An interventional medical system comprising a relatively compact implantable medical device and a catheter; the medical device comprising an electronic controller, a hermetically sealed housing containing the controller, an electrode electrically coupled to the controller, a fixation member mounted to a distal end of the housing, and a holding member, the holding member having a first contour, wherein the first contour forms a neck and an enlarged head, the neck extending longitudinally from a proximal end of the housing to the head; the catheter comprising an elongate inner assembly, an elongate outer tubular member, and a handle, the handle comprising a first control member and a second control member, the outer tubular member comprising a proximal end coupled to the first control member, the outer tubular member defining an elongate lumen that extends from a proximal opening thereof, at the proximal end of the tubular member, to a distal-most opening thereof, at a distal end of the outer tubular member, the distal-most opening allowing passage of the medical device therethrough, the inner assembly extending along, and being in sliding engagement within the lumen of the outer tubular member; and the catheter inner assembly comprising: an elongate shaft including a proximal end secured to the handle, and a distal end having a receptacle formed therein, the receptacle being sized to receive the device holding member and having an interior contour configured to mate with the contour of the device holding member when the holding member is received therein, and the receptacle including a laterally facing opening, the laterally facing opening having a profile that matches a longitudinal profile of the neck and the enlarged head of the first contour of the device holding member so that the laterally facing opening allows only a properly oriented passage of the holding member therethrough, the passage being without deformation of either the device holding member or the laterally facing opening of the receptacle; and a lock and release member extending alongside the elongate shaft, the lock and release member including a proximal end and a distal end, the proximal end of the lock and release member being coupled to the second control member of the handle, and the distal end comprising an arcuate sidewall in sliding engagement around the shaft; and wherein, when the second control member of the catheter handle is in a first position, the arcuate sidewall of the distal end of the lock and release member overlays the laterally facing opening of the shaft receptacle; when the second control member of the catheter handle is in a second position, the arcuate sidewall of the distal end of the lock and release member is located proximal to the laterally facing opening of the shaft receptacle; when the first control member of the catheter handle is in a first position, the distal end of the shaft and the lock and release member of the inner assembly is contained within the lumen of the outer tubular member of the catheter; and when the first control member of the catheter handle is in a second position, the inner assembly protrudes distally from the distal-most opening of the lumen of the outer tubular member of the catheter. an elongate shaft including a proximal end secured to the handle, and a distal end having a receptacle formed therein, the receptacle being sized to receive the device holding member and having an interior contour configured to mate with the contour of the device holding member when the holding member is received therein, and the receptacle including a laterally facing opening, the laterally facing opening having a profile that matches a longitudinal profile of the neck and the enlarged head of the first contour of the device holding member so that the laterally facing opening allows only a properly oriented passage of the holding member therethrough, the passage being without deformation of either the device holding member or the laterally facing opening of the receptacle; and a lock and release member extending alongside the elongate shaft, the lock and release member including a proximal end and a distal end, the proximal end of the lock and release member being coupled to the second control member of the handle, and the distal end comprising an arcuate sidewall in sliding engagement around the shaft; and wherein, when the second control member of the catheter handle is in a first position, the arcuate sidewall of the distal end of the lock and release member overlays the laterally facing opening of the shaft receptacle; when the second control member of the catheter handle is in a second position, the arcuate sidewall of the distal end of the lock and release member is located proximal to the laterally facing opening of the shaft receptacle; when the first control member of the catheter handle is in a first position, the distal end of the shaft and the lock and release member of the inner assembly is contained within the lumen of the outer tubular member of the catheter; and when the first control member of the catheter handle is in a second position, the inner assembly protrudes distally from the distal-most opening of the lumen of the outer tubular member of the catheter.