acid base balance

Dr.Osama Ali Ibraheim,MDAssociate Professor, Consultant,

Department of Anesthesia , College of MedicineKing Saud University

04/11/23 1

When you first study clinical acid-base balance, this is the natural question!

"What do I need to know?" The Bird's Eye View

Let start with PhysiologyLet start with Physiology

ACID BASE HOMEOSTASISThe chemical processeschemical processes represent the

first line of defense to an acid or base load and include the extracellular and intracellularintracellular buffersbuffers

The physiologic processesphysiologic processes modulate acid-base composition by changes in cellular metabolism and by adaptive responses in the excretionexcretion of volatile acids by the lungslungs and fixed acids by the kidneyskidneys

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ACID-BASE HOMEOSTASIS

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AcidsAcids

Acids = BasesAcids = Bases

Acids > BasesAcids > BasesAcids < BasesAcids < Bases

AcidsAcids

Buffers

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ACID-BASE BALANCE

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ACID-BASE BALANCEAcid - BaseAcid - Base balance is primarily concerned with two ions:HydrogenHydrogen (H+) BicarbonateBicarbonate (HCO3

- )

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H+ HCO3-

ACID-BASE REGULATION

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ACID-BASE REGULATIONMaintenance of an acceptable pH range in

the extracellular fluids is accomplished by threethree mechanisms:1)1) Chemical BuffersChemical Buffers

React very rapidly(less than a second)

2)2) Respiratory RegulationRespiratory RegulationReacts rapidly (seconds to minutes)

3)3) Renal RegulationRenal RegulationReacts slowly (minutes to hours)

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ACID-BASE REGULATIONChemical BuffersChemical Buffers

The body uses pH buffers in the blood to guard against sudden changes in acidity

A pH buffer works chemically to minimize changes in the pH of a solution

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Buffer

ACID-BASE REGULATION

Respiratory RegulationRespiratory RegulationCarbon dioxide is an important by-product of

metabolism and is constantly produced by cellsThe blood carries carbon dioxide to the lungs where it is

exhaled

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CO2CO2 CO2

CO2 CO2

CO2

Cell Metabolism

ACID-BASE REGULATIONRespiratory RegulationRespiratory Regulation

When breathing is increased,the blood carbon dioxide leveldecreases and the bloodbecomes more BaseBase

When breathing is decreased,the blood carbon dioxide levelincreases and the blood becomes more AcidicAcidic

By adjusting the speed and depth of breathing, the respiratory control centers and lungs are able to regulate the blood pH minute by minute

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ACID-BASE REGULATIONKidney RegulationKidney Regulation

Excess acid is excreted by the kidneys, largely in the form of ammonia

The kidneys have some ability to alter the amount of acid or base that is excreted, but this generally takes several days

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ACIDS

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ACIDSAcids can be defined as a proton (HH++) donor Hydrogen containing substances which

dissociate in solution to release HH++

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Click Here



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H+OH-

H+

OH-

H+

OH-

H+

OH-

ACIDSPhysiologically important acids include:

Carbonic acid (HCarbonic acid (H22COCO33))Phosphoric acid (HPhosphoric acid (H33POPO44))Pyruvic acid (CPyruvic acid (C33HH44OO33))Lactic acid (CLactic acid (C33HH66OO33))

These acids are dissolved in body fluids

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Lactic acid

Pyruvic acid

Phosphoric acid

BASES

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BASESBases can be defined as:

A proton (HH++) acceptorMolecules capable of accepting a hydrogen ion (OHOH--)

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H+OH-

H+

OH-

H+

OH-

H+

OH-

BASESPhysiologically important bases include:

Bicarbonate (HCOBicarbonate (HCO33- - ))

Biphosphate (HPOBiphosphate (HPO44-2 -2 ))

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Biphosphate

pH SCALE

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PH

Expresses hydrogen ion concentration in water solutions

Water ionizes to a limited extent to form equal amounts of HH++ ions and OHOH-- ions

HH22OO HH++ + OH + OH--

HH++ ion is an acid

OHOH-- ion is a base

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H+ ion is an acid

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OH- ion is a base

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H+ ion is an acid

OH- ion is a base

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Pure water is NeutralNeutral ( H+ = OH- )

pH = 7AcidAcid

( H+ > OH- ) pH < 7

BaseBase ( H+ < OH- )

pH > 7Normal blood pH is 7.35 - 7.457.35 - 7.45pH range compatible with life is 6.8 - 8.06.8 - 8.0

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OH-

OH-

OH-

OH-

OH-

OH-

H+

H+

H+

H+

OH-

OH-

OH-

OH-OH-

H+

H+

H+

H+OH-

OH-

OH-

H+

H+

H+

H+H+

H+

H+

ACIDS, BASES OR NEUTRAL???ACIDS, BASES OR NEUTRAL???

1

2

3

pH equals the logarithm (log) to the base 10 of the reciprocal of the hydrogen ion (HH++) concentration

HH++ concentration in extracellular fluid (ECF)

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pH = log 1 / HH++ concentration

4 X 10 -8 (0.00000004)

Low pH values = high HH++ concentrationsHH++ concentration in denominator of formula

Unit changes in pH represent a tenfold change in HH++ concentrationsNature of logarithms

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pH = log 1 / HH++ concentration

4 X 10 -8 (0.00000004)

pH = 4 is more acidic than pH = 6pH = 4 has 10 times more free HH++

concentration than pH = 5 and 100 times more free HH++ concentration than pH = 6

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ACIDOSIS ALKALOSISNORMAL

DEATH DEATH

Venous Blood

Arterial Blood

7.3 7.57.46.8 8.0

ACIDOSIS / ALKALOSIS

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ACIDOSIS / ALKALOSIS (Academia/Alkalemia)An abnormality in one or more of the pH

control mechanisms can cause one of two major disturbances in Acid-BaseAcid-Base balanceAcidosisAcidosisAlkalosisAlkalosis

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ACIDOSIS / ALKALOSISpH changes have dramatic effects on

normal cell function1)1) Changes in excitability of nerve and muscle cells

2)2) Influences enzyme activity3)3) Influences KK++ levels

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CHANGES IN CELL EXCITABILITYpH decrease (more acidic) depresses the

central nervous systemCan lead to loss of consciousness

pH increase (more basic) can cause over-excitabilityTingling sensations, nervousness, muscle twitches

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INFLUENCES ON ENZYME ACTIVITYpH increases or decreases can alter the

shape of the enzyme rendering it non-functional

Changes in enzyme structure can result in accelerated or depressed metabolic actions within the cell

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AlkalosisAlkalosis

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H+ OH-

Acidosis Acidosis

H+ OH-

Normal ratio of HCOHCO33-- to HH22COCO33 is 20:1

HH22COCO33 is source of HH++ ions in the body

Deviations from this ratio are used to identify Acid-Acid-BaseBase imbalances

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BASE ACID

H2CO3

H+

HCO3-

AcidosisAcidosis and AlkalosisAlkalosis can arise in two fundamentally different ways:1) Excess or deficit of CO1) Excess or deficit of CO22

((Volatile AcidVolatile Acid))Volatile Acid Volatile Acid can be eliminated by the respiratory system

2) Excess or deficit of 2) Excess or deficit of Fixed AcidFixed AcidFixed AcidsFixed Acids cannot beeliminated by therespiratory system

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ACIDOSIS / ALKALOSISNormal values of bicarbonate (arterial)pH pH = 7.4PCOPCO22 = 40 mm Hg

HCOHCO33-- = 24 meq/L

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SOURCES OF HYDROGEN IONS

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C C C C C C

H H H H H H

HHHHHH

SOURCES OF HYDROGEN IONS 1) Cell Metabolism (CO1) Cell Metabolism (CO22))2) Food Products2) Food Products3) Medications3) Medications4) Metabolic Intermediate by-products4) Metabolic Intermediate by-products5) Some Disease processes5) Some Disease processes

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SOURCES OF HYDROGEN IONS1) Cellular Metabolism1) Cellular Metabolism of carbohydrates

release COCO22 as a waste productAerobic respiration

CC66HH1212OO66 CO CO22 + H + H22O + EnergyO + Energy

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SOURCES OF HYDROGEN IONSCOCO22 diffuses into the bloodstream where the reaction: COCO2 2 + H+ H22O O HH22COCO33 H H++ + HCO + HCO33

-- This process occurs in red blood cells

HH22COCO33 (carbonic acid)Acids produced as a result of the presence of COCO22 isreferred to as aVolatile acidVolatile acid

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Dissociation of HH22COCO33 results in the production of free HH++ and HCOHCO33

--

The respiratory system removes COCO22 thus freeing HCOHCO33

-- to recombine with HH++

Accumulation or deficit of COCO22 in blood leads to respective HH++ accumulations or deficits

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CO2 H+

CO2 H+

pH

pH

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COCO22

COCO22

Red Blood CellRed Blood Cell Systemic CirculationSystemic Circulation

COCO22 HH22OO HH++ HCOHCO33--++ ++

HCOHCO33--

ClCl--

(Chloride Shift)(Chloride Shift)

COCO22 diffuses into plasma and into RBC Within RBC, the diffuses into plasma and into RBC Within RBC, the

hydration of COhydration of CO22 is catalyzed by carbonic anhydrase is catalyzed by carbonic anhydrase

Bicarbonate thus formed diffuses into plasmaBicarbonate thus formed diffuses into plasma

carboniccarbonicanhydraseanhydrase

TissuesTissues

PlasmaPlasma

48COCO22


HH22OO

HH++ HCOHCO33--


PlasmaPlasma

COCO22 COCO22COCO22 COCO22 COCO22 COCO22

COCO22

Click for Carbon Dioxide diffusion

++ ++

TissuesTissues

HH++

ClCl--

HbHbHH++ is buffered by is buffered by

HemoglobinHemoglobin

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1) CO2 DIFFUSIONHemoglobin buffers H+

Chloride shift insures electrical neutrality

HbCl-

H+

H+

H+

H+

H+

H+

H+

H+

Cl-

Cl-

Cl-

Cl-

Cl-

Cl-

Red Blood Cell

Cl-

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CARBON DIOXIDE DIFFUSIONCARBON DIOXIDE DIFFUSION

COCO22

COCO22



HCOHCO33--

ClCl--

(Chloride Shift)(Chloride Shift)

COCO22 diffuses into the plasma and into the RBC diffuses into the plasma and into the RBC

Within the RBC, the hydration of COWithin the RBC, the hydration of CO22 is catalyzed by carbonic is catalyzed by carbonic

anhydraseanhydrase

Bicarbonate thus formed diffuses into plasmaBicarbonate thus formed diffuses into plasma


TissuesTissues

PlasmaPlasma

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Red Blood CellRed Blood Cell Pulmonary CirculationPulmonary Circulation


HCOHCO33--

ClCl--

AlveolusAlveolus

PlasmaPlasma

COCO22Bicarbonate diffuses back into RBC in pulmonary capillaries and Bicarbonate diffuses back into RBC in pulmonary capillaries and reacts with hydrogen ions to form carbonic acidreacts with hydrogen ions to form carbonic acid

The acid breaks down to COThe acid breaks down to CO2 2 and waterand water

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Red Blood CellRed Blood Cell Pulmonary CirculationPulmonary Circulation

COCO22 HH22OO

HH++

++ ++ HCOHCO33--

ClCl--

AlveolusAlveolus

PlasmaPlasma

COCO22

COCO22 HH22OO

Basic Concepts The hydrogen ion concentration [H+] in extra

cellular fluid is determined by the balance between the partial pressure of carbon dioxide (PCO2)/HCO3 in the fluid. This relationship is expressed as follows :

[H+] (nEq/L) = 24 x (PCO2/HCO3)

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Using a normal arterial PCO2 of 40 mm Hg and a normal serum HCO3 concentration of 24 mEq/L, the normal [H+] in arterial blood is 24 x (40/24) = 40 nEq/L.

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pHThe body constantly tries to maintain a neutral

environment.All electrolytes maintain a positive or negative charge.

Water disassociates into H+ and OH- ions.pH (French for the power of hydrogen) is the percentage of

hydrogen ions (H) in a solution. Acids: substances that donate hydrogen ions (H) to a

solution. Ex: carbonic acid .Bases: substances that accept hydrogen ions. Ex:

bicarbonate (HCO3).

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pHA solution with more base than acid has fewer

hydrogen ions so has a higher pH. A pH than 7 makes the solution a base.

A solution that contains more acid than base has more hydrogen ions so has a lower pH. A pH 7 makes the solution an acid.

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pH is regulated by (1) chemical buffers, (2) respiratory system (3) kidneysChemical Buffers: (First system within minutes)Bicarbonate-buffer-systemPhosphate buffer-systemProtein-buffer-systemChange strong acids to weak acids (hydrochloric

acid to carbonic acid) or neutralize them.04/11/23 57

Respiratory system:Chemoreceptors in the medulla of brain sense pH changes

and vary the rate and depth of breathing to compensate for pH changes.

The lungs combine CO2 with water to form carbonic acid. carbonic acid leads to a in pH.

Increased breathing blows off CO2 pH.Decreased breathing retains CO2 pH.

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Kidneys: within daysKidneys can retain bicarbonate (HCO3) or eliminate it.Normal level in arterial blood is 22 to 26 mEq/L.

HCO3 and pH values increase or decrease together (when pH is , HCO3 is . When pH is , HCO3 is .

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pHIf acidosis or alkalosis are caused by faulty breathing,

they are respiratory acidosis or alkalosis.

If acidosis or alkalosis are caused by vomiting, diarrhea, ineffective bicarbonate buffering or kidney disorders, they are metabolic acidosis or alkalosis.

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Question...Is the average pH of the blood lower in:

a) arteriesb) veins Veins! Veins!

Why?Why?

Because veins pick up the Because veins pick up the byproducts of cellular metabolism, byproducts of cellular metabolism,

including…including…COCO22!!

Because veins pick up the Because veins pick up the byproducts of cellular metabolism, byproducts of cellular metabolism,

including…including…COCO22!!

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Acute (minutes to hours) Ventilation

Buffering

Long term

Renal excretion

Hepatic metabolism

Henderson-Hasselbalch EquationpH = pKa + log [base]/[acid]

Ex: = 6.1 + log 20/1= 6.1 + 1.3= 7.4

Key ratio is base: acidHCO3

- : CO2 (standing in for H2CO3)

ABGsAn arterial blood gas (ABG) is a sample of arterial blood

that reports:pH: 7.35 -7.45 (H ion concentration)PaCO2: 35-45 mm Hg. (dissolved CO2 in blood or

ventilatory effectiveness)HCO3: 22 to 26 mEq/L (metabolic effectiveness)PaO2: 80-100 mm Hg (O2 content of blood)SaO2 = 95% - 100% (% of hemoglobin saturated)

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Acid Base Control The initial changes in PCO2 or HCO3 is calling the

primary acid-base disorder, and the subsequent response is called the compensatory or secondary acid-base disorder.

Compensatory responses are not strong enough to keep the pH constant (they do not correct the acid-base derangement) they only to limit the change in pH that results from a primary change in PCO2 or HCO3.

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Compensation for Metabolic Acidosis: The ventilatroy response to a metabolic acisosis will

reduce the PaCO2 to a level that is defined by equation (The HCO3 in the equation is the measured bicarbonate concentration in plasma, expressed in mEq/L)Expected PaCO2 = 1.5 X HCO3 + (8 ± 2)

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For example, if a metabolic acidosis results in a serum HCO3 of 15 mEq/L the expected PaCO2 is (1.5 x 15) + (8 ± 2) = 30.5 ± 2mm Hg .

If measured CO2 equal to 30?Adequate compensation

If it is above 30? Respiratory acidosisIf it is below 30? Respiratory alkalosis

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Compensation for Metabolic Alkalosis: The compensatory response to metabolic alkalosis

have varied in different reports, but the equation shown below has proven reliable, at least up to HCO3 level of 40 mEq/L.

Expected PaCo2 = (0.7 X HCO3) + (21 ± 2)

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For example, if a metabolic alkalosis is associated with plasma HCO3 of 40 mEq/L, the expected PCO2 is (0.7 X 40) + (21 ± 2) = 49 ± 2 mm Hg.

If the measured PaCO2 is equivalent to the expected PaCO2 then the respiratory compensation is adequate

If the measured PaCO2 is higher than the expected PaCO2 (>51 mm Hg in this example), the respiratory compensation is not adequate, and there is a respiratory acidosis in addition to the metabolic alkalosis. This condition is call a primary metabolic Alkalosis with a superimposed respiratory acidosis.

If the PaCO2 is lower than expected primary metabolic alkalosis with a superimposed respiratory alkalosis.

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CompensationAcute Respiratory Δ pH = 0.008 x Δ PaCO2 Acidosis Expected pH = 7.40 –[0.008x(PaCO2 -40)]Chronic Respiratory Δ pH = 0.003 x Δ PaCO2 Acidosis Expected pH = 7.40 –[0.003x(PaCO2 -40)]Acute Respiratory Δ pH = 0.008 x Δ PaCO2 Alkalosis Expected pH = 7.40 + [0.008 x (40-PaCO2)]Chronic Respiratory Δ pH = 0.003 x Δ PaCO2 Alkalosis Expected pH = 7.40 + [0.003 x (40-PaCO2)]

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A Stepwise Approach to Acid-Base Interpretation: Stage I . identify the Primary Acid Base

Disorder:

Rule 1: An acid base abnormality is present if either the PaCO2 or the

pH is outside the normal range. (A normal pH or PaCO2 does not exclude the presence of an acid base abnormality, as explained in Rule 3).

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Rule 2: If the pH and PaCO2 are both abnormal, compare the

directional change. If both change in the same direction (both increase or decrease), the primary acid base disorder is metabolic, and if both change in opposite direction, the primary acid base disorder is respiratory.

Example: Consider a patient with an arterial pH of 7.23 and a

PaCO2 of 23 mm Hg. The pH and PaCO2 are both reduced (indicating a primary metabolic problem) and the pH is low (indicating academia), so the problem is a primary metabolic acidosis.

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Rule 3: If either the pH or PaCO2 is normal, there is mixed metabolic

and respiratory acid base disorder (one is an acidosis and the other is an alkalosis).

If the pH is normal, the direction of change in PaCO2 identifies the respiratory disorder, and if the PaCO2 is normal, the direction of change in the pH is normal identifies the metabolic disorder

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Remember that the compensatory responses to a primary acid base disturbance are never strong enough to correct the pH, but act to reduce the severity of the change in pH.

Therefore, a normal pH in the presence of an acid base disorder always signifies a mixed respiratory and metabolic acid base disorder. (It is sometimes easier to think of this situation as a condition of overcompensation for one of the acid base disorder.)

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Stage II Evaluate Compensatory Responses: If Acid base disturbance was identified in stage I, go directly to stage

III .The goal of stage II is to determine if compensatory responses are adequate and if there are additional acid base derangement.

Rule 4 if there is a primary metabolic acidosis or alkalosis, use the

measured bicarbonate concentration in the equation and identify expected PaCO2. If the measured and expected PaCO2 are equivalent, the condition is fully compensated.

If the measured PaCO2 is higher than the expected PaCO2, there is superimposed respiratory acidosis. If the measured PCO2 is less than the expected PCO2, there is a superimposed respiratory alkalosis.

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Stage III: Use the "Gaps" to evaluate Metabolic Acidosis. Anion GapIs the difference between the unmeasured anions UA and unmeasured

cations UC AG= Na -- CL – HCO3 (3-11 mEq/L)Urinary anion gap AG= urinary UA -urinary UC = U na + U k+ — U clIt is useful in patients with non anion gap acidosis to determine renal or

gastrointestinal loss of HCO3+ ve urinary AG- ----- Type I renal tubular acidosis (RTA)- ve urinary AG ------- Diarrhea

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To sort out high AG acidosis mixed with other acid base disorders ,measure the Δ Gap

If Δ AG Gap + HCO3 is normal (24) high AG metabolic acidosis)

If Δ AG Gap + HCO3 is higher than normal > (24) high AG metabolic acidosis + metabolic alkalosis

Δ AG Gap + HCO3 is lower than normal < (24) high AG metabolic acidosis + non AG metabolic acidosis

the gap-gap: can uncover mixed metabolic disorders (e.g, a metabolic acidosis and alkalosis) that would otherwise go undetected.

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Steps to Assess ABGs1. Check the pH. Is it normal (7.35-7.45), acidotic ( 7.35) or

alkalotic ( 7.45)?

2. Check the PaCO2. Is it normal (34-35 mm Hg), low (below 35 mm Hg) or high ( 45 mm Hg)?

Is it opposite from the pH? If it is, problem is respiratory origin.

Example: pH = 7.30 (low or acidotic); PaCO2 = 48 mm Hg (high)

Example: pH = 7.50 (high or alkalotic); PaCO2 = 28 mm Hg (low).

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ABGs3. Check the HCO3. Is it normal (22 to 26 mEq/L), low (below

22 mEq/L) or high (above 26 mEq/L)?Does the HCO3 correspond with pH (if pH is high, is HCO3

high, etc.). If it does correspond, the problem is metabolic origin.

Example: pH = 7.30 (low); HCO3 = 20 mm Hg (low)Example: pH = 7.50 (high): HCO3 = 30 mm Hg (high)

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ABGspH = 7. 60; PaCO2 = 28 mm Hg; HCO3 = 24 mEq/L.Respiratory alkalosis (asthma)

pH = 7.30; PaCO2 = 34 mm Hg; HCO3 = 19 mEq/L.Metabolic acidosis: diarrhea

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ABGsWhat if both PaCO2 & HCO3 are abnormal?Example: pH = 7.27 (low)PaCO2 = 27 mm Hg (low)HCO3 = 10 mEq/L (low)

One represents the primary disorder; the other represents compensation. Which is which?

The value that is moving in the right abnormal relationship is the primary problem.

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ABGsopH = 7.27 (low or acidosis)oPaCO2 = 27 mm Hg (low or alkalosis)oHCO3 = 10 mEq/L (low or acidosis)

oHCO3 corresponds with pH.oThis is metabolic acidosis with compensatory respiratory alkalosis.

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ABGso pH = 7.27o PaCO2 = 70 mm Hgo HCO3 = 45 mEq/L

o pH = 7.27 (low or acidosis)o PaCO2 = 70 mm Hg (high or acidosis)o HCO3 = 45 mEq/L (high or alkalosis)

o PaCO2 agrees with pH. Respiratory acidosis partially compensated by metabolic alkalosis.

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ABGsopH = 7.52oPaCO2 = 47 mm HgoHCO3 = 36 mEq/L

opH = 7.52 (high)oPaCO2 = 47 mm Hg (high or acidosis)oHCO3 = 36 mEq/L (high or alkalosis)oThe HCO3 is in agreement with the pH (HCO3 is when pH is ). This is metabolic alkalosis compensated by respiratory acidosis.

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Problem Consider a hypothetical situation in which thePCO2 is 80 mmHg and [HCO3] 48 mM of arterial plasma.

Which of the following statements is correct?A. There is no acid-base disturbance.B. Metabolic alkalosis is the primary acid-base

disturbance.C. Respiratory acidosis is the primary acid-base

disturbance.D. Intracellular pH is lower than normal.E. Intracellular pH is higher than normal.

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Questions?

04/11/23 86

acid base balance

Documents

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concentration h

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