A 63-year-old female presented with high fever and abdominal pain. She has gram- negative bacteraemia and septic shock. The following data are from an arterial blood gas analysis:

Parameter

Patient Value

Normal Range

FiO2

0.3

pH

7.43

7.35 – 7.45

PaCO2

23* mmHg (3.0 kPa)

35 – 45 (4.6 – 6.0)

PaO2

107 mmHg (14 kPa)

HCO3

15* mmHg

22 – 26

Standard base excess

-8.6* mmol/l

-2 – +2

Sodium

147* mmol/l

135 – 145

Potassium

6.7* mmol/l

3.2 – 4.5

Chloride

95* mmol/l

100 – 110

Lactate

23.0* mmol/l

< 2

a)  Describe the abnormalities on the above arterial blood gas profile

b)  List three causes of a raised lactate in sepsis

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College Answer


a)  Describe the abnormalities on the above arterial blood gas profile

  • Raised A-a DO2 (78)
  • High anion gap (37) metabolic acidosis with markedly raised lactate
  • Metabolic alkalosis (delta ratio > 3)
  • Respiratory alkalosis (PCO2 lower than expected for HCO3)

b)  List three causes of a raised lactate in sepsis

  • Tissue hypoperfusion and hypoxia
  • Use of adrenaline (increased glycolytic flux)
  • Down regulation of pyruvate dehydrogenase by inflammatory mediators
  • Underlying ischaemic tissue

Discussion

This is a straighforward question about ABG interpretation; it presents the candidate with a triple disorder.

This question is frequently repeated. The chapter on the causes of lactic acidosis in sepsis contains a list of past SAQs which are either identical or very similar.

Yes, the A-a gradient is raised: (0.3 x 713) - 18.4 = 195.5, thus the A-a difference is 195.5-107 = 88.5.

How they got 78 I am not sure.

The anion gap is (147) - (95 + 15) = 37, or 43.7 when calculated with potassium.
The delta ratio, assuming a normal anion gap is 12 and a normal bicarbonate is 24, would therefore be (37 - 12) / (24 - 15) = 2.77. This suggests there is also a metabolic alkalosis.

The PaCO2 expected for this sort of bicarbonate level is 30.3 (15 x 1.5 + 8) which suggests that there is also a respiratory alkalosis.

As for the causes of raised lactate in sepsis... The question asks for 3 causes. In the past, the examiners have been firm in saying that if you produce more than the [requested number] of answers, you will only be marked on the first [requested number] answers.

In any case one can usually come up with a few reasons a septic patient might have a raised lactate. The topic of pyruvate dehydrogenase being inhibited by inflammatory mediators is explored in this article.

References

References

D C Gore, F Jahoor, J M Hibbert, and E J DeMaria Lactic acidosis during sepsis is related to increased pyruvate production, not deficits in tissue oxygen availability. Ann Surg. 1996 July; 224(1): 97–102.