Abstract: A portable positive displacement respirator/ventilator is provided for both a pressure sigh and a volume sigh of predetermined duration and frequency by providing a variable length crank arm connecting a piston through a connecting rod by mounting the connecting rod on a circular toothed gear, pivoting the gear for movement at its center and driven on its outer edge with a separate motor driven pivot gear which when moved relocates the effective length of movement of the point of attachment of the connecting rod thereby changing the volume swept by the piston for the same arc of reciprocal movement of the larger circular toothed gear enabling both a volume change or a volume sigh with the device. A cooperating multiplicity of adjustable pressure regulating valves where at least one can be intermittently closed during the operation of the device regulates ventilation pressure or breathing pressure and sigh pressure.