Standard Base Excess and Strong Ion Theory

8/3/2019 Standard Base Excess and Strong Ion Theory

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Base Excess & Strong Ion TheoriesBase Excess & Strong Ion Theories

(the real truth about ABGs)

Steve Anisman MD

Renal Wolf Pack Jan 31, 2003


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But firstBut first

Whats up with asparagus pee?

40% of the British population (based on a 1989 paper inBritish Journal of Clinical Pharmacology in which 115 people

were studied) or 100% of the French population (103 subjects)make metabolites of asparagusic acid, a substance foundonly in asparagus. These metabolites are variants on methylmercaptan, aka methanethiol, which is a sulfur-containing

derivative of the amino acid methionine.Methyl mercaptan can also be metabolized via other

pathways, and is responsible for the characteristic odors ofgarlic, onions, rotten eggs, and skunk secretions.


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And another thingAnd another thing10% of people (300 Israelis were tested) cant smell theodor in their urine or in other peoples urine even if the odor is present.

So before you get all high & mighty andconvinced that your pee doesnt smell, get a fewpeople to accompany you to the bathroom.

And be aware that there is a widely held belief that people with asparapee tend to have higher IQsthen those bland folks who cant mount an appropriateresponse to an asparagus challenge.


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..


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The caseThe caseA 50 yo M psychiatrist is brought to the ED with recentvomiting, diarrhea, SOB/COPD exacerbation. He was believedto have been unconscious on the floor for two days after alithium overdose. He has a long history of abuse of loop and

thiazide diuretics. He has been self-medicating withbicarbonate tablets and Renagel. He has been taking 30 Oscal-D pills/day, in addition to 200mg of lisinopril in the belief thatthey would be renally protective protection he needs, given

his chronic use of Gentamicin and 5g Motrin/day.He has a 120-pack/year smoking Hx, is S/P pulmonary

lobectomy, is on 5L home O2, uses CPAP at night for his OSA,and has recently been experiencing hemoptysis. V/Q scan is

high prob.


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The caseThe caseIn order to prevent his frequent episodes of DKA, he hasbegun a regimen of glyburide and lantus, the dosages ofwhich he calculates daily after consulting a formula which

he has devised, incorporating both the phase of the moonand the color of his aura.

How would you quantify the acid and base

components in this mans blood?

He is scheduled for MWF hemodialysis inBelchertown, but has not been for 2 weeks because he

believes the nurses are putting too much acid in hisdialysate.


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Exam & LabsExam & LabsExam reveals an imaginary person. No JVD, no

extra heart or lung sounds, no edema.

There is a way


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Respiratory = PCO2Respiratory = PCO2

End of story. pCO2 is directly measured (notcalculated), and is a reliable indicator of respiratory

acid-base disturbances. The correlation betweenpCO2 and respiratory pH is direct, consistent, andlinear.

And theres an equivalent

measurement for metabolic


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and its NOT bicarb and its NOT bicarbIn pure metabolic disorders, bicarb is a usefulmeasurement, but if youll remember the equilibrium

formula: H20 + CO2 H2CO3 H

+

+ HCO3-

youll notice that HCO3 can be affected by

respiratory (CO2) or metabolic (H+) components,

and therefore isnt a specific marker for either.

In fact, the relationship between metabolic acidosis

and bicarbonate is neither consistent nor linear.


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Two approaches:Two approaches:

Strong Ion TheoryBase Excess

Lets begin, shall we?


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Strong IonsStrong Ions

You need electroneutrality, or you would glow.

If your blood was saline, Na+ would have to equal Cl-.

If you added potassium bicarbonate

And then added a bunch of other ions

Na+ = Cl-K+ + + HCO3-H+ + + OH-

Which ones of these matter, and which are clutter?


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Strong IonsStrong Ions

These are the Strong Ions, so-called because they do

not readily combine with other ions or lose their charge.

Conversely, H+ and HCO3- readily combine, and are

called weak ions.

Na+ Cl-K+

The difference between the strong cations andstrong anions is called the Strong Ion Difference (SID),

and indicates the net ionic charge of the weak anions;

so it indicates the relative strength of H+

and HCO3-

.

Mg++ Ca++ Others (lactate, etc)


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Strong Ion TheoryStrong Ion TheoryAlright, already so far its basically anion gap with a

new name. But, the Strong Ion Theory goes a step

further and adds a few other factors: pCO2, SID, andnon-volatile weak acids (buffers). Additionally, the

Gibbs-Donnan equilibrium needs to be considered

[Just for the record weve already considered pCO2and were done with it. More on SID coming up 2 slides

from now...]


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The Non-Volatile BuffersThe Non-Volatile BuffersIn blood, were dealing mainly with hemoglobin,

albumin, and phosphate. Stick with me here

A- = Ionized weak acid buffer

HA = Non-ionized weak acid buffer

ATOT = Total weak acid buffer

Dissociation for these HAs is: HA A-

+ H+

ATOT = A- + HA

so: HA * K A- * H+

so: You can calculate A- if you know pH & ATOT


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Strong Ion DifferenceStrong Ion DifferenceRemember, SID = strong cations strong anions. It

indirectly measures weak ions (HCO3 and A-).

SID = HCO3- + A-

[Just for the record, HCO3 + A- was called Buffer Base as far

back as the 1950s SID was invented by Stewart in 1980.]

You can get HCO3 by Henderson-Hasselbach if you

know pCO2 and pH (this is the calculated ABG value).

You can get A- if you know ATOT

and pH.


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Base ExcessBase ExcessDefinition: The amount of a strong acid (like HCl)

needed to bring blood to 7.40. Assumes 100% oxygenation, 37oC, and pCO2 of 40.

Normal = 0

Used to calculate the metabolic component of

an acid-base disturbance.


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Base Excess calculationsBase Excess calculationsCalculated the same way, in practice, as SID:

Buffer Base = HCO3- + A-

HCO3 calculated by pH & pCO2 (blood gas machine)

BE = BufferBase expected bufferbase

(expected if pH = 7.4 and pCO2 = 40)

A- calculated using pH & hemoglobin (whole blood)

ORA- calculated using albumin & phos (plasma)


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Membranes & Ions you guys

should feel right at home!

Membranes & Ions you guys

should feel right at home!There are flavors of Base Excess: Base

ExcessErythrocyte

; Base ExcessPlasma

; Base

ExcessECF (entire extracellular fluid); Base ExcessWhole Blood

how do we decide what to use?

The Gibbs-Donnan equilibrium describes thebehavior whenever a membrane separatesimpermeable ATOT buffer (Hgb) while allowingpassage of other ions (Cl-, HCO3-).


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Gibbs-DonnanGibbs-Donnan

It means, for our purposes, that you can predict what

will happen to plasma pH if you add acid to wholeblood.

Trust me on this part Base Excess ofExtracellular Fluid is the one that we like. Extensively

studied, reliable, good stuff. Siggaard-Andersenvalidated it extensively in actual people lots of them

during the polio epidemics of the 1950s. Really,trust me. In fact, it even has its own name


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Standard Base ExcessStandard Base Excessaka Base Excess of ECF. ECF includes plasma,red cells, and the surrounding interstitial fluid. Itswhere the action takes place in the body regardingacid-base movement.

Blood-gas machines calculate SBE as:

SBE = 0.9287 * (HCO3

- - 24.4 + (14.83 * (pH 7.4)))

And guess what it turns out that ATOT, whilefascinating, doesnt really matter clinically. A nice

advantage for SBE.


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And SBE makes a pretty nomogram.And SBE makes a pretty nomogram.

Compare & Contrast:


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And the math is easier.And the math is easier.

How do you figure out if compensation is normal?

In metabolic acidosis, IfpCO2 = SBE, then its normal.

In metabolic alkalosis, IfpCO2

= SBE * 0.6, then its

normal.


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And the math is easier.And the math is easier.

In chronic respiratory disorders, ifpCO2 * 0.4 = SBE,

then its normal.

In acute respiratory disorders, ifSBE = 0 (+/- 5), then its

normal.


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How about an improvedAnionGap?How about an improvedAnionGap?

If you apply the same logic that weve already used,

you can guess that there might be fancier ways to

calculate anion gap than what we now do. This methodshould give you a gap of 12 (not 12-19):

AG = pH * ((1.16 * alb) + (0.42 * phos)) (5.83 * alb) (1.28 * phos)

Turns out this doesnt work so well, but its a version of

a new trend called Strong Ion Gap. A nice version of

this involves the


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Fencl-Stewart corrections!!Fencl-Stewart corrections!!Theres an interesting paper in Critical Care

Medicine by Balasubramanyan [1999;27(8):1577-

81] showing that this version of the Strong Ion Gapworks. Fencl & Stewart basically said that BE had

some virtues, but that it missed the beautiful things

of the strong ion theory and that you could applythe strong ion concepts to Base Excess and pick

disorders you might have missed otherwise.


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Fencl-Stewart corrections!!Fencl-Stewart corrections!!BE caused by free water (BEfw) = 0.3 * (Na 140)

BE caused by changes in Cl- (BEcl) = 102 (Cl * 140/Na)

BE caused by changes in albumin (BEalb) = 3.4 * (4.5 - albumin)

BEnet = BEfw + BEcl + BEalb + BEUA [ua = unmeasured anions]

BEUA = BEnet (BEfw + bEcl + BEalb)

If there were no abnormalities in sodium,choride, albumin, or unmeasured anions, then BE

would be equivalent to BEUA.


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Fencl-Stewart corrections!!Fencl-Stewart corrections!!In a PICU, checking for BEUA picked up about 25%

more abnormalities than BE alone, and about 15%

more abnormalities than BE with normal gap (!!!!).And BEUA was the strongest predictor of mortality

stronger than gap, stronger than BE, stronger than

lactate.Even stronger than BEUA was the act of

checking a lactate, but thats a different

conversation


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ControversiesControversies

Says Siggaard-Andersen:

this reveals that the Stewart approach isabsurd and anachronistic

this interpretationis contrary to all

previous rational thinking...

The use of SBE was accepted decades ago by

Europe (who published in Lancet), but shunned by

the US (particularly Boston, NEJM), leading to the

Great Trans-Atlantic Debate. This disagreement hassince been replaced by derision for Strong Ions


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And a few words aboutHasselbachAnd a few words aboutHasselbach

Henderson: [H+] * [HCO3-] = K * [CO2] * [H2O] (1908)

Henderson was the real McCoy. Hasselbach was an

miscreant, a rabble-rouser, and neer-do-well.

No muss, no fuss, no bother, no inverse relationships,

nothing that a high-school student couldnt understand.

In fact, if you remove H2O, which doesnt vary, and

change [CO2] to pCO2, you get:

Modified Henderson: [H+] * [HCO3-] = K * p C O2

Hasselbach: pH = pK + log ([HCO3-] / [CO2]) (1916)


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And a few words about tight controlAnd a few words about tight control

Youll read everywhere that pH is tightly controlled

by the body in a narrow range. What a load of

malarkey.

Logarithms introduce a false sense of tightclustering. When the pH changes by 0.3 units, (say from

7.5 to 7.2), this represents a doublingof the hydrogen

ion concentration (from 40 nMol/L to 80). Even normal

variation between 7.35 and 7.45 represents 25%

variation in [H+].That is nottight.

Sorry, I just had to get that off my chest.


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..


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Example #1Example #1

7.13 / 19 / 109 / -20 (SBE)

21 yo M with cardiomyopathy, admitted with acute

abdomen. Normal lytes, normal gap. Next day,

cyanotic & hypotensive.

Acidosis.

Base Deficit = 20 (Base Excess = - 20), so metabolic.

BE = 20, pCO2 = 21. No respiratory component.


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First Intubation7.19 / 70 / 249 / 0

Second Intubation7.10 / 85 / 50 / 0

67 yo F with COPD admitted for dyspnea, intubated,

soon extubated. Eats lunch, becomes lethargic,

reintubated.

Acidosis.

Base Deficit = 0, so purely respiratory.


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7.49 / 44 / 90 / 6 (SBE)

Na=1

48 Cl=9

8 Alb=2.1

51 yo M nephrologist. Thats all we know.

Free water effect = 0.3 * (Na 140) = -2.4

Explained ion effect = -2.4 + 9.3 + 8.16 = 15.06

Cl effect = 102 (Cl * 140/Na) = 9.3

Albumin effect = 3.4 * (4.5 alb) = 8.16

BEua (Unexplainedions) = 6 15.06 = -9


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7.49 / 44 / 90 / 6 (SBE)

Na=1

48 Cl=9

8 Alb=2.1

51 yo M nephrologist. Thats all we know.

BEua = -9

Unexplained ions are anions

metabolic acidosis is also present

need to check lactate, -OHb

Alkalosis.

SBE = pCO2 * 0.6, so pure metabolic.


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Thank you!Thank you!

At this time, Im happyAt this time, Im happy

to refer any questions toto refer any questions toDr. OSheaDr. OShea

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Standard Base Excess and Strong Ion Theory