what happened Tenesma, you left a power void on the surgury forum when you vanquished womansurg.

 

[Docxter]

Sorry if it's being annoying. Won't post on this topic anymore.