Ensure that the sheet is the correct size to fit a deep-cell, air-filled mattress so that stretching or ‘hammocking’ does not compromise the benefit of pressure-redistribution associated with cell deflation. Use a 4-way stretch sheet for minimal interference of cell deflation in active/alternating therapy surfaces.  Make sure the sheet does not hamper observation and access to mattress controls such as Wound Valves. The sheet must not interfere with access, visibility or operation of CPR controls. If using a fitted sheet for the first time, with a patient previously nursed on a loose non-fitted sheet, do not assume that repositioning and skin assessment intervals will remain the same.