SA-node rhythm vs artificial pacing · VVI / AAI / DDD · demand & asynchronous modes
An artificial pacemaker delivers a small electrical stimulus that captures heart tissue and forces a beat whenever the heart's own pacemaker (the sino-atrial node) is too slow or conduction has failed. Each chamber tracked by the device has an escape interval — the longest the pacemaker will wait for an intrinsic beat before pacing. The escape interval is simply 60000 / pacing rate milliseconds.
Demand pacing (sensing ON): the device watches for the heart's own activity. If an intrinsic beat is sensed before the escape interval elapses, the pacemaker is inhibited and resets its timer. Asynchronous pacing (sensing OFF) ignores the heart and paces at a fixed rate regardless — useful conceptually but risky because a paced spike can land on a vulnerable part of the cardiac cycle.
Modes: VVI paces and senses the ventricle; AAI paces and senses the atrium; DDD is dual-chamber — it senses the atrium, waits an AV delay, then paces the ventricle if intrinsic AV conduction fails, preserving the natural atrial-then-ventricular sequence. Try the Complete heart block preset: the atria beat on their own but no impulse reaches the ventricles, so the pacemaker rescues every ventricular beat.