A cannula for providing respiratory therapy to a patient includes a first nasal prong having a proximal end attached to a cannula body and a distal end for insertion into a nare of the patient. The first nasal prong defines a lumen for a flow of breathing gas from a source of breathing gas to the nare of the patient, and the first nasal prong has a variable geometry such that a cross-sectional area of the lumen at the distal end of the first nasal prong varies with a flow rate of the breathing gas. Varying the cross-sectional area of the first nasal prong lumen with the flow rate of the breathing gas enables the first nasal prong to maintain a high velocity flow to the nare for effective flushing of the patient's airway.