Cardiac Pacemaker: Timing the Heartbeat Electrically

The heart's own pacemaker

A healthy heart does not need an external signal to beat. The sinoatrial (SA) node, a cluster of specialised cells in the right atrium, has the fastest intrinsic rate of spontaneous depolarisation of any cardiac tissue and so sets the pace for the whole organ — around 60 to 100 beats per minute at rest. Its impulse spreads through the atria, is delayed briefly at the AV node to let the atria finish emptying, then races down the His-Purkinje system to trigger a synchronised ventricular contraction. An artificial pacemaker exists to take over one or more of these jobs when disease damages the SA node, the AV node, or the conduction pathway.

Demand pacing: only fire when needed

Early pacemakers fired at a fixed rate regardless of the heart's own activity — a design that can be dangerous, because a pacing pulse landing during the heart's vulnerable repolarisation window can trigger a lethal arrhythmia. Modern devices use demand pacing: they continuously sense intracardiac electrical activity through the same lead used to pace, and only deliver a pulse if no intrinsic beat is sensed within a timed window.

on every intrinsic beat sensed:  reset escape-interval timer
on timer reaching escape interval with no sensed beat:
  deliver pacing pulse  →  reset timer

escape interval = 60,000 / programmed lower rate limit (ms)

The escape interval is simply the maximum time the device will wait before assuming the heart needs help — set from the programmed lower rate limit, typically corresponding to 50-70 beats per minute. A patient whose SA node still works most of the time will rarely see a pacing spike; the device becomes a safety net rather than the primary driver.

The NBG code: what AAI, VVI and DDD mean

Pacing modes are described with a standardised five-letter code (NBG); the first three letters matter for most everyday cases:

Letter 1: chamber PACED     A=atrium  V=ventricle  D=dual (both)
Letter 2: chamber SENSED    A=atrium  V=ventricle  D=dual (both)
Letter 3: RESPONSE to a sensed event
          I = Inhibited (a sensed beat withholds the next pace)
          T = Triggered (a sensed beat immediately fires a pace)
          D = Dual (both inhibited and triggered responses used)

AAI  paces & senses the atrium only, inhibited by a sensed atrial beat
     → for sick sinus syndrome with intact AV conduction
VVI  paces & senses the ventricle only, inhibited by a sensed beat
     → simplest mode, used when atrial pacing is not needed/possible
DDD  paces & senses both chambers, both inhibited and triggered
     → closest artificial mimic of normal AV-synchronised conduction

AV delay and dual-chamber synchrony

A dual-chamber device like DDD adds a second timer: the AV delay, which mimics the natural pause the AV node imposes. After an atrial event (sensed or paced), the device waits out the AV delay; if no intrinsic ventricular signal arrives before it expires, the device paces the ventricle. This preserves the mechanically important sequence — atria contract first, ventricles follow — which raises cardiac output substantially over ventricle-only pacing, particularly during exercise when that sequencing matters more.

Reading the ECG: capture, spikes and heart block

A pacing pulse shows up on the ECG as a brief, narrow vertical spike immediately before the chamber's own waveform — a spike before a P wave for atrial pacing, a spike before a wide QRS complex for ventricular pacing (ventricular pacing produces a wide QRS because the impulse spreads through ordinary muscle rather than the fast Purkinje network). A spike followed by the expected waveform is called capture; a spike with no following waveform is failure to capture, usually from a dislodged lead or a pulse below threshold. Heart block — a delayed or absent conduction from atria to ventricles — is exactly the condition dual-chamber pacing is designed to bridge: when the AV delay times out without a sensed ventricular beat, the device steps in.

Frequently asked questions

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