A mechanical ventilator for ventilating a patient employs an air displacement member mounted for oscillating motion in a chamber; preferably the chamber is semi-cylindrical and the air displacement member is a vane mounted for oscillating movement about an axis of rotation such that a free outer edge surface of the vane is maintained in closely spaced apart relationship with the interior wall of the chamber throughout the oscillating; the ventilator permits significant variation in the ventilation flow waveform.