An implantable medical device for delivering a therapeutic substance to a delivery site in a patient. A reservoir holds a supply of the fluid therapeutic substance. A catheter has a proximal end, a delivery region and a lumen extending from the proximal end to the delivery region. The proximal end of the catheter is operatively coupled to the reservoir. The delivery region of the catheter is adapted to be placed proximate the delivery site in the patient. The therapeutic substance is adapted to be delivered through the lumen to the patient. A sensing device is operatively coupled with the lumen of the catheter being capable of detecting a pressure of the therapeutic substance in the lumen. A controller is operatively coupled to the sensing device, the controller being capable of taking an action in response to the pressure in the lumen.