Lab case 410 interepretation


PH = 7.56, that is moderate alkalaemia

We have low pCO2 (34 mmHg) and high HCO3 (30 mmol/L), both these values can lead to alkalosis. Most likely combined alkalosis. However, it will be easier to calculate the compensation with metabolic part (Then we don’t need to think acute vs chronic) and ultimately we will end up with the same results.

To calculate the expected compensation for metabolic alkalosis, we use the following formula:

Expected pCO2 = 0.7 x HCO3 + 20 (+/- 5)

For the is patient the expected compensation will be 41 mmHg, with a normal range between 36 to 46. This patients pCO2 is (34 mmHg), below the expected normal range so she has additional respiratory alkalosis.

It is a good practice to calculate the anion gap in every blood gases. So, anion gap for this patient. Anion gap is calculated as AG = Na – (Cl + HCO3) = 13. So we have additional HAGMA.

This patient has K and Cl levels are in the low side of the normal range. Combing this with metabolic alkalosis. This tells us that most likely the cause of this patient’s metabolic alkalosis is upper GI vomiting.

Question 2:

Most common causes of vomiting post colonoscopy are medication side effects and distention of the proximal colon that lead to stimulation of the vomiting centres via the vagus nerve afferent fibres.

Treatment, still the same:

  • Anti-emetics that work primarily by suppressing the vomiting centres (Ondansteron or stemetil)
  • Hydration and electrolytes replacement
  • Look for other possible causes of vomiting (i.e UTI)