endotension , the continued expansion and tensioning of a blood vessel at an aneurysm site after the repair of the aneurysm by placement of a stent graft therethrough , typically caused by refilling of the aneurysmal location by blood leakage thereto , is detected . the detection is accomplished by locating a detector in the aneurysmal vessel , between the blood vessel wall and the stent graft , and monitoring signals generating therefrom which are indicative of the arrival of fresh blood in the region , and thus a leakage of blood therein . the detector may be an oxygen sensor , and the monitoring can include maintaining a history of periodic blood oxygen signals indicative of the oxygen content of blood between the stent graft and the vessel wall , and the periodic analysis of these signals to determine whether the oxygen level is increasing , indicating the existence of endoleak and thus likely endotension .