The device is designed for alternatingly supplying air at superatmospheric and respectively at subatmospheric pressure into a variable pressure chamber of a lungs ventilator for pulsingly and meteredly supplying fresh gas into a patient's lungs. The device comprises a pump having a rotary piston adapted to unidirectionally and pulsingly feeding air at a rate whose variation in each pulse is substantially defined by the function of "sin2 alpha" ("alpha" being the amplitude of the rotational motion), and a negative pressure limiting valve including a valve member electromagnetically biased by a biasing force which decreases with the second power of the displacement of said member, to provide a venting passage the cross-sectional area of which varies concurrently with the speed of air through said passage for levelling the variable resistance encountered by said air traversing said passage at different speeds.
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