Question 3.1

Last updated 26/07/2019 - 01:13
 Topic: Acid-Base Disorders
Pass rate
100%
Other SAQs in this topic:
Other SAQs in this paper:

The following data are from the arterial blood gas analysis of a 71-year-old male with necrotising fasciitis:

Parameter Patient Value Normal Adult Range
Barometric pressure 760 mmHg (100 kPa) .
FiO2 0.3 .
pH 7.43 7.35 – 7.45
PCO2 23 mmHg (3.1 kPa)* 35 – 45 (4.6 – 5.9)
PO2 107 mmHg (14.3 kPa) .
Bicarbonate 15 mmol/L* 22 – 26
Standard Base Excess -8.6 mmol/L* -2.0 – +2.0
Lactate 23.0 mmol/L* 0.2 – 2.5
Sodium 147 mmol/L* 137 – 145
Potassium 6.7 mmol/L* 3.2 – 4.5
Chloride 95 mmol/L* 100 – 110

List the acid-base abnormalities. (30% marks)

[Click here to toggle visibility of the answers]

College Answer

Lactic acidosis
Anion gap elevation (37 mEq/L)
Metabolic alkalosis
Respiratory alkalosis

Discussion

This is a triple disorder.

Let us dissect these results systematically.

  1. The A-a gradient is high; ~78mmHg
  2. There is neither alkalaemia nor acidaemia
  3. The PaCO2 is low, which is a move in the appropriate direction given the metabolic acidosis
  4. The SBE is -8.6, suggesting a metabolic acidosis
  5. The respiratory compensation is excessive - the expected PaCO2 (15 × 1.5) + 8 = 30.5mmHg, and so there is also a respiratory alkalosis according to the Boston rules.
    According to the Copenhagen rules, the the expected PaCO2 = (40 - SBE) = 31.4mmHg.
    So, in this case there is no Trans-Atlantic disagreement.
  6. The anion gap is (147) - (95  + 15) = 37, or 43.7 when calculated with potassium.
  7. The delta ratio (assuming an albumin of 40) would therefore be (37 - 12) / (24 - 15) = 2.77 thus there is a high anion gap metabolic acidosis which co-exists with a metabolic alkalosis.
    Note that without taking this step, the candidates would still have guessed that there is an underlying metabolic alkalosis. How else would you have a normal pH with a lactate of 23?
 

[Submit a comment or correction]