An apparatus to detect a hypopnea from a measured flow of breathable gas and a method of classifying the severity of a hypopnea are disclosed. The apparatus comprises a controller having at least one processor to access data representing a measured flow of breathable gas. The controller is further configured to: determine a long-term measure of ventilation from the measured flow； determine a short-term measure of ventilation from the measured flow； determine a threshold as a proportion of the long-term measure of ventilation； measure an area bounded by first and second crossings of the short-term measure of ventilation and the threshold (bounded area A)； and detect the hypopnea with the measured area. The controller is further configured to indicate a severity of the hypopnea with the measure of area
