The stem
A 74-year-old woman with atrial fibrillation has had severe central abdominal pain for six hours. Her abdomen is soft and only mildly tender. An arterial blood gas is taken on room air.
Laboratory reportSpecimen: blood
| Test | Result | Units | Reference |
|---|---|---|---|
| pH | 7.24 L | 7.35 – 7.45 | |
| PaCO2 | 28 L | mmHg | 35 – 45 |
| PaO2 | 88 | mmHg | 80 – 100 |
| Bicarbonate | 12 L | mmol/L | 22 – 26 |
| Base excess | −14 L | mmol/L | −2 – +2 |
| Lactate | 7.2 H | mmol/L | 0.5 – 2.2 |
Typical adult reference ranges. Always read results against your own laboratory’s ranges.
The examiner’s questions
Question 1Interpret the blood gas.Show the model answer
A metabolic acidosis (low pH, low bicarbonate, base deficit) with a high lactate, and respiratory compensation (low PaCO2). Oxygenation is normal.
Question 2Is the respiratory compensation appropriate?Show the model answer
Yes. By Winter’s formula the expected PaCO2 is 1.5 × bicarbonate + 8 ± 2, that is 24 to 28 mmHg. The measured 28 mmHg is within that range.
Question 3What is the most likely diagnosis?Show the model answer
Acute mesenteric ischaemia, most likely an embolus from atrial fibrillation. Severe pain with few abdominal signs, “pain out of proportion”, is the classic presentation.
Question 4What are your next steps?Show the model answer
Resuscitation with fluids and oxygen, urgent CT angiography, heparin on senior advice, and immediate surgical and vascular review for revascularisation or laparotomy.
After you answerThe teaching pointShow
A raised lactate means tissue is short of oxygen. A normal lactate does not exclude early bowel ischaemia, so the history must still lead.
The patients and results are written for teaching and do not belong to real patients.