Passmedicine - Acid Base Disorders

Question 1

What things cause a metabolic acidosis with a normal anion gap?

Answer 1

GI loss - diarrhoea, ureterosigmoidostomy, fistula
Renal tubular acidosis
Drugs, e.g. acetazolamide
Ammonium chloride injection 
Addison's disease

Question 2

What things cause a metabolic acidosis with a raised anion gap?

Answer 2

Lactate: shock, hypoxia
Ketones: DKA, alcohol
Urate: renal failure
Acid poisoning: salicylates, methanol

Question 3

What are the two types of lactic acidosis?

Answer 3

Lactic acidosis A: perfusion disorders, e.g. shock, hypoxia, burns

Lactic acidosis B: metabolic, e.g. metformin toxicity

Question 4

What are causes of metabolic alkalosis?

Answer 4

Vomiting/aspiration
Diuretics
Liquorice
Hypokalaemia
Primary hyperaldosteronism
Cushing's syndrome
Bartter's syndrome
Congenital adrenal hyperplasia

Question 5

What is the mechanism of metabolic alkalosis?

Answer 5

Activation of RAAS is key factor
Aldosterone -> reabsorption of Na in exchange for H in the DCT
ECF depletion (as in vomiting, diuretic use) --> Na and Cl loss --> activation of RAAS --> raised aldosterone 
In hypokalaemia, K shifts from cells --> ECF, alkalosis is caused by shift of H into cells to maintain neurality

Question 6

What CNS injuries can lead to respiratory alkalosis?

Answer 6

Stroke
SAH
Encephalitis

Question 7

What is the recommended 5 step approach to ABG interpretation?

Answer 7

1. how is the patient?
2. is the patient hypoxaemic? (PaO2 should be >10kPa on air)
3. Is the patient acidaemic (pH <7.35) or alkalaemic (pH >7.45)?
4. What has happened to the PaCO2? (>6.0 suggests resp acidosis/resp compensation for met alk, <4.7 suggests resp alk/comepnsation for met acid)
5. What is the bicarb/base level? (bicarb <22 suggests met acid/renal compensation for resp alk, >26 suggests met alk or renal compensation for resp acid)

Question 8

What is a good mnemonic to work out acid base disorders?

Answer 8

ROME
Respiratory = opposite (e.g. low pH + high pCO2 = acidosis, high pH, low pCO2 = alkalosis)

Metabolic = equal
(e.g. low pH, low bicarb = acidosis, high pH, high bicarb = alkalosis)