Acid-Base Balance Disturbances

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 – 7.45

Constant H+ concentration within physiological limits is physiologically important to preserve the enzyme activity and metabolism

Hydrogen ion homeostasis

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 change

Buffering of acids (H+) in blood

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 Acidosis

No 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

Relation between pH & buffer

Henderson-Hasselbach Equation

[HCO3-]

pH = 6.1 + log --------------------------------- pCO2 + 0.225

Assessment of Acid-Base Balance

Normal pH of blood is not an indication of acid-base balance.

Accordingly, in order to assess acid-base balance (status) of blood , we should assess pH & buffer concentration of blood

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

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 renal failure.

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

↓↓ HCO3-

Compensatory mechanisms of metabolic acidosis

1- 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

LABORTORY INVESTIGATION: Sample: Arterial Blood Equipment: Blood Gas Analyzer pH : Low

HCO3: Low

PCO2 : Low: as CO2 is produced then exhaled by lungs by rapid respiration

PO2: Normal

  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 drugs

2-Respiratory Acidosis ↑↑ CO2

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

  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 ↑↑ HCO3-

LABORTORY INVESTIGATION: Sample: Arterial Blood Equipment: Blood Gas Analyzer pH: High

HCO3: High

PCO2 : High (due to compensatory respiratory depression)

Compensation: by lungs via inhibition of respiration which leads to ↑ PCO2

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 ---------------------- PCO2 X 0.225

↓↓ CO2

Compensation : by kidney by ↓ HCO3 reabsorption and ↓ H+ secretion

LABORTORY INVESTIGATION: Sample: Arterial Blood Equipment: Blood Gas Analyzer

pH: High

HCO3: Low

PCO2 : Low

Respiratory Alkalosis

RESP

Practice Problems Acid-Base Imbalances

interpretation of Arterial Blood Gases (ABG)

Getting an arterial blood gas sample

Blood Gas Report

•Acid-Base Information• pH• PCO2

• HCO3 [calculated vs measured]

•Oxygenation Information• PO2 [oxygen tension]

• SO2 [oxygen saturation]

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 up

PRIMARY AND SECONDARY ACID-BASE DERANGEMENTS

[HCO3-]

pH = 6.1 + log --------------------------------- PCO2 + 0.225

Steps for interpretation of ABG

1. Know normal values of pH, (PCO2), and (HCO3-).

2. Look at the patient's ABG's to determine what's abnormal high or low.

3. Correlate the abnormal values of PCO2 and HCO3- to the abnormality of pH.

4. Name the disorder, the cause, and the source of any compensation.

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:

• If only one of the two parameters (CO2 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:

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:

• Normal values for ABG's: pH range 7.35 - 7.45 PCO2 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

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

pH – low = acidosis

HCO3 - low = metabolic acidosis

PCO2 – normal; no compensation

Practice Problem 3 ABG's:

pH 7.31 PCO2 35 mm Hg HCO3- 20 mEq/L

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

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

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

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

pH – high = alkalosis

HCO3 – high = metabolic alkalosis

PCO2 – high = respiratory compensation

Practice Problem 8 ABG's:

pH 7.44 PCO2 55 mm Hg HCO3- 33 mEq/L