The stem
A 55-year-old man has vomited large volumes of undigested food every day for three weeks and has lost weight. He is dehydrated, and a succussion splash is heard over the upper abdomen.
Laboratory reportSpecimen: blood
| Test | Result | Units | Reference |
|---|---|---|---|
| Sodium | 131 L | mmol/L | 135 – 145 |
| Potassium | 2.8 L | mmol/L | 3.5 – 5.0 |
| Chloride | 82 L | mmol/L | 98 – 107 |
| Bicarbonate | 36 H | mmol/L | 22 – 29 |
| Urea | 78 H | mg/dL | 17 – 43 |
| Creatinine | 1.6 H | mg/dL | 0.7 – 1.2 |
| pH (venous gas) | 7.53 H | 7.35 – 7.45 | |
| PCO2 | 48 H | mmHg | 35 – 45 |
Typical adult reference ranges. Always read results against your own laboratory’s ranges.
The examiner’s questions
Question 1Describe the abnormality.Show the model answer
A hypochloraemic, hypokalaemic metabolic alkalosis with partial respiratory compensation (raised PCO2), and pre-renal kidney injury from dehydration, with the urea raised more than the creatinine.
Question 2Explain why the potassium is low.Show the model answer
Hydrochloric acid is lost in the vomit, causing the alkalosis. Dehydration stimulates aldosterone, so the kidney holds on to sodium and excretes potassium and hydrogen ions in exchange. This worsens both the hypokalaemia and the alkalosis, and the urine can become acid despite the alkalosis (paradoxical aciduria).
Question 3What is the likely diagnosis, and what are its causes?Show the model answer
Gastric outlet obstruction. In an adult the main causes are cancer of the stomach antrum, scarring from a peptic ulcer near the pylorus, and pancreatic cancer compressing the duodenum.
Question 4How do you correct it?Show the model answer
Intravenous 0.9% sodium chloride with potassium chloride added according to local protocol, never as a bolus. A nasogastric tube to drain the stomach, with output measured and replaced. Correct the biochemistry before any operation, then arrange endoscopy.
After you answerThe teaching pointShow
Chloride is the key. Without enough chloride and volume the kidney cannot correct the alkalosis, which is why saline, not just potassium, is the treatment.
The patients and results are written for teaching and do not belong to real patients.