Di erent Interpretation Approaches To Acid Base...

Different Interpretation Approaches To Acid BaseDisturbances

Mohamad Atef Radwan

April 27, 2011

[email protected]

In The Name Of ALLAH

Mohamad Atef Radwan Different Interpretation Approaches To Acid Base Disturbances

Introduction

”If anyone killed a person not in retaliation of

murder, or (and) to spread mischief in the land - it

would be as if he killed all mankind, and if anyone

saved a life, it would be as if he saved the life of all

mankind”

Al-Maidah-32


Objectives

Understanding basic concept of pH, and Hydrogen ion activity.

Understanding different definitions of acidosis, alkalosis.

Development of traditional approach using combination ofHenderson-Haselblach equation, the base excess and its clinicalapplication.

Development of Stewart approach, its mathematical concept and itsclinical application.

Unification of acid-base physiology.


Introduction

The body produces more acids than bases (food, protein, lipidmetabolism,.... ).

Its critical to keep hydrogen ion concentration in certain range.

Management of Acidbase disorders begins with accurate diagnosis.

Main organs responsible for controlling hydrogen ion concentration arelungs, kidney, GIT, liver.


Traditional Approach

Boston School ”Henderson-Hasselbalch”

Copenhagen School ”Base Excess”

Acid And Base

Acid: Proton donor

Base: Proton acceptor

pH: Negative logarithm of hydrogen ion concentration


Boston School

Henderson-Hasselbalch equation

pH = 6.1 + log10[HCO−3 ]

0.03× PaCO2

Described six primary states of acid-base imbalance

Chronic/Acute respiratory acidosis

Chronic/Acute respiratory alkalosis

Metabolic Acidosis

Metabolic Alkalosis


Winters Rules

Rules established interrelationships among the degree of primary reductionof the metabolic component

The clinical question the rules are designed to answer in this situation is,”whether the patients respiratory compensation is within the range tobe expected or whether there is an additional component of respiratorydisturbance, too”

Winter Equation

PCO2 = 1.54× [HCO−3 ] + 8± 2

Albert MS, Dell RB, Winters RW. Quantitative displacement of acid-base equilibrium in metabolic acidosis. Ann. Intern.

Med. 1967 Feb;66(2):312-322.


Anion Gap

The term anion gap (AG) represents theconcentration of all the unmeasuredanions in the plasma.

Na+−(Cl−+HCO3−) = UA−UC = AnionGap

Value : 10 to 12 mEq/L

Help differentiate between causes of ametabolic acidosis: high anion gap versusnormal anion gap.


Copenhagen Approach

Acid-base disorders are classified as being of respiratory origin (primary changein PaCO2) or of metabolic origin (primary change in fixed acids). Some basicquestions to be answered by any approach are:

How can the magnitude of a respiratory disorder be determined?

How can the magnitude of a metabolic disorder be determined?


Van Slyke

Henderson persuaded his friend Van Slyke to placehis equation on quantitative footing.

Van Slyke realized that the plot of log PaCO2 VsPlasma pH was Linear.

By adding known amount of acid or base andreading value of pH vs log PaCO2, A line of ”nonrespiratory pH” could be obtained which wasknown as ”base excess”.


Siggaard-Andersen Nomogram

1960,Ole Siggard Anderson 25 year old, rotating intern, helped to produce analignment nomogram relating PaCO2 and pH to base Excess

Point A : measured pH at high PaCO2

Point B : measured pH at low PaCO2

Point F : actual pH of the anaerobicallydrawn blood and allows the calculation ofthe actual PaCO2.

Point C : the BE ”Base Excess”

Point D : the Buffer Base


BE

Definition

The miliequivalents of strong acid or bases that is needed to titrate one liters(in vitro) of blood or plasma

Has been equlibrated to PaCO2 = 40 mmHg

At physiological pH of 7.4

At temperature 37 C

Full O2 saturation


BE & SBE

By using the previous informations Siggard Andersonproduced formula for calculating BE as re-expressionof data and called it ”Van Slyke equation”

BE

Base excess = 0.93×([

HCO−3]− 24.4 + 14.8× (pH − 7.4)

)SBE

SBE = 0.9287× (HCO−3 − 24.4 + 14.83× ([pH − 7.4]))


Base excess using computing methods.


Algorithm

Anticipate the acid base disturbance from the causes and history that mayhave effect.

Check pH, PaCO2, HCO3, If one of these are abnormal:Check pH

If pH less than 7.4, the primary disorder is acidosisIf pH more than or equal 7.4, The primary disorder is alkalosis

Metabolic Or RespiratoryIf HCO3− is responsible for changing the pH , the cause is MetabolicIf PaCO2 is responsible for changing pH, the cause is Respiratory


Algorithm Cont.

Check pH acceptance (pH range from 7.3 to 7.5):In respiratory disorder

If pH is accepted, it is chronic respiratory disorder

If pH is Unaccepted, it is acute respiratory disorder

In metabolic disorderAccepted pH , indicates compensated metabolic disorder

Unaccepted pH, without change in PaCO2 indicates uncompensated metabolic disorder

Unaccepted pH, with change in PaCO2 indicates partially compensated metabolic

disorder

Check appropriateness using winter rules (for diagnosis of Mixed disorder)

Further analysis for specific disorders:

In Metabolic acidosis, calculate Anion Gap

In metabolic alkalosis, check chloride in urine.


Stewart Approach

What is the role of bicarbonate inacid-base balance?

The answer is simply:

None!


Stewart Approach

Quantitative analysis of pH deviation

How much each element of acid base controllersubstances will affect pH deviation

Variables

Independent variables (PaCO2, SID, ATOT )

Dependent variables (pH, H, HCO3−,......)


Mathematical Concept Of Stewart Approach

The Simplest Acid-Base System : Pure water

[H2O]⇐⇒ H+ + OH−

[H+]x[OH−] = KW × [H2O]

As KW is highly temperature dependent and very small

K ′W = KW × [H2O]

[H+]× [OH] = K ′W

H+ − OH− = 0

H+ = OH−


Mathematical Concept Of Stewart Approach

Cont.[H+]× [H+] = K ′W

[H+] =√

K ′W

[OH−] =√

K ′W

The following definitions were introduced :

1 Solution is acid-base neutral if the hydrogen ionconcentration is equal to the square root of the K ′W .

2 A solution is acidic if [H+] >√

(K ′W )

3 A solution is basic if [H+] <√

(K ′W )


Strong Ion Difference ”SID”

Adding specified amount of NaCl to Water [H2O],so solution will only contain Na+,Cl−,H+ AndOH−

By application of electrical neutrality:

Na+ − Cl− + H+ − OH− = 0

[H]× [OH−] = K ′W

By substitution of OH− by [K ′W ]/[H]

H+ − (K ′W /H+) + Na+ − Cl− = 0

By multiplying the previous equation by H+ andrearrangement

[H+]2 + [H+]([Na+]− [Cl−])− K ′W = 0


SID Cont.

ax2 + bx + c = 0

the quadratic equation can by solved as

[H+] = −([Na+]−[Cl−])2 +

√(([Na+]− [Cl−])2/4 + K ′W )

SID

[H+] =√

(K ′W + SID2/4)− SID/2

And by application to OH−

[OH−] =√

(K ′W + SID2/4) + SID/2


SID Cont.

Reproduced from original Stewart textbook using JAVA programming language and Gnuplot


SID

Strong Ion Difference

”The sum of all strong base cation concentration minus the sum of all stronganion concentration, all expressed in equivalents per Liter.”

SID = (∑

StrongBaseCation)− (∑

StrongAcidAnions)

Sodium minus Chloride = 40-42


SID Cont.

What is really happening ???

Adding HCL to water ??Traditionaly : H+ ion was added .., so solutionbecame more acidic .Stewart : Cl− was added, SID decreased , so solutionbecame more acidic .

Why !!


Adding Weak Acid

Weak electrolytes

Substance that partially dissociate when dissolved in water, i.e the moleculesof parent substance as well as the product of dissociation will exist .

Weak acid solution contains molecular species [HA] and [A−].


ATOT

Weak acid dissociationHA⇐⇒ H+ + A−

Water dissociation

[H+]× [OH−] = K ′W

Weak acid dissociation

[H+]× [A−] = KA × [HA]

Weak acid conversion

[HA] + [A−] = [ATOT ]

For achieving electrical neutrality

[H+] + [OH−] + [SID] + [A−] = 0


ATOT

Cubic equation like

f (x) = ax3 + bx2 + cx + d

[H+]3 + KA + [SID]× [H+]2 + KA

×([SID]− [ATOT ])− K ′W × [H+]− KA × K ′W = 0


pH VS SID plus ATOT



SID plus CO2

[SID] + [H+]− [OH−]− [HCO−3 ]− [CO−23 ] = 0

by substituting and clearing, cubic equation like syntax will be produced

H3 + [SID]× H2 − (KC x PC + K ′W )× H − K3 x KC x PC = 0


SID + CO2



SID + ATOT + CO2

SID + H+ + HCO−3 − A− − CO−23 − OH− = 0

H4 + KA + SID ∗ H3 + KA × (SID)− ATOT )

- (KC ×PC + K ′W ) ∗H2−KA × (KC × PC + K ′W + K3

×KC × PC × H − KA × K3 × KC × PC = 0


Full Picture

Acid-base Balance :is set of mechanisms by which parts of the body, notablylungs, kidneys, and gastrointestinal track, control thecomposition of circulating blood plasma, so its H+ liesgenerally within range from 2× 10−7 to 1× 10−7 Eq/L orpH 7.7 to 7.0

Regulators

Lung

Kidney

GIT


Lungs

::::::

CO2 regulator

::::::


Lungs Cont.

Alveolar ventilation could be changes inseconds.

Elevation of CO2 level lead to elevation of H+

value ”Respiratory Acidosis”.

Decrease of CO2 level lead to decrease of H+

value ”Respiratory Alkalosis”.

Sustained change in CO2 level leads to changeof SID Value. Role which is played by thekidneys.


Kidney

SID=(Na++K+)-Cl−

1 Circulating plasma is perfusing the kidneys atan average rate of about 500 mL/min.

2 Every Cl− filtered but not reabsorbed meanscorresponding increase in plasma SID.

3 Every Na+ or K+ not reabsorbed means adecrease in plasma SID.


Kidney

SID=(Na++K+)-Cl−

Sustained change of PaCO2 level

1 Acute Respiratory Acidosis = PaCO2 upbriefly, so plasma H+ is up

2 Acute Respiratory Alkalosis = PaCO2 downbriefly, so plasma H+ is down

3 Chronic Respiratory Acidosis = PaCO2 up”sustained” , SID up ,H+ up slightly

4 Chronic Respiratory Alaklosis = PaCO2 down”sustained”, SID down, H+ down slightly


GIT

Cl− is removed from the plasma circulating throughthe gastric mucosa and secreting into the lumen asgastric acid.

SID in the plasma is increasing (Na+-Cl−).

that effect on total circulating plasma is small, butdetectable, the classic name for this phenomena is(Alkaline tide).

Disturbance:

transferred Cl− is lost from body and never returned to plasma.

Plasma SID is elevated.

Decrease of H+ ”Rise in pH”.


Figge and Fencl

Figge and Fencl concentrated in their work on major species of weak acid andtheir conjugate base.

Weak acids

phosphoric acid (phosphate system)

Citric acid (citrate system)

Dissociatable amino acid side chain of albumin

Fencl Model

pH = f(pH){SID,PCO2, [PiTOT ], [Albumin], [CitrateTOT ]}


Figge and Fencl. Cont.

Important values to be caculated

SID And ATOT


ATOT

Albumin & Phosphorus participation :

[Alb] = [Alb]× (0.123× pH − 0.631)

[Pi ] = [Pi ]× (0.309× pH − 0.469)

New SID : SIDe

SID = [HCO3−] + [Alb] + [Pi ]


SID

Electrical neutrality

Na++K++Ca+2+Mg+2 =HCO3−+Alb−+Pi−+Cl−+XA−

SIDa

SIDa=Na++K++Ca+2+Mg+2-Cl−

XA−

XA−=SIDa-SIDe=SIG


Classification of disturbances

FENCL V, JABOR A, KAZDA A, FIGGE J. Diagnosis of Metabolic Acid-Base Disturbances in Critically Ill Patients. Am. J. Respir. Crit. Care Med.2000 Dec 1;162(6):2246-2251.


Algorithm

Check history for detecting expected acid base deviation

The following data are required for interpretation

Na+,Cl−,K +,Albumin,PCO2

Calculate corrected chloride ”Corrected Cl” for assessment of volumestatus incorporation in acid base status

Corrected Cl− = Observed Cl− × (Normal Na+/Observed Na+)

Calculate apparent SID ”SIDa”

SIDa = (Na+ + K + + 6)− Cl−

(6) = value for presentation of Ca+2 and Mg+2 value ”provided theyare normal”


Algorithm Cont.

Calculated effective SID ”SIDe”

SIDe = HCO3− + 2.8× Albumin in g/dL + 2

(2) in SIDe equation for substitution of phosphorus [Pi] value as it notroutinely measured

HCO3− usually measured by arterial blood gas machine or it could becalculated using Hasselbalch equation

Calculate strong ion gap (SIG) or effect exerted by unknown anions”XA−”

SIG = XA− effect = SIDa − SIDe

Make comparison for calculated data to its reference range


Stewart application using computing method


Summay

It was long way for achieving accurate interpretation of acid basedisorders.

Story started since 100 year ago by Handerson-Haselblach Equation .

Later Boston school tried to classify different disorder as respiratory andmetabolic.

BE approach concentrated on metabolic element, Introduced BE andBuffer base definitions .

80s, Stewart presented his radical theory for explaining disturbances .

For rapid interpretation traditional approach may be used, for moredetailed interpretation quantitative approach could be used .


Idea From acidbase.org


Thank You!


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