Acid-Base Balance & Disorders

Question 1

Arterial blood pH is closely regulated in health to 7.35-7.45 by various mechanisms including bicarbonate, other plasma buffers such as deoxygenated haemoglobin, and the kidney.

For metabolic acidosis, to figure out the underlying cause, it is important to figure out the anion gap. How is the anion gap claculated?

Answer 1

• Difference between plasma cations (Na + K) and anions (Cl + HCO₃)
• Normal range = 10-18 mmol/L

Question 2

Metabolic acidosis can present with a) an increased anion gap or b) a normal anion gap.

What are the causes of metabolic adiosis with an increased anion gap?

Answer 2

Due to increased production, or reduced excretion, of fixed/organic acids. HCO₃ falls and unmeasured anions associated with the acids accumulate

• Lactic acidosis → shock, sepsis, tissue ischaemia
• Urate → renal failure
• Ketones → diabetes mellitus, alcohol
• Drugs→ salicylates, biguanides (eg metformin), ethylene glycol, methanol

Question 3

What are the causes of metabolic acidosis with a normal anion gap?

Answer 3

Due to loss of bicarbonate or ingestion of H+ ions

• Renal tubular acidosis
• Diarrhoea
• Drugs (acetazolamide)
• Addison’s disease
• Pancreatic fistula

Question 4

What are signs and symptoms of metabolic acidosis?

Answer 4

• Mild acidaemia → asymptomatic
• More severe (pH <7.1) → nausea, vom, malaise, confusion, chest/abdo pain
• Most characteristic sign is hyperpnoea: long deep breaths at normal rate
• Predisposed to cardiac dysfunction → hypotension, shock, ventricular arrhythmias, palpitations, coma

Question 5

What is metabolic alkalosis?

Answer 5

• primary increase in bicarbonate +/- compensatory increase in CO₂ pressure
• pH may be high or nearly normal

Question 6

What can cause metabolic akalosis?

Answer 6

• Prolonged vomiting
• K+ depletion (diuretics)
• Burns
• Mineralocorticoid excess (CAH, Cushing’s)

Question 7

What are the signs and symptoms of metabolic alkalosis?

Answer 7

• Usually due to underlying disorder
• Severe alkalosis increases protein binding of ionized Ca⁺
  
  • → hypocalcaemia, headache, lethargy, confusion, neuromuscular excitability
• Alkalosis also lowers threshold for anginal symptoms and arrhythmias
• Hypoventilation

Question 8

What is respiratory acidosis?

Answer 8

CO₂ accumulation due to decrease in RR and/or resp volume

Question 9

What causes respiratory acidosis?

Answer 9

• T2RF due to any lung, neuromuscular or physical cause
• COPD commonest cause → PaO₂ low + use controlled O₂ therapy venturi mask
• Decompensated resp disease → asthma, pneumonia, pulm oedema
• Foreign body aspiration
• Sedative drugs
• Cerebral tumours
• Flail chest
• Obesity

Question 10

What causes respiratory alkalosis?

Answer 10

As a result of hyperventilation of any cause:

• CNS → stroke, SAH, meningitis
• Other → mild/mod asthma, anxiety, altitude, pregnancy, PE, severe anaemia, salicylate poisoning

Question 11

What are signs and symptoms of respiratory alkalosis?

Answer 11

• Depends on rate and degree of fall in PCO₂
• Acute resp alkalosis →
  
  • light-headedness and confusion
  • peripheral and circumoral paraesthesias
  • cramps
  • syncope

Question 12

What are the signs and symptoms of respiratory acidosis?

Answer 12

Depends on rate and degree of PCO₂ increase as CO₂ rapidly diffuses across BBB

• hypoventilation
• headache
• confusion
• anxiety
• drowsiness
• stupor
• accessory muscle use