Question 22.2

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) - 28.8 = 185.1, thus the A-a difference is 195.5-107 = 78.1.

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

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.

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