Following laparotomy for haemoperitoneum, a patient is transferred to ICU. Blood biochemistry and arterial blood gas analysis on admission to ICU are as follows:

Test

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

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

24 – 32

Standard Base Excess*

-9.0 mmol/L

-2.0 – +2.0

  • Describe the acid-base status.
  • What is the likely cause of this disturbance?
  • What is the underlying biochemical mechanism?

[Click here to toggle visibility of the answers]

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.


Discussion

Let us dissect these results systematically.

  1. The A-a gradient is probably high; the FiO2 is not supplied, but the patient is hypoxic.
  2. There is acidaemia
  3. The PaCO2 is compensatory
  4. The SBE is -9, suggesting a metabolic acidosis
  5. The respiratory compensation is adequate - the expected PaCO2 is (16 × 1.5) + 8 = 32mmHg.
  6. The anion gap is normal:
     (147) - (124 + 16) = 7, or 10.6 when calculated with potassium
  7. Urinary pH and electrolytes would be interesting, but are not supplied.

Thus, this is a normal anion gap metabolic acidosis, which is well compensated. One possible reason for this might be an enthusiastic overabundance of saline.

Judging from the answer, the college was expecting a  Stewart-based physicochemical approach to acid-base analysis, which is rather refreshing to see. Normal saline intoxication is dealt with elsewhere.

References

References

Story DA. Hyperchloraemic acidosis: another misnomer? Crit Care Resusc. 2004 Sep;6(3):188-92.

Skellett, S., et al. "Chasing the base deficit: hyperchloraemic acidosis following 0.9% saline fluid resuscitation." Archives of Disease in Childhood 83.6 (2000): 514-516.

Constable, Peter D. "Hyperchloremic acidosis: the classic example of strong ion acidosis." Anesthesia & Analgesia 96.4 (2003): 919-922.

Reid, Fiona, et al. "Hartmann’s solution: a randomized double-blind crossover study." Clinical Science 104 (2003): 17-24.