Question 24.3

The following blood gas was taken from a 35-year-old patient who presented with weakness and lethargy.

Parameter	Patient Value	Adult Normal range
pH	7.49*	7.35 – 7.45
pO2	85 mmHg (11.3 kPa)
pCO2	35 mmHg (4.7 kPa)	35 – 45 (4.7 – 6.0)
Standard bicarbonate	28 mmol/L	22 – 29
Base excess	4 mmol/L*	-3 to +3
Sodium	148 mmol/L*	135 – 145
Potassium	1.8 mmol/L*	3.5 – 5.2
Ionised Calcium	1.09 mmol/L*	1.15 – 1.30
Chloride	111 mmol/L*	95 – 110
Glucose	9.1 mmol/L	4.0 – 11.0
Lactate	1.2 mmol/L	0.5 – 1.3
Creatinine	63 µmol/L	45 – 90

a) List the important biochemical abnormalities and show calculations where appropriate. (25% marks)

b) List three clinical scenarios, which may produce this pattern of abnormalities.
(15% marks)

[Click here to toggle visibility of the answers]

College answer

Not available.

Discussion

In detail:

The A-a gradient is unknown, as the FiO₂ is not given, but from the stem it kind of sounds like this person just walked into the ED, so this could be a room air gas. In that case, the A-a gradient is (713 × 0.21) - (35 / 0.8) - 85 = 21 mmHg, which is pretty normal.
The patient is mildly alkalaemiac
There is a mild metabolic alkalosis (SBE is 4.0)
The CO₂ is essentially normal, or at the lower border of normal. Some might say that, in the context of a borderline metabolic alkalosis, the CO₂ should be at the upper border of normal. To do this scientifically, the application of the SBE rule for metabolic alkalosis would yield an expected CO₂ of (0.6 × 4) + 40 = 42.4 mmHg, or (0.7 × 28) + 20 = 39.6 mmHg using the original bicarbonate formula. In either case, one could say that the CO₂ is inappropriately depressed, i.e. there is also a mild respiratory alkalosis.
The anion gap is (148 - 111 - 28) = 9, i.e. it is low, if the anion gap you expect is 12 (i.e. if the albumin is normal).
The most striking abnormality, in terms of how far it strays from the normal range of values, is potassium here: it is 1.8, which is life-threatening (i.e. about to start causing some afterdepolarisations).
Sodium is also borderline elevated, suggesting a little bit of dehydration, or perhaps something more.
The ionised calcium is borderline low, but not impressively so. Similarly, there is some trivial hyperglycaemia.

So: the pattern of abnormalities is:

Hypokalemia
Hypernatremia
Hyperchloraemia
Metabolic alkalosis
Hyperglycaemia
Hypocalcemia

The hypokalemia is by far the worst of them, which leaves the trainee to sift through their mental silo of causes of hypokalemia, finding ones associated with enough of the other findings. These could be:

Conn's syndrome (primary hyperaldosteronism)
Diuretic or laxative abuse
Hypokalemic periodic paralysis
Bartter syndrome or Gitelman syndrome
Cushings disease

The BSL is given as slightly high, which would exclude things like insulinoma.

References

Glover, P. "Hypokalaemia." Critical Care and Resuscitation 1999; 1: 239-251.

Gennari, F. John. "Hypokalemia." New England Journal of Medicine 339.7 (1998): 451-458.

Assadi, Farahnak. "Diagnosis of hypokalemia: a problem-solving approach to clinical cases." Iranian journal of kidney diseases 2.3 (2008): 115-122.

[Submit a comment or correction]