An implantable medical device configured to detect physiological events, comprising: a plurality of first memory locations (210a, 210b, 210c) allocated to each of a plurality of physiological event types; a second single memory location adapted to store entries of physiological signal data corresponding to all of the plurality of physiological event types; an allocation table configured to form a plurality of fields; and a control module configured to store event data in a memory location of the plurality of first memory locations in response to detecting a physiological event of a first event type, the memory location of the plurality of first memory locations corresponding to the first event type, store an entry of physiological signal data in the second memory location, and enter a location of the stored entry of physiological signal data in an allocation table field linked to the stored event data in the memory location of the plurality of first memory locations; and characterized in that storing the entry of physiological signal data comprises: means adapted to overwrite a previously stored entry of physiological signal data in response to the single second memory location being full; and wherein the control module is adapted to identify the previously stored entry as an oldest stored entry of an event type for which at least a predetermined number of previously stored entries of the same event type as the oldest stored entry will remain if the oldest stored entry is overwritten. a plurality of first memory locations (210a, 210b, 210c) allocated to each of a plurality of physiological event types; a second single memory location adapted to store entries of physiological signal data corresponding to all of the plurality of physiological event types; an allocation table configured to form a plurality of fields; and a control module configured to store event data in a memory location of the plurality of first memory locations in response to detecting a physiological event of a first event type, the memory location of the plurality of first memory locations corresponding to the first event type, store an entry of physiological signal data in the second memory location, and enter a location of the stored entry of physiological signal data in an allocation table field linked to the stored event data in the memory location of the plurality of first memory locations; and characterized in that storing the entry of physiological signal data comprises: means adapted to overwrite a previously stored entry of physiological signal data in response to the single second memory location being full; and wherein the control module is adapted to identify the previously stored entry as an oldest stored entry of an event type for which at least a predetermined number of previously stored entries of the same event type as the oldest stored entry will remain if the oldest stored entry is overwritten. means adapted to overwrite a previously stored entry of physiological signal data in response to the single second memory location being full; and wherein the control module is adapted to identify the previously stored entry as an oldest stored entry of an event type for which at least a predetermined number of previously stored entries of the same event type as the oldest stored entry will remain if the oldest stored entry is overwritten.