Acid-Base balance

Question 1

Describe and explain the effect of aspirin (salicyclate) overdose on acid-base balance

Answer 1

Initially causes respiratory alkalosis by directly stimulating the medullary respiratory centre to increase ventilation (more so in adults than children).

Later, progressive metabolic acidosis develops (more so in children than adults). This occurs 4–6 hours after ingestion in a young infant or 24 hours or more after ingestion in an adolescent or adult.

Question 2

What is normal pH?

Answer 2

7.4

Question 3

What level of bicarbonate is normal?

Answer 3

24mmol/L

Question 4

What is the normal range for PCO2?

Answer 4

4.9 - 6.1 pKa

Question 5

Are pH, bicarbonate and PCO2 high or low in respiratory acidosis? Which is the primary abnormality?

Answer 5

Low pH
High PCO2 - primary abnormality
High bicarbonate - compensates for CO2

Question 6

Are pH, bicarbonate and PCO2 high or low in respiratory alkalosis? Which is the primary abnormality?

Answer 6

High pH
Low PCO2 - primary abnormality
Low bicarbonate - compensates for low CO2

Question 7

Are pH, bicarbonate and PCO2 high or low in metabolic acidosis? Which is the primary abnormality?

Answer 7

Low pH
Low bicarbonate - primary abnormality
Low bicarbonate - compensates for low bicarbonate

Question 8

Are pH, bicarbonate and PCO2 high or low in metabolic alkalosis? Which is the primary abnormality?

Answer 8

High pH
High bicarbonate - primary abnormality
High PCO2 - compensates for high bicarbonate

Question 9

What mechanism causes high PCO2 in respiratory acidosis? Give three acute causes

Answer 9

Reduced ventilation
Acute causes:
 - drugs e.g. opiates
 - major airway obstruction
 - infection

Question 10

What is the most common cause chronic cause of respiratory acidosis?

Answer 10

Chronic lung disease e.g. COPD

Question 11

Describe how to manage respiratory acidosis

Answer 11

Treat underlying cause (e.g. give bronchodilators)
CPAP/BiPAP or intubation
Oxygen sats are likely to be low as well so the patient may need to be given supplementary oxygen

Question 12

What mechanism causes respiratory alkalosis? Give three causes.

Answer 12

Increased ventilation
Causes:
 - first ascent to altitude
 - voluntary hyperventilation
 - aspirin

Question 13

Give three mechanisms that can cause metabolic acidosis and suggest possible examples of each

Answer 13

Increased hydrogen ion production
 - DKA, lactic acidosis
 - shock, infection, ischaemia
Failure to excrete hydrogen ions
 - renal failure
Direct loss of bicarbonate
 - diarrhoea (failure to reabsorb intestinal bicarbonate)

Question 14

Describe how to manage metabolic acidosis

Answer 14

Treat underlying cause

Give sodium bicarbonate (with caution)

Question 15

What is the most common cause of metabolic alkalosis? Give two examples

Answer 15

Loss of hydrogen ions

• vomiting (loss of gastric HCl)
• aldosterone excess (increased renal proton loss)

Question 16

Describe how to manage metabolic alkalosis

Answer 16

Treat underlying cause
IV 0.9% saline
Severe cases may need to be treated by haemodialysis or haemofiltration