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Acid Base Balance

The main determinant of acid-base disturbances is the principal buffering system represented by the following equation:

H₂O+CO₂ « H₂CO₃ « H⁺+HCO3^-

When analysing acid base disturbances proceed in  an orderly fashion analysing each step as one proceeds

 

pH

pH may be:

Ñ    Normal

Ñ    Low  (acidosis)

Ñ    High (alkalosis)

 

Acidosis

Ñ    Acidosis refers to an increase in H⁺ ions in the body

Acidosis may be:

Ñ    Respiratory acidosis

Ñ    Metabolic acidosis

 

Title:              

Ñ    Respiratory Acidosis

Definition

Ñ    Increase in H+ ion in the body due to hypoventilation

Pathogenesis

Pathological Process

Ñ    Respiratory acidosis is due to retention of carbon dioxide driving the production of carbonic acid and thereby producing excess H⁺ ions.

Causes

The causes of respiratory acidosis are:

RS

Ñ    Barotrauma

Ñ    Obstructive airways disease (COPD, asthma, obstructive sleep apnoea)

Ñ    Pneumoconiosis,

Ñ    ARDS

CNS

Ñ    Muscular dystrophies 

Ñ    Myasthenia gravis

Ñ    Guillain Barre syndrome

Ñ    Poliomyelitis

Ñ    Head injury

Ñ    CNS depression due to drugs eg.  Morphine, Anaesthetic

Ñ    Stroke

LMS

Ñ    Kyphoscoliosis

Ñ    Ankylosing spondylitis

E&M

Ñ    Obesity

 

Title:              

Ñ    Metabolic Acidosis

Definition:

Ñ    Acidosis caused by a primary metabolic problem

Pathogenesis

Pathological Process

Ñ    Metabolic acidosis is caused by ingestion or generation of acids or inability to excrete or utilise acid

Causes

Metabolic acidosis may be:

Ñ    High Anion Gap

Ñ    Normal Anion Gap

 

High Anion Gap Acidosis

E&M

Ñ    Ketoacidosis:

Diabetic

Alcoholic

Starvation

Ñ    Lactic acidosis

KUS

Ñ    Renal failure

Toxins

Ñ    Salicylate

Ñ    Ethylene glycol

Ñ    Methanol

Ñ    Ethanol

 

Normal Anion Gap Acidosis

GIT

Gastrointestinal HCO3 loss

Ñ    Diarrhoea

Ñ    Fistulae

External

Pancreatic

Small bowel drainage

Ñ    Uretero-sigmoidostomy, jejunal loop, ileal loop

KUS

Ñ    Renal tubular acidosis

Ñ    Pyelonephritis

Ñ    Tubulointerstitial disease

Drugs

Ñ    Acetazolamide

Ñ    Rapid saline administration

 

Alkalosis

This may be

Ñ    Respiratory Alkalosis

Ñ    Metabolic Alkalosis

 

Title:              

Ñ    Respiratory Alkalosis

Definition:

Ñ    Alkalosis caused by hyperventilation

Pathogenesis

Pathological Process

Ñ    Respiratory alkalosis is caused by hyperventilation which decreases the amount of carbon dioxide in the blood stream thus reducing carbonic acid and H⁺ ion formation

Causes

RS

Ñ    High altitude

Ñ    Pneumonia

Ñ    Aspiration

Ñ    Pulmonary oedema

Ñ    Pulmonary embolism

Ñ    Pneumothorax

CNS

Ñ    Hysteria

Ñ    Anxiety

Ñ    Pain

Ñ    Cerebrovascular event

Ñ    Encephalitis

Ñ    Meningitis

Ñ    Raised intracranial pressure

E&M

Ñ    Hyperthyroidism

Ñ    Fever

HS

Ñ    Gram-negative septicaemia

Drugs

Ñ    Early salicylate poisoning

 

Title:              

Ñ    Metabolic Alkalosis

Definition:

Ñ    Alkalosis caused by a primary metabolic condition

Pathogenesis

Pathological Process

Ñ    Metabolic alkalosis is caused by increased elimination of H⁺ ions or neutralisation of H⁺ ions

Causes

GIT

Ñ    Antacids

Ñ    Vomiting

Ñ    Gastric aspiration

KUS

Ñ    Diuretics

Ñ    Hypokalaemia

Ñ    Bartter’s syndrome

Ñ    Gitelman’s syndrome

E&M

Ñ    Cushing’s

Ñ    Hyperaldosteronism

 

pCO₂

pCO₂ may be:

Ñ    Normal

Ñ    High

Ñ    Low

Title:              

Ñ    High pCO₂

Definition:

Ñ    Increase in the arterial carbon dioxide concentration

Pathogenesis

Pathological Process

Ñ    This is caused by hypoventilation:

Causes

Either

Ñ    Respiratory compensation for metabolic alkalosis

Or

RS

Ñ    Obstructive airways disease (COPD, asthma, obstructive sleep apnoea)

CNS

Ñ    Myasthenia gravis

Ñ    Guillain Barre syndrome

Ñ    Head injury

Ñ    Sedation

LMS

Ñ    Kyphoscoliosis

Ñ    Ankylosing spondylitis

E&M

Ñ    Obesity

 

Title:              

Ñ    Low pCO₂

Definition:

Ñ    Decrease in the arterial carbon dioxide concentration

Pathogenesis

Pathological Process

Ñ    This is caused by hyperventilation:

Causes

Either

Ñ    Respiratory compensation for metabolic acidosis

Or

RS

Ñ    High altitude

Ñ    Pneumonia

Ñ    Pulmonary oedema

Ñ    Pulmonary embolism

Ñ    Pneumothorax

CNS

Ñ    Hysteria

Ñ    Anxiety

Ñ    Pain

Ñ    Cerebral haemorrhage

Ñ    Encephalitis

Ñ    Meningitis

Ñ    Raised intracranial pressure

E&M

Ñ    Hyperthyroidism

Ñ    Fever

HS

Ñ    Gram-negative septicaemia

Drugs

Ñ    Early salicylate poisoning

 

pO₂

pO₂ may be:

Ñ    Normal

Ñ    Low

Ñ    High

 

Title:              

Low pO₂

Definition:

Ñ    Decreased arterial oxygen saturation

Pathogenesis

Pathological Process

Ñ    Caused by decreased concentration of inspired oxygen, failure of ventilation, distribution or diffusion 

Ñ    It could also be due to right to left shunting

Causes

Ñ    Respiratory compensation for metabolic alkalosis

Or

RS

Ñ    High altitude

Ñ    Pneumonia

Ñ    Pulmonary oedema

Ñ    Pulmonary embolism

Ñ    Pneumothorax

Ñ    Obstructive airways disease (COPD, asthma, obstructive sleep apnoea)

CNS

Ñ    Myasthenia gravis

Ñ    Guillain Barre syndrome

Ñ    Head injury

Ñ    Sedation

LMS

Ñ    Kyphoscoliosis

Ñ    Ankylosing spondylitis

E&M

Ñ    Obesity

 

Title

Ñ    High pO2

Cause

Ñ    Increased in percentage of inspired oxygen

 

 

HCO₃^-

HCO₃^- may be:

Ñ    Normal

Ñ    Increased

Ñ    Decreased

 

Title:              

Ñ    Increased HCO₃^-

Definition:

Ñ    Increased concentration of bicarbonate in arterial blood

Pathogenesis

Pathological Process

This may be due to:

Ñ    Metabolic alkalosis

Retention of bicarbonate produced in the generation of H+ ions that are subsequently eliminated or neutralised

Ñ    Compensated respiratory acidosis

Compensatory increase in bicarbonate in order to neutralise respiratory acidosis (bicarbonate generated by the kidney)

 

Title:              

Ñ    Decreased HCO₃^-

Definition:

Ñ    Decreased concentration of bicarbonate in arterial blood

Pathogenesis

Pathological Process

This may be due to:

Ñ    Metabolic acidosis

Loss of bicarbonate through the gastrointestinal tract or kidneys

Ñ    Respiratory alkalosis

Washout of carbon dioxide reduces generation of bicarbonate

 

Anion Gap

Ñ    The anion gap refers to the gap difference between measured cations and measured anions

Ñ    (Na⁺  + K⁺) – (Cl^- + HCO₃^-)

Ñ    Normal range: 10-16 mmol/L

Ñ    Reflects concentration of

·          Unmeasured anions

Plasma proteins, phosphates, sulphur, organic acids

·          Unmeasured cations

Magnesium, calcium

 

Title:              

Ñ    Increased Anion Gap

Definition:

Ñ    Increased unmeasured anion

Pathogenesis

Pathological Process

Ñ    Ingestion or generation of unmeasured anion

Causes

KUS

Ñ    Renal failure

E&M

Ñ    Diabetic ketoacidosis

Ñ    Lactic acidosis

Toxins

Ñ    Alcoholic ketoacidosis

Ñ    Methanol

Ñ    Ethylene glycol

Ñ    Salicylates

 

Title:              

Ñ    Decreased Anion Gap

Definition:

Ñ    Relative decrease in unmeasured anion

Pathogenesis

Pathological Process

Ñ    Result of either decrease in unmeasured anion or increase in unmeasured cation

Causes

Ñ    Increase in unmeasured cation

Magnesium

Calcium

Decreased unmeasured anion

Ñ    Hypoalbuminaemia

 

Title:              

Ñ    Lactic Acidosis

Definition:

Ñ    Increased concentration of lactic acid in blood

Pathogenesis

Pathological Process

This may be due to:

Ñ    Increased production of lactic acid

Ñ    Decreased utilisation of lactic acid

Causes

Increased production of lactic acid

CVS

Ñ    Cardiac arrest

Ñ    Shock

GIT

Ñ    Bowel infarction

RS

Ñ    Carbon monoxide poisoning

Drugs

Ñ    Antiretrovirals (mitochondrial toxicity)

Decreased utilisation of lactic acid

E&M

Ñ    Diabetes mellitus

GIT

Ñ    Liver failure

Drugs

Ñ    Metformin, phenformin

Toxins

Ñ    Alcohol

 

 

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