Acid-Base Disorders - Anion gap, ABG, Metabolic acidosis/alkalosis, Respiratory acidosis/alkalosis

Question 1

How is metabolic acidosis classified?

Answer 1

By anion gap:

Normal anion gap (hyperchloraemic)
Raised anion gap

Question 2

What are the causes of normal anion gap metabolic acidosis?

Answer 2

Gastrointestinal bicarbonate loss: Diarrhoea, ureterosigmoidostomy, fistula
Renal tubular acidosis
Drugs: e.g., acetazolamide
Ammonium chloride injection
Addison’s disease

Question 2

What are common causes of respiratory alkalosis?

Answer 2

Anxiety leading to hyperventilation
Pulmonary embolism
Salicylate poisoning
CNS disorders: Stroke, subarachnoid haemorrhage, encephalitis
Altitude
Pregnancy

Question 2

What causes metabolic alkalosis?

Answer 2

Loss of hydrogen ions or gain of bicarbonate
Vomiting/aspiration
Diuretics
Liquorice, carbenoxolone
Hypokalaemia
Primary hyperaldosteronism
Cushing’s syndrome
Bartter’s syndrome
Congenital adrenal hyperplasia

Question 2

What are the causes of raised anion gap metabolic acidosis?

Answer 2

Lactate: Shock, hypoxia
Ketones: Diabetic ketoacidosis, alcohol
Urate: Renal failure
Acid poisoning: Salicylates, methanol

Question 2

What are common causes of respiratory acidosis?

Answer 2

COPD
Decompensation in respiratory conditions (e.g., severe asthma, pulmonary oedema)
Sedative drugs (e.g., benzodiazepines, opiate overdose)

Question 2

What are common causes of respiratory alkalosis?

Answer 2

Anxiety leading to hyperventilation
Pulmonary embolism
Salicylate poisoning
CNS disorders: Stroke, subarachnoid haemorrhage, encephalitis
Altitude
Pregnancy

Question 3

How is the anion gap calculated?

Answer 3

Anion gap = (Sodium + Potassium) - (Bicarbonate + Chloride)

Question 4

What is the normal range for the anion gap?

Answer 4

8-14 mmol/L

Question 5

What is a potential cause of a raised anion gap metabolic acidosis related to chronic paracetamol use?

Answer 5

5-oxoproline

Question 6

What is the first step in the Resuscitation Council’s 5-step approach to arterial blood gas interpretation?

Answer 6

How is the patient?

Question 7

How can you assess if a patient is hypoxaemic?

Answer 7

Check if PaO2 on air is >10 kPa

Question 8

What does a PaCO2 > 6.0 kPa indicate?

Answer 8

Respiratory acidosis
Or respiratory compensation for metabolic alkalosis

Question 9

What does a PaCO2 < 4.7 kPa indicate?

Answer 9

Respiratory alkalosis
Or respiratory compensation for metabolic acidosis

Question 10

What bicarbonate level suggests metabolic acidosis?

Answer 10

Bicarbonate < 22 mmol/L

Or base excess < -2 mmol/L

Question 11

What bicarbonate level suggests metabolic alkalosis?

Answer 11

Bicarbonate > 26 mmol/L

Or base excess > +2 mmol/L

Question 12

What does ROME stand for in arterial blood gas interpretation?

Answer 12

Respiratory = Opposite
Low pH + High PaCO2 = Acidosis
High pH + Low PaCO2 = Alkalosis
Metabolic = Equal
Low pH + Low bicarbonate = Acidosis
High pH + High bicarbonate = Alkalosis

Question 13

What clues might indicate the need to calculate the anion gap in metabolic acidosis?

Answer 13

Supplied chloride level

Question 14

What are the subtypes of lactic acidosis and their causes?

Answer 14

Type A: Sepsis, shock, hypoxia, burns
Type B: Metformin

Question 15

What is a common cause of normal anion gap metabolic acidosis that can also cause hypokalaemia?

Answer 15

Prolonged diarrhoea

Question 16

What condition is associated with a normal anion gap and is often treated with ammonium chloride?

Answer 16

Addison’s disease

Question 17

What type of metabolic acidosis is linked to high lactate levels and associated with sepsis and shock?

Answer 17

Lactic acidosis Type A

Question 18

What is a key drug-related cause of raised anion gap metabolic acidosis?

Answer 18

Metformin (Type B lactic acidosis)

Question 19

Which syndrome is associated with metabolic alkalosis and involves a loss of potassium?

Answer 19

Primary hyperaldosteronism

Question 20

How does activation of the renin-angiotensin II-aldosterone (RAA) system contribute to metabolic alkalosis?

Answer 20

Aldosterone causes Na+ reabsorption in exchange for H+, leading to alkalosis when Na+ and Cl- are lost (e.g., due to vomiting or diuretics)

Question 21

What role does hypokalaemia play in metabolic alkalosis?

Answer 21

In hypokalaemia, K+ shifts from cells to ECF, causing H+ to move into cells to maintain neutrality, leading to alkalosis.

Question 22

Which type of neuromuscular disease is associated with respiratory acidosis?

Answer 22

Neuromuscular diseases (e.g., amyotrophic lateral sclerosis)

Question 23

What syndrome related to obesity can contribute to respiratory acidosis?

Answer 23

Obesity hypoventilation syndrome

Question 24

Which sedative drugs can cause respiratory acidosis?

Answer 24

Benzodiazepines and opiate overdose

Question 25

What other respiratory conditions might lead to respiratory acidosis if they decompensate?

Answer 25

Life-threatening asthma or pulmonary oedema

Question 26

What is a common CNS disorder that can lead to respiratory alkalosis?

Answer 26

Stroke, subarachnoid haemorrhage, or encephalitis

Question 27

How can pregnancy contribute to respiratory alkalosis?

Answer 27

Pregnancy can cause respiratory alkalosis due to increased respiratory rate and decreased CO2 levels.

Question 28

How does salicylate poisoning affect blood gas levels?

Answer 28

Salicylate poisoning causes mixed respiratory alkalosis and metabolic acidosis; early stimulation of the respiratory centre leads to respiratory alkalosis, while later effects and acute renal failure lead to acidosis.