Blood gases

Question 1

Causes of respiratory alkalosis

Answer 1

Anxiety = hyperventilation
PE
Salicylate poisoning 
CNS disorders: stroke, SAH, encephalitis 
Altitude 
Pregnancy

Question 2

Causes of respiratory acidosis

Answer 2

COPD
Decompensation in other resp conditions e.g. life-threatening asthma/pulmonary oedema
Sedative drugs: benzos, opiate overdose

Question 3

Causes of metabolic alkalosis

Answer 3

Loss of H+ ions or gain of HCO3-
Mainly due to problems of the kidney/ GIT

Causes:
Vomiting
Diuretics 
hypoK
1 hyperaldosteronism 
Cushing's syndrome 
Bartter's syndrome 
CAH

Question 4

Causes of metabolic acidosis

Answer 4

Commonly classified according to the anion gap

Normal anion gap (hyperchloraemic metabolic acidosis)

• GI bicarb loss: diarrhoea, fistula
• Renal tubular acidosis
• Drugs: acetazolamide
• Ammonium chloride injection
• Addison’s

Raised anion gap

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

Question 5

Explain the anion gap

Answer 5

Works out if metabolic acidosis is due to increased acid production OR ingestion VS decreased acid secretion or loss of HCO3

Normal anion gap:
4 - 12 mmol/L

Formula:
(Na + K) - (Cl + HCO3)

High anion gap:
- increased acid production/ ingestion e.g. DKA, lactic acidosis, aspirin overdose, ethylene glycol

Low anion gap:
- decreased acid secretion/ loss of HCO3 e.g. GI loss (diarrhoea, ileostomy), renal tubular acidosis (retaining H+), Addison’s (retaining H+)