Mixed acid–base disorder with a normal pH in a dog
7-year-old Labrador with intractable vomiting for 4 days due to a pyloric foreign body. Weak, pale mucous membranes, capillary refill time 3 s and weak femoral pulses. Venous blood gas:
| Parameter | Result | Reference |
|---|---|---|
| pH | 7.40 | 7.35–7.45 |
| pCO₂ | 37 mmHg | 35–45 |
| HCO₃⁻ | 22 mmol/L | 20–25 |
| Sodium | 146 mmol/L | 142–152 |
| Potassium | 3.1 mmol/L | 3.6–5.2 |
| Chloride | 92 mmol/L | 108–118 |
| Anion gap (Na + K − Cl − HCO₃) | 35 mmol/L | 12–24 |
| Lactate | 7.2 mmol/L | < 2.0 |
What is the correct acid–base interpretation?
chevron_right Show answer and explanation
A. Normal acid–base balance; the high lactate is a sampling artefact
A normal pH and bicarbonate rule nothing out: an anion gap of 35 mmol/L and marked hypochloraemia show two opposing disorders that cancel each other out. With clinical hypoperfusion, a lactate of 7.2 is credible.
B. High anion gap metabolic acidosis fully compensated by respiration
Respiratory compensation lowers pCO₂ and never returns pH or bicarbonate to normal. Here both pCO₂ and bicarbonate are normal, which is impossible in a pure metabolic acidosis of this size.
C. High anion gap metabolic acidosis plus hypochloraemic metabolic alkalosis
Correct. The anion gap has risen by about 17 mmol/L (lactate), so in a single disorder bicarbonate would have fallen markedly; that it remains at 22 reveals a concurrent metabolic alkalosis. Very low chloride with normal sodium (low corrected chloride) points to its origin: loss of gastric acid.
D. Chronic respiratory alkalosis with compensatory hyperchloraemic metabolic acidosis
There is no hypocapnia, so there is no respiratory alkalosis, and chloride is low, not high. Renal compensation for respiratory alkalosis would be hyperchloraemic with a normal anion gap.
lightbulb Key takeaway
Always calculate the anion gap and compare its rise with the fall in bicarbonate (the delta): if bicarbonate falls much less than expected, a metabolic alkalosis is also present. In pyloric obstruction with shock both coexist; restoring volume with 0.9% saline plus potassium corrects both.
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Updated: November 1, 2026 · Educational content for veterinary professionals. It does not replace current clinical guidelines or the judgement of the attending veterinarian.