<prompt> Pressure sores usually occur when external pressure exceeds capillary occlusive pressure over20 mmHg25 mmHg30 mmHg35 mmHg  <response>: When external pressure exceeds the capillary occlusive pressure over 30mmHg, blood flow to skin ceases leading to tissue anoxia, necrosis and ulceration. Ref: Bailey and love 27th edition Pg no: 29 