A ventilatory muscle training apparatus is disclosed which utilizes a gas rebreathing system having a reservoir and a mouthpiece for use in conveying gas into and out of the reservoir during inhalation and exhalation. An adjustable valve-like aperture is located in the system in a position to vent some...http://www.google.es/patents/US4301810?utm_source=gb-gplus-sharePatente US4301810 - Ventilatory muscle training apparatus
