An inflatable mattress 1 configured to reduce the incidence of bed sores by providing intermittent pressure relief to a patient sleeping on it. The mattress comprises adaptations to improve the pressure relief provided around the vulnerable heel area including active deflation of the cells (A, B, figure 4) in the heel region and passive deflation in the body region. The mattress further comprises first and second compressors (11, figure 5), where the second compressor is responsible for active evacuation of cells in the heel region. A pump unit suitable for an alternating pressure mattress, comprising first and second compressors, a plurality of valves and a control unit is further disclosed.
