The septic venous gas — interpretation and the lactate-guided resuscitation

1. The sample — a venous gas, with its known limits. This is a venous gas drawn from a cannula. The venous pH, HCO3, base excess, and lactate closely agree with the arterial values — in Middleton's central-venous agreement study the mean differences were 0.03 pH units, 0.52 mmol per litre bicarbonate, 0.19 base excess and 0.08 lactate, with narrow limits of agreement — supporting their use for screening and for monitoring. The venous PaO2 is meaningless for assessing oxygenation, so if hypoxaemia or hypercapnia is suspected, an arterial sample is taken.^[8]

2. Step 1 — the pH. pH 7.25 is a moderate acidaemia. ^[10]

3. Step 2 — the primary disorder. pH is low and pCO2 is low — same direction — so the primary disorder is metabolic acidosis, with the low pCO2 representing compensatory hyperventilation. ^[10]

4. Step 3 — the compensation. Winter's formula: expected pCO2 = 1.5 times HCO3 plus 8, plus or minus 2 mmHg = 1.5 times 11 plus 8 = 24.5 mmHg (about 3.3 kPa). The measured pCO2 is 3.2 kPa, which falls within the predicted range — appropriate compensation, no coexisting respiratory disorder.^[11]

5. Step 4 — the anion gap. Anion gap = Na minus (Cl plus HCO3) = 138 minus (110 plus 11) = 138 minus 121 = 17 mmol per litre. This is mildly raised (normal 8 to 12), consistent with a high-anion-gap metabolic acidosis driven by the lactate of 6.4.

6. Step 5 — the delta ratio. (AG minus 12) divided by (24 minus HCO3) = (17 minus 12) divided by (24 minus 11) = 5 divided by 13 = 0.38. A ratio under 1 means a coexisting normal-anion-gap (hyperchloraemic) acidosis layered on the high-anion-gap acidosis — the classic septic pattern of lactate accumulation plus large-volume normal-saline resuscitation (which raises chloride and suppresses bicarbonate). This is a mixed metabolic disorder, easily missed.^[10]

7. Step 6 — the lactate. Lactate 6.4 mmol per litre is severe hyperlactataemia. In septic shock, a lactate over 4 mmol per litre signals a high mortality, and the lactate is a strongly prognostic value on the gas.^[5]

8. The synthesis. A severe, mixed high-anion-gap plus normal-anion-gap metabolic acidosis in a septic, shocked patient, with appropriate respiratory compensation and a severe hyperlactataemia. The gas confirms septic shock and predicts a high mortality; it drives the resuscitation.