How a Pacemaker Works
Basic Mechanism
A pacemaker is an electronic device that delivers electrical impulses through electrodes to the heart muscle, causing it to contract and maintain an adequate heart rate when the heart's natural pacemaker (sinus node) fails or when electrical conduction is blocked. 1
The device consists of two essential components:
- Impulse generator: Contains the battery and electronic circuitry that creates the electrical signals 2
- Lead(s): Insulated wires that carry electrical impulses from the generator to the heart muscle 2
Sensing and Pacing Functions
Modern pacemakers perform two critical functions:
Sensing: The device continuously monitors the heart's intrinsic electrical activity through the leads 3. When the pacemaker detects a natural heartbeat, it inhibits its output to avoid competing with the heart's own rhythm 4.
Pacing: When no intrinsic heartbeat is detected within a programmed time interval, the device delivers an electrical impulse to stimulate cardiac contraction 5. This maintains heart rate above a minimum threshold, typically preventing rates from falling below 60 beats per minute 4.
Pacing Modes and Synchronization
The sophistication of modern pacemakers lies in their ability to maintain physiologic timing:
Single-chamber pacing (VVI): Paces and senses only the ventricle, delivering impulses when ventricular activity falls below the programmed rate 4. This is the simplest mode but lacks atrial-ventricular coordination 4.
Dual-chamber pacing (DDD): Paces and senses both atria and ventricles, maintaining the natural sequence of atrial contraction followed by ventricular contraction 4. This preserves the atrial contribution to ventricular filling, which can account for 15-30% of cardiac output 6.
Rate-responsive pacing: Incorporates sensors (detecting motion, minute ventilation, or other physiologic parameters) to increase heart rate during physical activity, mimicking the natural response of the sinus node 4.
Programmability and Optimization
All modern pacemakers are externally programmable, allowing cardiologists to adjust parameters without surgery 1, 3. Key programmable features include:
- Pacing rate: The minimum heart rate maintained by the device 3
- AV delay: The time interval between atrial and ventricular pacing, optimized to maximize ventricular filling 7
- Sensing thresholds: How sensitive the device is to detecting intrinsic cardiac activity 3
- Output energy: The strength of electrical impulses delivered 3
Clinical Importance of Mode Selection
The choice of pacing mode significantly impacts patient outcomes beyond simply maintaining heart rate 6. Dual-chamber pacing (DDD) reduces atrial fibrillation risk by 20% (HR 0.80), decreases stroke risk by 19% (HR 0.81), and prevents pacemaker syndrome in up to 83% of patients compared to single-chamber ventricular pacing 6.
Pacemaker syndrome occurs when ventricular-only pacing causes loss of AV synchrony, resulting in decreased stroke volume by up to 50% and increased left atrial pressure by up to 25% 6, 7. Patients experience dizziness, fatigue, and reduced exercise capacity 7.
Evolution and Current Technology
Pacemaker technology has evolved from simple fixed-rate devices to sophisticated multiprogrammable systems that can pace multiple chambers, respond to metabolic demands, and even combine defibrillation capabilities 8, 3. Current devices feature telemetry for remote monitoring and diagnostic algorithms to detect arrhythmias 8, 3.