Question 4.3

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.

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

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

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

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College answer

Not available.

Discussion

When invited to "comment on the abnormalities", one is always tempted to write "damn, those are some hideous abnormalities". As "comment" does not appear in the GLOSSARY OF TERMS at the beginning of the question paper, one must interpret this direction themselves. What they probably meant was "describe". "Describe the abnormalities on this arterial blood gas". 

Let's go through this ABG in some detail.

  • The oxygenation is impaired. The A-a gradient is elevated: the expected alveolar O2 is (713 × 0.6) - (31  / 0.8) = 389, which means the gradients is 297 mmHg. This patient with vomiting and abdominal pain possible either atelectatic collapse of his lung bases, or aspiration, or probably both.
  • There is acidaemia. The pH is 6.98
  • There is a metabolic acidosis. SBE is -22
  • The CO2 is trying to be helpful. However, the expected CO2 would be 18 by the SBE method, or (1.5×7)+8 = 18.5 using the Boston rules. Thus, there is also a respiratory acidosis.
  • The anion gap is hugely elevated (146-103-7 = 36).
  • The delta ratio is therefore (36-12)/(24-7) = 1.41, i.e. this is a high anion gap metabolic acidosis
  • The lactate is markedly raised (14.5)
  • The creatinine is raised, which suggests that some of the anion gap is also accounted for by the accumulation of the nonvolatile acids of renal failure

So, for a measly 15% of the marks, in 1.5 minutes of writing, what could you possibly comment here?

a)  The abnormalities on this gas are:

  • a high A-a gradient
  • renal failure
  • severe high anion gap metabolic acidosis most likely due to a combination of lactic acidosis and uraemia,
  •  with inadequate respiratory compensation.

b) Five causes for this picture are easy to find. Most likely any reasonable answer would have been accepted. From AF being given as background history, one would absolutely have to include ischaemic gut due to embolic phenomena in their differentials (in fact it would have to be the first differential). Thus:

  • Ischaemic gut
  • Septic shock
  • Aspiration pneumonia
  • Pancreatitis
  • Myocardial infarction with cardiogenic shock

Ketoacidosis with dehydration could also go in there, but the glucose just looks too normal.

References