Question 7.1

Highest mark

7.7

The following arterial blood gas results are from a 72-year-old male admitted for investigation of nausea, vomiting and severe abdominal pain. He has a history of type 2 diabetes and atrial fibrillation.

a) Comment on the abnormalities on this arterial blood gas. (15% marks)

b) List five likely causes for the acid-base disturbance. (15% marks)

Parameter	Patient Value	Adult Normal Range
FiO2	0.6
pH	6.98*	7.35 – 7.45
pO2	92 mmHg (12.3 kPa)
pCO2	31.0 mmHg (4.1 kPa)*	35.0 – 45.0 (4.6 – 6.0)
SpO2	99%
Bicarbonate	7.0 mmol/L*	22.0 – 26.0
Base Excess	-22.0 mmol/L*	-2.0 – +2.0
Lactate	14.5 mmol/L*	0.5 – 1.6
Sodium	146 mmol/L*	135 – 145
Potassium	5.3 mmol/L*	3.5 – 5.0
Chloride	103 mmol/L	95 – 105
Glucose	7.7 mmol/L*	3.5 – 6.0
Creatinine	711 μmol/L*	60 – 110
Haemoglobin	108 g/L*	135 – 180

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College answer

a)
Elevated Aa gradient
Profound lactic acidosis
High Anion Gap Metabolic Acidosis (36)
Associated respiratory acidosis or incomplete compensation
Delta ratio 1.41 – suggests pure elevated anion gap acidosis
Renal impairment

b)
Metformin induced
Ischaemic gut
Pancreatitis
Sepsis
Cardiogenic shock

Discussion

Let us dissect these results systematically:

The A-a gradient is raised (the expected alveolar O₂ concentration is = (0.6 × 713) - (3 ×0.8) = 389, so the A-a gradient must be around 297)
There is acidaemia.
There is a metabolic acidosis (SBE is -22). Lactate is responsible for virtually all of it (lactate is 14 mmol/L)
The CO₂ is appropriately decreased, but not enough. The expected CO₂ for "full compensation" would be 28 mmHg by the Copenhagen rules or (7 × 1.5) +8 = 18.5 mmHg by Winter's rule. Thus, there is also a respiratory acidosis
The anion gap is raised; it is 36 if you omit potassium and 41.3 if you include it (the consequences of using different equations for this are discussed elsewhere).
Assuming an albumin value of 40g/L, the delta ratio is either 1.41 or 1.72, but either way there is a pure HAGMA.
Urea is not available, but the creatinine of over 700 strongly suggests that something is wrong with the kidneys.

The college answer gives up only five causes, so one would be expected to at least include these in their list. Here they are again with justifications:

Ischaemic gut (AF, nausea, vomiting, abdominal pain)
Sepsis (safe differential for everything)
Pancreatitis (abdominal pain)
Cardiogenic shock (diabetic, AF suggests structural cardiac disease)
Metformin (diabetic, therefore presumably on metformin)

Other unofficial possibilities include:

Portal vein thrombosis with ascites and hepatorenal syndrome (explains lactate, abdominal pain, and to some extent the creatinine)
Malignancy (eg. perforated colorectal cancer with bowel obstruction, which explains everything)

References

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