Acid-base abnormality (A&E)

Question 1

What are acid-base abnormalities?

Answer 1

Conditions characterised by changes in the concentration of [H+] or [HCO3-], leading to changes in arterial blood pH

Question 2

How do we calculate anion gap?

Answer 2

([Na+] + [K+]) - ([Cl-] + [HCO3-])

Question 3

What is the normal anion gap vs raised anion gap?

Answer 3

• normal: 6-16mmol/L
• raised: >16mmol/L

Question 4

What can cause a normal anion gap metabolic acidosis? (5)

Answer 4

Normal anion gap metabolic acidosis (6-16mmol/L)= hyperchloraemic metabolic acidosis

• GI bicarbonate loss: diarrhoea, (ureterosigmoidostomy, fistula)
• renal tubular acidosis
• drugs e.g. acetazolamide (carbonic anhydrase-inhibiting diuretic) - idiopathic intracranial hypertension
• ammonium chloride injection
• Addison’s disease

Question 5

What can cause a raised anion gap metabolic acidosis? (4)

Answer 5

Raised anion gap metabolic acidosis (>16mmol/L)

• lactic acidosis e.g. shock, sepsis, hypoxia, metformin
• ketones: DKA, alcoholic ketoacidosis (metabolic ketoacidosis with normal/low glucose = alcoholic)
• urate: renal failure –> uraemia
• acid poisoning: salicylates, methanol

Question 6

What may metabolic alkalosis be caused by?

Answer 6

• loss of H+ or gain of HCO3-
• mainly due to problems of kidney or GI tract

Question 7

What are some causes of metabolic alkalosis? (8)

Answer 7

• vomiting / aspiration (e.g. peptic ulcer –> pyloric stenos, nasogastric suction)
• diuretics
• hypokalaemia
• primary hyperaldosteronism (hypokalaemia = alkalosis)
• Cushing’s syndrome (hypokalaemia = alkalosis)
• liquorice, carbenoxolone
• Bartter’s syndrome
• congenital adrenal hyperplasia

Question 8

What may respiratory acidosis be due to?

Answer 8

Hypoventilation

Question 9

What can cause respiratory acidosis? (5)

Answer 9

• COPD
• decompensation in other respiratory conditions e.g. life-threatening asthma/pulmonary oedema
• sedatives e.g. benzodiazepines, opiate OD
• airway obstruction
• weakening of respiratory muscles

Question 10

What may respiratory alkalosis be due to?

Answer 10

Hyperventilation

Question 11

What can cause respiratory alkalosis? (6)

Answer 11

• anxiety/panic attack –> hyperventilation (–> hypocalcaemia –> tingling in lips and fingers)
• pulmonary embolism
• salicylate poisoning + aspirin OD (early stages)
• CNS disorders: stroke, subarachnoid haemorrhage, encephalitis
• altitude (hypoxia)
• pregnancy

Question 12

What are the compensatory responses for the different acid-base abnormalities? (4)

Answer 12

• metabolic acidosis: hyperventilation (immediate)
• metabolic alkalosis: hypoventilation (immediate)
• respiratory acidosis: increase renal [HCO3-] reabsorption (delayed)
• respiratory alkalosis: decrease renal [HCO3-] reabsorption (delayed)

Question 13

How do we investigate acid-base abnormalities?

Answer 13

Arterial blood gas

Question 14

How do we evaluate ABGs?

Answer 14

1. evaluate blood pH (<7.35 = acidosis, >7.45 = alkalosis)
2. evaluate pCO2 (if pH and CO2 change in opposite direction = respiratory disorder; if pH and CO2 change in same direction = metabolic disorder)
3. evaluate HCO3- (high = metabolic alkalosis/compensated respiratory acidosis; low = metabolic acidosis/compensated respiratory alkalosis)
   
   • base excess: >+3 = metabolic alkalosis, <-3 = metabolic acidosis
4. compensation
5. check hypoxia using pO2 (<10kPa)

Question 15

How do we manage respiratory acidosis and alkalosis?

Answer 15

Treat underlying cause

Question 16

How do we manage metabolic acidosis?

Answer 16

IV sodium bicarbonate

Question 17

How do we manage metabolic alkalosis?

Answer 17

Acetazolamide (carbonic anhydrase-inhibiting diuretic)