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1. A device for providing resistance in an air pathway for a patient exhaling, the device comprising:
a) a central tube region; b) a first passageway for passing air into said central tube region when a patient breathing through the device inhales; c) a second passageway for passing air out of said central tube region when a patient breathing through the device exhales; d) a third passageway for passing air from said central tube region and into a patient when the patient breathing through the device inhales, and for passing air from said patient to said central tube when the patient breathing through the device exhales; e) a valve in the first passageway that allows air to flow in through the first passageway to the central tube when a patient using the device inhales, and that prevents air from flowing out through the first passageway when a patient using the device exhales; f) a valve in the second passageway that allows air to flow out from the second passageway when a patient using the device exhales with an expiratory air pressure greater than a selected pressure, that prevents air from flowing out through the second passageway when a patient exhales with an expiratory air pressure that is less than said selected air pressure, and that prevents air from flowing in through the second passageway when a patient using the device inhales, wherein said valve in said second passageway includes: i) a stopper to close the passageway and prevent air from flowing through the passageway when the stopper is biased to its closed position, and ii) a stopper-biasing spring to maintain the stopper in a fixed and closed position unless the expiratory air pressure in the passageway is greater than a selected pressure; wherein the volume available for expiratory air to occupy remains fixed and constant as long as the valves in the first and second passageways are closed, and wherein said first passageway, said second passageway, and said third passageway are separate and distinct from each other.
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