An implantable medical device for treating a dissection, the device including: a graft element provided with: a first graft section of tubular form and having a first diameter; an intermediate graft section of generally tubular form and having a second diameter greater than the first diameter; a third graft section of tubular form and having a third diameter less than the second diameter, the intermediate graft section being disposed between the first and third graft sections; and a first transition graft section between the first and intermediate graft sections, and a second transition section between the intermediate and third graft sections; at least one first stent ring associated with the first graft section and having a first expanded operating diameter, at least one second stent ring associated with the intermediate graft section and having a second expanded operating diameter greater than said first expanded operating diameter, and at least one third stent ring associated with the third graft section; the device having a first expanded diameter in which the first, intermediate and third graft sections have substantially the same expanded diameters, the device further having a second expanded diameter in which the intermediate graft section is expanded to said second diameter, being greater than the expanded diameters of the first and third graft sections characterised in that the expanded intermediate graft section includes a side flush with the first and third graft sections. a graft element provided with: a first graft section of tubular form and having a first diameter; an intermediate graft section of generally tubular form and having a second diameter greater than the first diameter; a third graft section of tubular form and having a third diameter less than the second diameter, the intermediate graft section being disposed between the first and third graft sections; and a first transition graft section between the first and intermediate graft sections, and a second transition section between the intermediate and third graft sections; a first graft section of tubular form and having a first diameter; an intermediate graft section of generally tubular form and having a second diameter greater than the first diameter; a third graft section of tubular form and having a third diameter less than the second diameter, the intermediate graft section being disposed between the first and third graft sections; and a first transition graft section between the first and intermediate graft sections, and a second transition section between the intermediate and third graft sections; at least one first stent ring associated with the first graft section and having a first expanded operating diameter, at least one second stent ring associated with the intermediate graft section and having a second expanded operating diameter greater than said first expanded operating diameter, and at least one third stent ring associated with the third graft section; the device having a first expanded diameter in which the first, intermediate and third graft sections have substantially the same expanded diameters, the device further having a second expanded diameter in which the intermediate graft section is expanded to said second diameter, being greater than the expanded diameters of the first and third graft sections characterised in that the expanded intermediate graft section includes a side flush with the first and third graft sections.