Acid-Base Balance Disturbances - PowerPoint Presentation

Slide1

Acid-Base Balance Disturbances

Slide2

Acids are produced continuously during normal metabolism.

(provide H+ to blood) H+ ion concentration of blood varies between narrow limits pH of the extracellular fluid = 7.35 – 7.45Constant H+ concentration within physiological limits is physiologically important to preserve the enzyme activity and metabolism

Hydrogen ion

h

omeostasis

Slide3

H

+ is generated during intracellular metabolism from several sources (~ 150 000 mmol H+ is produced every day)They are continuously neutralized by buffers resulting in no gain of H+ ions = No pH changeBuffering of acids (H+) in blood

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Disorders of Acid-Base Balance

 Increase in H+ concentrations results in a decrease in pH of blood (acidosis)Decrease in H+ concentrations results in an increase in pH of blood (alkalosis)  Alkalosis or Acidosis describes any abnormality in H+ balance whether :1- Compensated Alkalosis or AcidosisNo

blood pH

changes

(

pH

of blood is

within normal

range

).

Buffer concentrations are

abnormal

Compensatory mechanisms

try to restore pH to normal if pH is changed.

 

2-

Uncompensated

alkalosis or acidosis (

alkalaemia

or

acidaemia

)

Abnormal

pH

of blood (above or below normal range)

Buffer concentrations are

abnormal

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Relation between pH & buffer

Henderson-Hasselbach Equation [HCO3-] pH = 6.1 + log --------------------------------- pCO2 + 0.225Assessment of Acid-Base Balance

Normal pH of blood is

not

an indication of acid-base balance.

Accordingly, in order to assess acid-base

b

alance (status) of blood ,

we should assess

pH

&

buffe

r

concentration of blood

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Acid-base

1- Acidosis : - Metabolic ↓↓ HCO3- - Respiratory ↑↑ CO2 2- Alkalosis : - Metabolic ↑↑ HCO3 - Respiratory ↓↓ CO2

DIAGNOSIS IS CONFIRMED

BY LABORATORY INVESTIGATIONS OF

pH, pCO2 & pO2 & HCO3-

Sample

: Arterial Blood using

Procedure

: Blood gas analysis

Acid-Base Balance Disturbances

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1-

Metabolic Acidosis Causes: I- Increased production of H+  Common Causes of increased H+ (acids) in the blood: 1- Increased endogenous acid production. - Diabetic ketoacidosis (increased ketone bodies in blood) - Lactic acidosis (increased lactic acid in blood). 2-Ingestion of acids (or substance that produces an acid) - Poisons: as salicylate (aspirin) overdose - Methanol ingestion

- High

protein

diet.

 

3

-decreased

acid (H

+

) excretion by the

kidney: in

r

enal

f

ailure

.II- Loss of bicarbonate: e.g. in diarrhea

↓↓

HCO

3

-

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Compensatory mechanisms of metabolic acidosis1- Exhaustion of bicarbonate buffer with shift of reactions to CO2 production. Stimulation of the respiratory centre to eliminate excess CO2 formed(CO2 wash)2- Increase in renal acid excretion of H+  Metabolic Acidosis cont

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LABORTORY INVESTIGATION

: Sample: Arterial Blood Equipment: Blood Gas Analyzer pH : Low HCO3: LowPCO2 : Low: as CO2 is produced then exhaled by lungs by rapid respirationPO2: Normal

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CausesImpaired carbon dioxide excretion and thus blood pCO2 increases. caused by any pulmonary (lung) cause resulting in hypoventilation. 1-Chronic respiratory acidosis: occurs due to chronic obstructive airway diseases. Chronic bronchitis Emphysema Bronchial asthma 2-Acute respiratory acidosis: occurs due to acute respiratory failure Cardiac arrest Neuromuscular disorders of chest wall Depression of the respiratory centre in the brain by: cerebral disease or drugs2-Respiratory Acidosis

↑↑

CO2

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LABORTORY INVESTIGATION

: Sample: Arterial Blood Equipment: Blood Gas Analyzer pH: Low PCO2: High (due to the respiratory problem)HCO3: High (due to compensation) PO2 : Low (due to the respiratory problem)Compensation: by kidney via

↑

HCO3- reabsorption

↑

H+ excretion

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 The primary abnormality in metabolic alkalosis is the increased plasma bicarbonate level. (HCO3-). Causes:Less common1- Intake of a large amounts of alkali as sodium bicarbonate: (if intake is more than 1000 mmol/day)More common2- Loss of H+ (acids) from the body:   1- From the kidneys (increased excretion of acids, H+ ions): a- Mineralcorticoid (aldosterone) excess b- Severe potassium deficiency 2- From the GIT (increased loss acids, H+ ions): vomiting and gastric wash

3-Metabolic

ALkalosis

↑↑

HCO

3

-

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LABORTORY INVESTIGATION

: Sample: Arterial Blood Equipment: Blood Gas Analyzer pH: HighHCO3: HighPCO2 : High (due to compensatory respiratory depression)Compensation: by lungs via inhibition of respiration which leads to ↑ PCO2

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4- Respiratory Alkalosis

 Causes:The PCO2 is reduced due to: Hyperventilation which may be due to:1- Respiratory centre stimulation as in cases of: Anxiety Cerebral disease (infection, tumour)2- pulmonary embolism.3- Fevers4- Hepatic failure Mechanism The decrease in PCO2 leads to an increase in pH as follows:  [HCO3-] pH = 6.1 + Log ---------------------- P

CO

2

X 0.225

↓↓

CO2

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Compensation

: by kidney by ↓ HCO3 reabsorption and ↓ H+ secretionLABORTORY INVESTIGATION: Sample: Arterial Blood Equipment: Blood Gas Analyzer pH: HighHCO3: LowPCO2 : LowRespiratory Alkalosis

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RESP

Practice Problems Acid-Base Imbalancesinterpretation of Arterial Blood Gases (ABG)

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Getting an arterial blood gas sample

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Blood Gas Report

Acid-Base Information pH PCO2 HCO3 [calculated vs measured]Oxygenation Information PO2 [oxygen tension] SO2 [oxygen saturation]

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Acid-Base Disorder Primary Change Compensatory Change

Respiratory acidosis PCO2 up HCO3 upRespiratory alkalosis PCO2 down HCO3 downMetabolic acidosis HCO3 down PCO2 downMetabolic alkalosis HCO3 up PCO2 upPRIMARY AND SECONDARY ACID-BASE DERANGEMENTS [HCO3-] pH = 6.1 + log --------------------------------- PCO2 + 0.225

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Steps for interpretation of ABG

Know normal values of pH, (PCO2), and (HCO3-). Look at the patient's ABG's to determine what's abnormal high or low.Correlate the abnormal values of PCO2 and HCO3- to the abnormality of pH. Name the disorder, the cause, and the source of any compensation.

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2- Evaluate the patient's ABG's:

is the pH normal? Is it too high or too low? Is it acidosis or alkalosis? Is the HCO3- normal? Is it too high or too low? Will it cause acidosis or alkalosis? Will it correct acidosis or alkalosis? Is the CO2 normal? Is it too high or too low? Will it cause acidosis or alkalosis? Will it correct acidosis or alkalosis? 1- Normal values for ABG's:

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If only one of the two parameters (CO

2 or HCO3-) is abnormal, then its value should be consistent with the pH (for example, if the CO2 is high, since that causes a drop in pH, the pH should be low). If both of the parameter are abnormal, then usually one is CAUSING the problem, and the other is trying to CORRECT (COMPENSATE FOR) the problem. (For example, if the CO2 is high, and is causing the pH imbalance, then the pH must be low, since CO2 behaves as an acid. If HCO3- level is also abnormal, then usually it will be high, to compensate for the low pH, since it is a base.) 3-Correlate the abnormal values:

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Respiratory acidosis (with or without renal compensation) Respiratory alkalosis(with or without renal compensation) Metabolic acidosis (with or without respiratory compensation) Metabolic alkalosis (with or without respiratory compensation)5- Suggest a possible causeFor example, a cause of chronic respiratory acidosis is emphysema. 4- Name the disorder:

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Normal values for ABG's: pH range 7.35 - 7.45 PCO

2 35 - 45 mm Hg HCO3-, 22 - 26 mEq/L pH is too low - acidosis;  PCO2 is too high, would cause acidosis or correct alkalosis; HCO3- is normal, neither causing nor correcting imbalance high PCO2 is correlated with low pH, which is consistent with patient's report because PCO2 is causing the problem, this is respiratory acidosis; because bicarbonate is normal, there is no compensation Practice Problem 1ABG's:  pH 7.31   PCO2 55 mm Hg   HCO3- 28 mEq/L

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pH – low = acidosis

PCO2 – high = respiratory acidosis HCO3 - high = renal compensation Practice Problem 2 ABG's: pH 7.34 PCO2 55 mm Hg HCO3- 35 mEq/L

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pH – low = acidosis

HCO3 - low = metabolic acidosisPCO2 – normal; no compensation Practice Problem 3 ABG's: pH 7.31 PCO2 35 mm Hg HCO3- 20 mEq/L

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pH – low = acidosis

HCO3 - low = metabolic acidosis PCO2 – low = respiratory compensation Practice Problem 4 ABG's: pH 7.35 PCO2 25 mm Hg HCO3- 20 mEq/L

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pH – high = alkalosis

PCO2 – low = respiratory alkalosis HCO3 - normal; no compensation Practice Problem 5 ABG's: pH 7.48 PCO2 25 mm Hg HCO3- 24 mEq/L

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pH – high = alkalosis

PCO2 – low = respiratory alkalosis HCO3 - low = renal compensation Practice Problem 6 ABG's: pH 7.44 PCO2 25 mm Hg HCO3- 20 mEq/L

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pH – high = alkalosis

HCO3 – high = metabolic alkalosis PCO2 – normal; no compensation Practice Problem 7 ABG's: pH 7.48 PCO2 40 mm Hg HCO3- 33 mEq/L

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pH – high = alkalosis

HCO3 – high = metabolic alkalosisPCO2 – high = respiratory compensation Practice Problem 8 ABG's: pH 7.44 PCO2 55 mm Hg HCO3- 33 mEq/L