ABGs

Question 1

Normal ranges in an ABG

Answer 1

pH: 7.35 – 7.45
PaO2: 10.7 – 13.3 kPa
PaCO2: 4.7 – 6.0 kPa
HCO3 (bicarbonate): 22 – 26 mmol/L
Base excess: -2 to +2
Lactate: 0.5 - 1 mmol/L

Question 2

ABG interpretation: ROME

Answer 2

Respiratory = Opposite
low pH + high PaCO2 i.e. acidosis, or
high pH + low PaCO2 i.e. alkalosis

Metabolic = Equal
low pH + low bicarbonate i.e. acidosis, or
high pH + high bicarbonate i.e. akalosis

Don’t let this confuse you. Diagram saved on computer

Question 3

How to interpret ABGs

Answer 3

• How is the patient?
• Is the patient hypoxaemic? (the PaO2 on air should be >10 kPa)
• Is the patient acidaemic (pH <7.35) or alkalaemic (pH >7.45)

Acidosis:
* High PaCO2 → Respiratory acidosis (or respiratory compensation for a metabolic alkalosis)
* Low HCO3- (or base excess < -2 mmol/l) → Metabolic acidosis (or renal compensation for a respiratory alkalosis)

Alkalosis:
* Low PaCO2 → Respiratory alkalosis (or respiratory compensation for a metabolic acidosis)
* High HCO3- (or base excess > +2 mmol/l) → Metabolic alkalosis (or renal compensation for a respiratory acidosis)

Question 5

How is the anion gap calcaulated?

Answer 5

(Sodium + potassium) - (bicarbonate + chloride)

Normal anion gap is 8-14 mmol/L

Question 6

How is metabolic acidosis classified?

Answer 6

Metabolic acidosis is commonly classified according to the anion gap:
* Normal anion gap (= hyperchloraemic metabolic acidosis)
* Raised anion gap

This can be calculated by: (Na+ + K+) - (Cl- + HCO-3).

If a question supplies the chloride level then this is often a clue that the anion gap should be calculated.
The normal range = 10-18 mmol/L

Question 7

Name 2 causes of metabolic acidosis with a normal anion gap (= hyperchloraemic metabolic acidosis)

Answer 7

Gastrointestinal bicarbonate loss:
* prolonged diarrhoea**: may also result in hypokalaemia
* ureterosigmoidostomy
* fistula

Renal tubular acidosis

Drugs: e.g. acetazolamide

Ammonium chloride injection

Addison’s disease

Question 8

Name 2 causes of metabolic acidosis with a raised anion gap

Answer 8

Lactate:
* Shock
* Sepsis
* Hypoxia

Ketones:
* Diabetic ketoacidosis
* Alcohol

Urate: renal failure

Acid poisoning: salicylates, methanol

Question 9

Metabolic acidosis secondary to high lactate levels may be subdivided into two types - what are they?

Answer 9

• Lactic acidosis type A: sepsis, shock, hypoxia, burns
• Lactic acidosis type B: metformin

Question 10

What are the 2 organ systems that are mainly the cause of metabolic alkalosis?

Answer 10

• Kidney
• Gastrointestinal tract

Question 11

Name 2 causes of metabolic alkalosis

Answer 11

Metabolic alkalosis may be caused by a loss of hydrogen ions or a gain of bicarbonate

Causes:
* Vomiting / aspiration (may also lead to hypokalaemia)
* Diuretics
* Liquorice
* Hypokalaemia
* Primary hyperaldosteronism
* Cushing’s syndrome
* Bartter’s syndrome

Question 12

Name 2 causes of respiratory acidosis

Answer 12

• COPD
• Decompensation in other respiratory conditions e.g. life-threatening asthma / pulmonary oedema
• Neuromuscular disease
• Obesity hypoventilation syndrome
• Sedative drugs: benzodiazepines, opiate overdose

Question 13

Name 2 causes of respiratory alkalosis

Answer 13

• Anxiety leading to hyperventilation
• Pulmonary embolism
• Salicylate poisoning
• CNS disorders: stroke, subarachnoid haemorrhage, encephalitis
• Altitude
• Pregnancy

Salicylate overdose leads to a mixed respiratory alkalosis and metabolic acidosis