methods for detecting the condition of a catheter providing access to a patient &# 39 ; s blood vessel are disclosed in which the blood is supplied to an extracorporeal blood flow circuit . the method includes generating a pressure wave on one side of one of the access points for supplying blood to the extracorporeal blood flow circuit or returning the blood to the patient , then sensing the pressure wave on the other side of the access points . apparatus for detecting the condition of the catheter are also provided .