Smaill: Acid/Base Physiology 3

Question 1

What is the anion gap?

Answer 1

The anion gap is the difference between primary measured cations (sodium Na+ and potassium K+) and the primary measured anions (chloride Cl- and bicarbonate HCO3-) in serum.

Anion Gap = (Na⁺ + K⁺) - (Cl^- + HCO₃^-)

• If K⁺ is not used in the measurement the normal range is 8 - 16 mmol.L^-1

Question 2

What is the purpose of the anion gap calculation?

Answer 2

It is a useful way of identifying the cause of metabolic acidosis.

• If the anion of the non-volatile acid is Cl^- (metabolic acidosis due to diarrhoea or renal dysfunction) the anion gap will be normal.
• If the anion of the non-volatile acid is not Cl^- (e.g lactate or beta-hydroxybutyrate) the anion gap will increase.

Question 3

What value difference between P_AO2 and P_aO2 indicates impaired oxygen diffusion in the lungs?

Answer 3

• Young Adults = >12mmHg at sea level.
• Elderly = >25mmHg at sea level.

Question 4

What are the steps involved in the analysis and interpretation of arterial blood gases?

Answer 4

1. pH
2. P_CO2, H_CO3^- and BE - Acidosis, Alkalosis, Respiratory or Metabolic?
3. P_CO2, H_CO3^- and BE - Compensated or Uncompensated?
4. Anion Gap - Normal or Increased?
5. P_O2 - Normal, Low or High?