a medical ventilator is disclosed for switching and mixing oxygen with air in a driving and oxygen mixing means and delivering the mixture to a gas flow control apparatus for application directly to a patient breathing apparatus or to storage in a bellows reservoir of a continuous positive airway pressure facility . the ventilator is equipped with a control module having directly set control adjustments for determining respiratory rate , inspiration - to - expiration ratio , oxygen concentration and the prescribed minute volume of the oxygen mixture to be delivered to a patient . the adjustment control circuitry which performs the calculations , logic and drive operations for switching and mixing oxygen in plural chambers of module , the switching of the mixture through module , and sensing fill and refill needs of reservoir . module includes apparatus for exerting a positive pressure on the oxygen in reservoir .