How a Pacemaker Works: A Complete, Clear, and Clinically Accurate Guide for Healthcare Professionals

A pacemaker is a small electronic device that keeps the heart beating at a safe and stable rhythm when the body’s own electrical system fails. In this complete, clinically accurate guide, you’ll learn exactly how a pacemaker works, when it is used, and what healthcare professionals should look for during a pacemaker check - including sensing, capture thresholds, and lead impedance. Whether you’re a clinician, student, or simply curious, this article provides a clear and reliable explanation of the mechanisms behind modern cardiac pacing.

Pacemakers are used to treat bradycardia and disorders of the cardiac conduction system. This overview provides a clear explanation of how pacemakers work, their main indications and the essential concepts clinicians should understand, especially DDD mode and the AV interval.

Pacemaker battery - individual headers for atrial and ventricular leads.

What is a pacemaker?

A pacemaker is a small electronic device implanted under the skin, usually beneath the left clavicle. Its purpose is to maintain an adequate heart rate when the intrinsic rhythm is too slow or when electrical signals between the atria and ventricles fail to conduct properly.

System components

A pacemaker consists of:

Pulse generator

Contains the battery, processor and telemetry system. It is implanted subcutaneously.

Leads (electrodes)

Inserted through a vein, commonly the axillary or cephalic vein, and advanced into the heart. The leads sense the heart’s intrinsic electrical activity and can deliver pacing stimuli when needed.

Why do patients need a pacemaker?

Common indications include:

• Sick sinus syndrome, where the heart’s natural pacemaker functions poorly

• AV block, where conduction from atrium to ventricle is impaired

• Heart failure with electrical dyssynchrony

• Permanent atrial fibrillation with slow ventricular response

  
  

Pacemaker modes: what DDD and VVI mean

Mode names consist of three letters:

• First letter: chamber paced (A = atrium, V = ventricle, D = both)

• Second letter: chamber sensed

• Third letter: response to sensing (I = inhibit, D = trigger or inhibit)

  
  

VVI: simple and robust

In VVI mode, the pacemaker senses and paces only the ventricle. It activates when the interval between intrinsic beats becomes too long.

Advantages:

• Simple and reliable

• Suitable for permanent atrial fibrillation

Limitations:

• No coordination between atria and ventricles (no AV synchrony)

  
  

DDD: more physiologic pacing

DDD mode is the standard option for most cases of sick sinus syndrome and AV block. It promotes a rhythm that mimics the heart’s natural conduction pattern.

How DDD mode works

• One lead is placed in the right atrium and one in the right ventricle.

• When the pacemaker senses an atrial event (a P wave), it starts an internal timing cycle: the AV interval, typically 120 to 180 ms.

• If no intrinsic ventricular activation occurs within this time, the pacemaker delivers a ventricular pacing stimulus so that the ventricle follows the atrium.

This is called tracking. The pacemaker follows the atrial rhythm and maintains correct timing between atrium and ventricle. DDD mode is therefore a tracking mode.

Advantages of DDD

• Maintains AV synchrony

• Improves ventricular filling and cardiac output

• Better exercise tolerance because the ventricle can follow atrial rate during activity

• Can switch between modes as needed (mode switch)

  
  

Understanding the AV interval

The AV interval is the waiting time after an atrial event before the device paces the ventricle if necessary. It mimics the natural delay of the AV node.

Conceptually:

• The ventricles need time to fill after atrial contraction.

• The AV interval provides this pause.

• If no intrinsic ventricular activation occurs during the interval, the pacemaker delivers a ventricular stimulus.

  
  

Max Tracking Rate (MTR)

DDD pacemakers have an upper limit on how many atrial beats per minute they will follow. This prevents excessively rapid ventricular pacing during atrial arrhythmias such as atrial flutter. The MTR is programmable, commonly 120 to 130 bpm in older patients, and higher in younger individuals.

If the atrial rate exceeds the programmed MTR, the atrium and ventricle will no longer conduct in a one to one pattern. Wenckebach behaviour can appear, such as 4:3 conduction. With further rate increase, two to one conduction may occur. This is often referred to as "upper-rate behaviour".

Mode switch (MS)

Mode switch is a safety feature in dual chamber pacemakers. It automatically stops tracking atrial activity when the patient enters a rapid atrial arrhythmia such as atrial fibrillation or atrial flutter.

In normal DDD mode, the pacemaker tracks atrial events. During an atrial arrhythmia, atrial rates become very high. If the device continued to track this, it would attempt to pace the ventricle at the same high rate, which could lead to dangerous pacing.

To prevent this, the device switches to a non tracking mode such as DDI.

As a result:

• The ventricle is protected from excessively high pacing rates

• The patient avoids uncomfortable or hemodynamically harmful rapid pacing

• The device returns to DDD automatically when sinus rhythm resumes

  
  

In short, mode switch prevents the ventricle from following chaotic atrial activity during atrial tachyarrhythmias.

What Is Assessed During a Pacemaker Check? (For Healthcare Professionals)

Pacemakers are routinely followed up to ensure both technical integrity and appropriate clinical function. During a device check, clinicians review:

• Recorded arrhythmia episodes (e.g., atrial fibrillation, high ventricular rates, noise on leads)

• Overall heart rate trends and pacing burden

• Estimated battery longevity

• Any alerts or safety notifications

These findings are generally intuitive. The aspects that most often create uncertainty are the technical parameters:sensitivity (amplitude), capture threshold, and impedance. Below is a concise and clinically practical explanation of each.

1. Amplitude / Sensitivity (Sensing)

What does it represent? Sensing measures the pacemaker’s ability to detect the heart’s intrinsic electrical activity. Values are expressed in millivolts (mV).

Typical acceptable sensing values:

• Atrial sensing: > 2 mV

• Ventricular sensing: > 5 mV

(These are practical reference points, not absolute cutoffs.)

Interpretation:

• Higher amplitude = better sensing → the device clearly detects intrinsic activity

• Low amplitude = risk of undersensing → may trigger inappropriate pacing or missed events

Clinical importance: Adequate sensing is essential for the device to inhibit pacing appropriately and avoid unnecessary impulses.

2. Capture Threshold

What does it represent? The capture threshold is the minimum amount of energy required for the pacemaker’s impulse to produce a true myocardial contraction.

Expressed as voltage with associated pulse width, e.g.:1.0 V @ 0.4 ms

Interpretation:

• Low threshold is ideal → effective myocardial response to pacing

• High threshold may indicate: lead micro-dislodgement– fibrosis around the lead tip– myocardial disease– transient postoperative changes (common in the early weeks)

  
  

A practical benchmark: A threshold around 1.0 V / 0.4 ms is considered excellent.

Clinical importance: Low thresholds reduce battery consumption and ensure reliable pacing.

3. Impedance

What does it represent?Impedance reflects electrical resistance in the pacing circuit: generator → lead → myocardium → return path.Values are expressed in ohms (Ω).

Typical normal range:~300–1,000 Ω, depending on lead design and manufacturer.

Interpretation:The emphasis is on trend over time, not isolated measurements.

• Stable impedance → good mechanical integrity of the lead and consistent contact

• Low impedance → may indicate insulation breach (possible short circuit)

• High impedance → may indicate conductor fracture, poor contact, or loose connection

  
  

Clinical importance: Impedance is primarily a marker of lead integrity and mechanical stability, not real-time physiologic function.

When is a pacemaker functioning well technically?

A pacemaker is considered technically sound when:

• Sensing values are adequate

• Capture thresholds are low and stable

• Impedance is stable and within a reasonable range

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If the patient still experiences symptoms despite normal technical parameters, consider:

• Suboptimal programming (e.g., incorrect mode, AV delays, LRL settings)

• Device algorithms that do not fit the patient’s rhythm

• Underlying arrhythmias or physiologic issues not corrected by pacing

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This requires a comprehensive clinical evaluation beyond the device interrogation.

What a pacemaker does not do

A pacemaker is designed to treat bradycardia and impaired conduction between atria and ventricles. It does not treat or correct tachyarrhythmias, including:

• Atrial fibrillation with rapid ventricular response

• Supraventricular tachycardias

• Ventricular arrhythmias

Management of these conditions may require:

• Medication

• Catheter ablation

• ICD or CRT-D in cases of ventricular arrhythmias

  
  

Summary: key points for clinicians

• Pacemakers support patients with bradycardia and conduction disease.

• VVI is often chosen for atrial fibrillation, while DDD is preferred when AV synchrony is needed.

• DDD mode provides a more physiologic rhythm by allowing the ventricle to follow the atrium through the AV interval.

• Understanding pacing modes, tracking and the AV interval improves interpretation of pacemaker ECGs and patient symptoms.