An implantable medical device comprising: a device module having a proximal end and a distal end, the device module comprising: a power source; a controller operably coupled to the power source, the controller configured to sense cardiac electrical activity via two or more electrodes (64, 65) and deliver pacing pulses via two or more electrodes (64, 65); a first part of a releasable connector; a fixation module comprising: a plurality of fixation elements for anchoring the fixation module to the patient's heart; a second part of the releasable connector, wherein the first part of the releasable connector and the second part of the releasable connector cooperate to releasably connect the device module with the fixation module; and the device module having one or more first fluoroscopic markers and the fixation module having one or more second fluoroscopic markers, wherein the one or more first fluoroscopic markers and the one or more second fluoroscopic markers are arranged so that a predefined alignment between one or more of the first fluoroscopic markers and one or more of the second fluoroscopic markers confirms that the releasable connector is in a fully connected state, characterised in that: the releasable connector comprises: one or more locking tabs; one more locking slots; wherein the one or more locking tabs are configured to be moved into the one or more locking slots against a bias mechanism, after which the one or more locking tabs are configured to be rotated relative to the one or more locking slots until one or more of the locking tabs are pushed into one or more retaining recesses by the bias mechanism, at which time the releasable connector is in the fully connected state, wherein at least one of the locking tabs and a location of at least one of the retaining recesses are marked by a corresponding fluoroscopic marker. a device module having a proximal end and a distal end, the device module comprising: a power source; a controller operably coupled to the power source, the controller configured to sense cardiac electrical activity via two or more electrodes (64, 65) and deliver pacing pulses via two or more electrodes (64, 65); a first part of a releasable connector; a power source; a controller operably coupled to the power source, the controller configured to sense cardiac electrical activity via two or more electrodes (64, 65) and deliver pacing pulses via two or more electrodes (64, 65); a first part of a releasable connector; a fixation module comprising: a plurality of fixation elements for anchoring the fixation module to the patient's heart; a second part of the releasable connector, wherein the first part of the releasable connector and the second part of the releasable connector cooperate to releasably connect the device module with the fixation module; and a plurality of fixation elements for anchoring the fixation module to the patient's heart; a second part of the releasable connector, wherein the first part of the releasable connector and the second part of the releasable connector cooperate to releasably connect the device module with the fixation module; and the device module having one or more first fluoroscopic markers and the fixation module having one or more second fluoroscopic markers, wherein the one or more first fluoroscopic markers and the one or more second fluoroscopic markers are arranged so that a predefined alignment between one or more of the first fluoroscopic markers and one or more of the second fluoroscopic markers confirms that the releasable connector is in a fully connected state, characterised in that: the releasable connector comprises: one or more locking tabs; one more locking slots; wherein the one or more locking tabs are configured to be moved into the one or more locking slots against a bias mechanism, after which the one or more locking tabs are configured to be rotated relative to the one or more locking slots until one or more of the locking tabs are pushed into one or more retaining recesses by the bias mechanism, at which time the releasable connector is in the fully connected state, wherein at least one of the locking tabs and a location of at least one of the retaining recesses are marked by a corresponding fluoroscopic marker. one or more locking tabs; one more locking slots; wherein the one or more locking tabs are configured to be moved into the one or more locking slots against a bias mechanism, after which the one or more locking tabs are configured to be rotated relative to the one or more locking slots until one or more of the locking tabs are pushed into one or more retaining recesses by the bias mechanism, at which time the releasable connector is in the fully connected state, wherein at least one of the locking tabs and a location of at least one of the retaining recesses are marked by a corresponding fluoroscopic marker.