The following data are taken from a 74-year-old female who has just been admitted to ICU following surgery for revision of an infected hip prosthesis.

Parameter Patient Value Normal Adult Range
Sodium 147 mmol/L* 135 – 145
Potassium 3.6 mmol/L 3.2 – 4.5
Chloride 124 mmol/L* 100 – 110
Haemoglobin 106 g/L* 115 – 155
FiO2 0.3 .
pH 7.32* 7.35 – 7.45
PCO2 32 mmHg (4.3 kPa)* 35 – 45 (4.6 – 5.9)
PO2 63 mmHg (8.4 kPa) .
Bicarbonate 16.0 mmol/L* 22.0 – 26.0
Standard Base Excess -9.0 mmol/L* -2.0 – +2.0

a)  Describe the acid-base abnormalities.(20% marks)

b)  What is the likely cause of this disturbance? (20% marks)

c)  What is the underlying biochemical mechanism?(10% marks)

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College Answer

a)
Normal anion gap metabolic acidosis with appropriate respiratory compensation.

b)
Resuscitation with large volume saline infusion.

c)
ECF dilution by fluid with strong ion difference of zero (or any reasonable explanation.)

Discussion

Let us dissect these results systematically.

  1. The A-a gradient is increased (if the atmospheric pressure is 760mmHg, the gradient is  110, with a P/F ratio of 210)
  2. There is acidaemia
  3. The PaCO2 is low, which is appropriate in acidaemia
  4. The SBE is -9.0, suggesting a metabolic acidosis
  5. The respiratory compensation is appropriate: the expected PaCO2 = (16 × 1.5) + 8 = 32 mmHg (according to the Boston rules) The Copenhagen rules agree: the expected PaCO= (40 - SBE) = 31mmHg.
  6. The anion gap is normal: (147) - (124 + 16) = 7, or 10.6 when calculated with potassium
  7. The delta ratio is therefore irrelevant.

It is pleasing to see the use of  Stewart's physicochemical approach to acid-base analysis in the college answer.