Interpretation of complex demand pacemaker arrhythmias.

Accurate knowledge ofpacemaker specifications provides the key to the understanding of complex demand pacemaker arrhythmias. This communication describes two examples of complex pacemaker arrhythmias related to the sensing mechanism of normally functioning Medtronic demand pulse generators. Oversensing of atrial voltage generated by an atrial parasystolic focus produced the first arrhythmia. The second arrhythmia was due to partial recycling (partial sensing) of an atrial demand pacemaker by an adequate QRS signal delivered in the 'relative refractory period' of the pacemaker, a short period that follows the refractory period after the emission of the pulse. These two cases illustrate the negligible refractory period after sensing in implantable Medtronic demand pacemakers. This feature enables such a pacemaker to detect two signals such as P and QRS in quick succession.