This is usually defined using duplex ultrasound which can measure flow in individual veins. Refluxive veins are defined as reverse flow in a superficial or saphenous vein which lasts more than half a second. Refluxive deep veins are defined when this reverse flow lasts more than 1 second. The normal direction of flow in a vein is antegrade and towards the heart. Refluxive flow is retrograde and away from the heart. If the valves in veins do not function they can leak causing refluxive flow. This results in impaired venous drainage. If the net flow in a vein is more refluxive than antegrade then the drainage impairment is usually severe. Measuring net flow in individual veins and within all leg veins simultaneously is a current challenge in venous research.