In an implantable pacemaker, pacing pulses are delivered to a ventricle in a P-wave synchronous mode as long as no atrial arrhythmia is detected, pacing pulses and a mode switch is made to deliver pacing pulses to the ventricle in a non-P-wave synchronous mode if atrial arrhythmia is detected. From an impedance signal measured in the atrium, atrial distention is determined and, in the non-P-wave synchronous mode, the delivery rate of the pacing pulses is increased to decrease the atrial distention during atrial arrhythmia.