The valve comprises a valve chamber 27 with a positive pressure port 34, a negative pressure port 32, a ventilation port 33 and an ambient port 31. In use a valve member 28 reversibly slides across a valve seat 37 containing the ports. A motor (38, fig 3) drives the valve member via gear 29 over a range of selected positions such that at any one time the ventilation port may receive variable negative or positive pressure conditions. The ports in the valve seat may have elongated or tear-drop shaped apertures (figs 5A-5D). The valve may be arranged as a rotary valve (figs 7-8) and is suitable for use in high frequency oscillation ventilation (HFOV) systems (figs 9-12).
