acid base balance june 2010

8/9/2019 Acid Base Balance June 2010

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By John Santangelo

Henderson-Hasselbach Equation

June 2010


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Blood pH is determined by a balance between

bicarbonate and CO2


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Blood pH = 7.36 7.44(slightly Alkaline)

Enzyme


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pH (potential of Hydrogen)

The logarithm of the reciprocal of

hydrogen-ion concentration in gramatoms per liter; provides a measure on ascale from 0 to 14 of the acidity or

alkalinity of a solution (where 7 isneutral and greater than 7 is more basicand less than 7 is more acidic);


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REGULATIONOFACIDBASEBALANCE

The bodyhas the remarkable ability tomaintain plasma pH within the narrow

normal range of7.36 to 7.44. It does so

by means of chemical buffering

mechanisms, by the kidneys, and by the

lungs. The pH is defined as hydrogen ion

concentration; the more hydrogen ions,

the more acidic the solution. The pH rangethat is considered to be compatible with

life is (6.87.8)


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Formulae to describe the carbonic acid -

bicarbonate buffer system.

The two headed arrows indicate that the processis reversible

H + HCO3 H2CO3 H2O + CO2

Hydronium Ion (H+) + Bicarbonate (HCO3-) Carbonic Acid (H2CO3) Water (H2O) +

Carbon Dioxide (CO2)

The Carbon Dioxide (CO2) and Water (H2O) are

blown off by the Lungs.Hyperventilation will speed up the reaction and a

blockage in the airways will slow down the

reaction (Hypoventilation)

Carbonic Andydrase


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Using the Hendersen-Hasselbach

equation,

pH = 6.10 + Log HCO3-

PCO2 X 0.030

In order to keep the pH of blood at 7.4,

and given pKa = 6.1 for bicarbonate, the

ratio of bicarbonate to 0.03 pCO2 should

remain constant. i.e. 20 to 1


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The Chief Mammalian Blood Buffer is a Mixture of

Bicarbonate and Carbon Dioxide.

All body fluids, inside or outside cells have bufferswhich defend the body against pH changes.

The most important buffer in extracellular fluids,

including blood, is a mixture of carbon dioxide (CO2)

and bicarbonate anion (HCO3)

CO2 acts as an acid (it forms carbonic acid when it

dissolves in water), donating hydrogen ions when they

are needed.

HCO3 is a base, soaking up hydrogen ions when there

are too many of them.There are also other buffers in blood, such as proteins

and phosphate, but they are less important.


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Abnormal acid-base balance

Acid-base imbalances can be defined as acidosis or

alkalosis.

Acidosis is a state of excess H+.

Acidemia results when the blood pH is less than 7.35.

Alkalosis is a state of excess HCO3-.

Alkalemia results when the blood pH is greater than 7.45.

When the acid-base disturbance results from a primary

change in HCO3-, it is a metabolic disorder; when the

primary disturbance alters blood pCO2, it is a respiratory

disorder.Compensation for these disturbances can be respiratory or

metabolic (i.e. renal) in nature and is intended to minimize

further pH changes. The following table mayhelp clarify

th

is for you.


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Acid-baseimbalance

Plasma pH Primarydisturbance

Compensation

R

espiratoryacidosis - low - increased pCO

2 - increased renalnet acid excretion

with resulting

increase in serum

bicarbonate

Respiratory

alkalosis

- high - decreased pCO2

- decreased renal

net acid excretion

with resulting

decrease in serum

bicarbonate

Metabolic acidosis - low - decreased HCO3- - hyperventilation

with resulting low

pCO2

Metabolic alkalosis - high - increased HCO3- - hypoventilation

with resulting

increase in pCO2


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Understanding the cause of an acid-base

imbalance is the key to treating it.

The simplified approach to understanding therelationship of acid and base starts with

carbonic acid (H2CO3).

Carbon dioxide is an acid when dissolved in

water.Carbon dioxide is produced by metabolism.

As long as cells are functioning, there will be

CO2 produced.

The respiratory mechanism affects the pHwithin minutes.

Metabolic changes can take days to affect

pH.


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Acid-base balance

H+ importance and concentration in the body

Chemistry of acid, base and buffers Sources of acids in the body

Buffer mechanisms in the body

The chemical buffers


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H+ importance and concentration in

the body Hydrogen ions are very small and reactive.

Normal concentration = 40nmole/L(compare with concentration of 4 and

140mmole for K and Na). H+ concentration is therefore given within

the pH scale: pH = -log [H +]

Normal range for pH ofarterial blood is:

7.35-7.45 Extreme ranges that may be tolerable with

life are: 6.9-7.8


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Chemistry ofacid, base and buffers -I

Acid - a substance that can donate [H+] Base - a substance that can receive [H+]

Strong acid - completely dissolved in liquid.

Weakacid - partially dissolved in liquid.


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Chemistry ofacid, base and buffers -II

Buffer - oppose big changes in the pHof a liquid

A buffer is usually composed of weakacid (HA) and conjugated base (A-).

The Hendeson-Hasselebach equation:

pH=pK+log([A-]/[HA])

Buffering is most effective for pHvalues within +/- 1.5 pH units of thepK.


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Sources of acids in the body

Volatile acids - CO2,

Non-volatile (Fixed) acids,


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Sources ofacids in the body

- volatile acids

End products of oxidation of glucose

and fats in aerobic metabolism Glucose, Fat +O2 -> ATP + CO2

CO2+H2O H2CO3 H++HCO3-

H2CO3 - carbonic acid is converted toCO2 and expired by the lung - Volatile

acid.


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Sources ofacids in the body

- Non-volatile (fixed) acids

End products of metabolism of sulfur

containing amino-acid, ph

osph

olipidsor phospoproteins.

Called Fixed acids because they cant

be expired by the lungs and are

secreted by the kidney.

Amount depends on diet.


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The main fixed acids

Sulfuric acid - oxidation of sulfurcontaining acids (e.g., cysteine)

Phosphoric acid - oxidation of phospho-

lipids or phospo-proteins. HCl- Conversion of ingested ammonium

chloride to urea.

Lactic acid - Anaerobic metabolism of

glucose Acetoacetic and Butyric acid - Diabetic

ketoacidosis


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The buffer systems of the body

Chemical buffers

Lung

Kidney


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The chemical buffers

The Bicarbonate buffer

The Non-bicarbonate buffers: Hemoglobin

Plasma proteins

The p

hosp

hate buffer

H2PO4 H+ + HPO4

2-


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The importance of the bi-carbonate

buffer

pK=6.1

The total concentration of the buffer pair(CO2, HCO3

-) is quite high: 24+1.2=~

26mmol/L The Bi-carbonate buffer is part of an open

system:

The lung holds the [CO2] constant by

adjusting alveolar ventilation The kidneys replace HCO3

- that is lostduring the buffering process


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Normal ValuesNormal Values

pH = 7.35 7.45

PCO2 = 38 42 mmHg (5.07 5.60 kPa)

Actual [HCO3-] = 23 27 mmol/l

Standard [HCO3-] = 23 27 mmol/l

Buffer bases = 46 52 mEq/l

Excess Base = - 2; +2 mEq/l

Total CO2 = 24 28 mmol/l

HCO3-/H2CO3 = 18 - 22


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Causes ofAcid Base disturbances

1. RespiratoryAcidosis.

2. RespiratoryAlkalosis.

3. Metabolic Acidosis.

4. Metabolic Alkalosis.


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Causes ofAcid Base disturbances

Respiratory Acidosis:

This is synonymous with CO2 retention and is

usually a sign of hypoventilation.

Causes:1. Central Nervous System (CNS).

2. Lung & Airway disorders.

3. Chest wall abnormality.

4. Muscle disorders.

5. Neuro Muscular transmission.

6. Peripheral neuropathy.


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Inhalation ofCO2 is another cause of respiratory

acidosis but is only likely to occur under

situations of rebreathing,

e.g. under anaesthesia.

RespiratoryAcidosis is associated with raised

alveolarCO2

, raised re-breath

ingCO2

andh

igh

PCO2 in the arterial blood.

Compensation for chronic respiratory acidosis is

loss of(H+)Cl- and retention of(Na+)HCO3- by

the kidneys.


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Normal Blood pH is 7.36 to 7.42

i.e. slightly alkaline

Either mechanism (respiratory or metabolic)

can cause an acidosis or an alkalosis.

Hyperventilation will lead to respiratoryAlkalosis causing Tetany.

Tetany is a condition of prolonged and painful spasms of the voluntary

muscles, especially the fingers and toes (carpopedal spasm) as well as

the facial musculature.

An airway blockage will lead to respiratory

Acidosis (Hypoventilation).


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Respiratory Alkalosis

is associated with Hyperventilation

Causes:

1. Hysterical hyperventilation.

2. Some cases ofCNS damage.

3.

Deliberate

hyperventilation duringanaesthesia.

4. Some cases ofhypoxia.


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RespiratoryAlkalosis is usually

acute so there is no time for Renal

Compensation, but if it prolonged,there will Renal excretion of an

increased quantity ofBase

(NaHCO3)


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Metabolic AcidosisDue to increased Acids

Causes

a. Increased intake (alimentary or

parenteral).

b. Increased production ofAcid.

c. And, Failure of excretion.


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Increased IntakeTh

eA

cid content of th

e blood can be raised by:Ingestion or injection ofNH4CL or diluteHCL.

The HCL directly increases the H+.

The NH4CL produces HCL by the NH3 beingsplit off and converted to Urea.

Adding NH4CL directly to blood would not

change the pH without the Liver intervening.


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Loss of intestinal contents by diarrhoea or smallbowel obstruction causes a loss of fluid of high pH,

i.e. containing an excess of Base (NaHco3.

The removal of Base allows the H+ to rise.

Ingestion of organic acids

Organic Acids would not usually produces changesin the pH because the liver would metabolize them

but liver disease could allow organic acids, if

ingested, to gain access to the systemic circulation.

Infusion of stored blood will add acid to the bodybecause it contains citric acid.


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Excess Acid might accumulate in the blood from processes ofmetabolism and cause a fall in blood pH.

There are two main mechanisms for this:

HypoxiaHypoxia from any cause, causes anaerobic glycolysis to increase.

This gives rise to Lactic Acid and not CO2.

The Lactic Acid lowers the pH

The causes of Hypoxia are:1. Low oxygen in inspired air

2. Lung disorders


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3. Hypoventilation.

4. Low cardiac output (including shock states)

5. Blood defect: hypovolaemia, anaemia or CO

poisioning.

6. Tissue toxins, e.g. cyanide.

Circulatory occlusion to any large area will

cause accumulation of organic acids in the area

supplied. On restoration of the circulation these

acids will be distributed systemically.Th

is is apossible cause of acidosis if the general

circulation and temperature are not

maintained.


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Diabetes and Starvation

In both these states keto acids are produced and therewill be some attempt at renal excretion (Ketourea).

Failure of excretion of acid could lead to acidosis.

Normally, during metabolism some inorganic acids areproduced, i.e. H2SO4 and H3PO4. These cations haveto be excreted by the kidneys covered either by, Na+,K+, (small amount) NH4+ (produced in the kidney)

Normally the amount involved is not great, but over along period if there is failure of excretion,accumulation will occur with a fall in pH.

Th

is is Renal Acidosis


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Any of these absolute or the relative

increases in acids can cause a fall in pHand this would be a metabolic acidosis.

Th

e final compensatory mech

anism formetabolic acidosis is induced respiratory

alkalosis produced byhyperventilation.

The low pH stimulates the respiratory

centre.


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MetabolicAlkalosisThis is due to ingestion or injection of excess

base. i.e NaHCO3 or NAOH or loss of gastric juice

containing HCL.

Acommon cause of alkalosis is excess loss of

CL-(i.e. HCL or NH4CL) from excessive and

improperly observed diuretic treatment.

Metabolic alkalosis is compensated by

respiratory depression which causes CO2retention but might also cause hypoxia. The pH

is usually raised but might be high normal if

there is muchCO2 retention.


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DiagnosisAcid Base disturbances

Clinical picture

Anybody with, Pneumonia, Sweating, Bounding Pulse,probablyhas CO2 retention.

Intestinal obstruction or severe diarrhoea probablyhas

acidosis due to loss of base. (NaHCO3)

A diabetic who is drowsy and hyperventilating with

urinary glucose and ketones probablyhas keto-acidosis.

Shock, with poor tissue perfusion mayhave lactic

acidosis.


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TreatmentRespiratoryAcidosis is corrected by increased

ventilation.

RespiratoryAlkalosis is corrected by reducing

ventilation or increasing the dead space.

Metabolic Acidosis

Treat the cause

Stop alimentary loss; correct hypoxia; reduce

renal load by diet; Give insulin; treat shock.

NaHCO3 is the most commonly used.


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Metabolic Alkalosis

Remove the cause.

1. relieve pyloric obstruction or modify diuretic

regime.

2. Ingestion or injection of sufficient NaCL forthe kidney to correct the alkalosis by excretion

ofNaHCO3.

3.

Direct correction of alkalosis wit

h

NH4

CL(orHCL) infusion or ingestion. This is only

indicated if the alkalosis is very severe or renal

or cardiac function are poor.


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Blood gases are measured usingArterial Blood,NOTVenous Blood.

A glass Syringe is usually used with a smallamount of Heparin as an anticoagulant.

Care should be taken not to include Air Bubbles

in the syringe as this would alter the values.

The blood and syringe should be transported onto the laboratory, on ice, as soon as possible.

The Radial, Brachial or Femoral Arteries areusually the preferred sites. The Radial artery

being the most common as it is easier to accessand less painful.


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Usually there is associated reduction of extracellular

volume so some Sodium has to be given in the form of

NaCL solution.

4. Control of respiratory failure if this is severe.

acid base balance june 2010

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