A hypopnea detection apparatus (11) comprises a controller having at least one processor to access data representing a measured flow of breathable gas. The controller (104) is further configured to determine a first measure from a short-term measure of ventilation of the data； determine a second measure from a long-term measure of ventilation of the data； compare the first measure with first and second proportions of the second measure； and indicate a detection of the hypopnea based on the comparing if the first measure falls below the first proportion and does not subsequently exceed a range of the first and second proportions during a first time period. The short-term measure of ventilation is a short term variance of the data and the long-term measure of ventilation is a long-term variance of the data.
