Patent ID: 7532929

Claim:
An implantable medical device, said implantable medical device comprising: a housing; an atrial sensing stage; a ventricular sensing stage; a ventricular stimulation pulse generator; an atrial stimulation pulse generator; a ventricular pacing pulse timer; an atrial tachycardia/fibrillation detector; a ventricular rate determination stage; a mode switching stage, wherein said mode switching stage causes said ventricular pacing pulse timer to switch from an atrial synchronized stimulation mode to a non-synchronized stimulation mode; and a PIZ determination stage for determining a physiological interval zone (PIZ) which determines a range of appropriate vetricular escape interval (VEI) duration values for VEI determination, and which is defined by an upper limit (ULPIZ) and a lower limit (LLPIZ), wherein said upper limit defines a longest VV-interval of the physiological interval zone, and wherein said lower limit defines a shortest VV-interval of the physiological interval zone, and wherein said physiological interval zone extends between said upper limit and said lower limit, and wherein, in an atrium-synchronous mode, a timing of ventricular pacing pulses is determined by an AV-interval triggered by an atrial event associated to an atrial contraction and wherein, in a non-synchronous ventricular pacing mode, said timing of ventricular pacing pulses is determined by said ventricular escape interval determined by a ventricular rate determination stage of said implantable medical device, and wherein the PIZ determination stage is operationally connected to said ventricular rate determination stage and wherein the ventricular rate determination stage is adapted to generate a ventricular escape interval duration value on a beat-to-beat basis for each cardiac cycle based upon a VV-interval duration of a preceding cardiac cycle with respect to the actual physiological interval zone upper and lower limits.