The rhythms that occur in a patient with a DDD pacemaker depend upon the underlying heart rate and AV nodal conduction. The pacemaker spike is represented in blue. First panel: The pacemaker may be completely inhibited when the sinus rate is greater than the lower rate limit of the pacemaker. Second panel: Atrial sensing pacing occurs when there is intrinsic AV nodal delay which is greater than the AV delay in the pacemaker. Third panel: Atrial pacing occurs when the sinus rate falls below the lower limit of the pacemaker and AV nodal conduction is intact. Fourth panel: If there is sinus bradycardia and AV nodal conduction delay, the pacemaker paces both atrium and ventricle, known as AV sequential pacing.
