In glycemic emergencies, an ETCO2 reading of less than 20 is indicative of which condition?

• A. Hyperosmolar state
• B. Hyperglycemia due to infection
• C. Diabetic ketoacidosis
• D. Lactic acidosis

Answer: (C)

A very low end-tidal CO2 in a glycemic emergency points to a metabolic acidosis with strong respiratory compensation. In diabetic ketoacidosis, insulin deficiency causes the production of ketoacids, creating a high anion-gap metabolic acidosis. To counteract the acidity, the patient hyperventilates (Kussmaul breathing), blowing off CO2. Because end-tidal CO2 reflects how much CO2 is being eliminated, this compensatory breathing drives ETCO2 down—often to well below 20 mmHg.

The other scenarios don’t typically present with this level of CO2 loss. Hyperosmolar states have little ketosis and only mild acidosis, so ETCO2 remains higher. Infections or other causes of hyperglycemia can raise glucose but don’t cause the same profound ketogenic acidosis with such marked CO2 washout. Lactic acidosis can lower ETCO2, but in the context of a glycemic emergency, the distinctive, severe drop points to diabetic ketoacidosis.