What Does a Pacemaker Do?
A pacemaker is an implantable electronic device that delivers electrical impulses to the heart muscle to maintain an adequate heart rate when the heart's natural pacemaker is too slow or when there is a block in the heart's electrical conduction system. 1, 2
Primary Functions
A pacemaker continuously monitors the heart's intrinsic electrical activity and delivers pacing stimuli only when needed to prevent bradycardia (abnormally slow heart rate). 3, 4 The device uses electrodes (leads) positioned in contact with the heart chambers to both sense the heart's own electrical signals and deliver electrical impulses when the intrinsic rate falls below a programmed threshold. 2, 5
Core Mechanisms
Sensing Function: The pacemaker detects the heart's own electrical activity through the leads, monitoring for spontaneous depolarizations in the atrium, ventricle, or both chambers depending on the pacing mode. 3, 6
Pacing Function: When no intrinsic cardiac activity is sensed within the programmed interval, the device delivers an electrical stimulus to trigger a heartbeat, ensuring the heart rate does not fall below a predetermined minimum. 3, 4
Inhibition Response: If the patient's own heart rate exceeds the programmed lower rate limit, the pacemaker remains inhibited and does not deliver unnecessary pacing impulses. 3
Clinical Indications and Benefits
Pacemakers are primarily implanted to treat symptomatic bradyarrhythmias, including sinus node dysfunction, acquired atrioventricular (AV) block, and certain fascicular blocks. 1 The American College of Cardiology and American Heart Association identify symptomatic bradycardia as the most common indication, defined as documented bradyarrhythmia directly responsible for syncope, near-syncope, dizziness, confusion from cerebral hypoperfusion, fatigue, reduced exercise capacity, or congestive heart failure. 1
Proven Clinical Outcomes
Symptom Relief: Pacemaker therapy relieves arrhythmia symptoms such as fatigue, fainting, and exercise intolerance. 1, 2
Quality of Life: Multiple studies document that pacing improves quality of life and enables patients with abnormal heart rates to resume more active lifestyles. 1, 2
Survival Benefit: Observational studies strongly suggest that permanent pacing improves survival in patients with complete AV block, especially when syncope has occurred. 1
Heart Failure Management: Biventricular pacing (cardiac resynchronization therapy) has been shown effective for advanced heart failure in patients with major intraventricular conduction defects, predominantly left bundle branch block. 1
Types of Pacing Modes
Modern pacemakers are programmable devices that can pace and sense in different cardiac chambers. 6, 7 The American College of Cardiology uses a standardized code where the first letter indicates the chamber(s) paced, the second letter indicates the chamber(s) sensed, and the third letter indicates the response to sensing. 6
Common Pacing Configurations
Single-Chamber Ventricular (VVI): Paces only the ventricle and is inhibited by sensed ventricular activity; used when atrial contribution is not hemodynamically significant, such as in permanent atrial fibrillation. 1, 6
Single-Chamber Atrial (AAI): Paces only the atrium and is inhibited by sensed atrial activity; appropriate for sinus node dysfunction when AV conduction is intact. 1, 6
Dual-Chamber (DDD): Paces and senses both chambers, maintaining AV synchrony across a wide range of heart rates; preferred in older patients because it preserves the atrial contribution to ventricular filling. 1, 6
Rate-Responsive Modes: Devices with an "R" designation (e.g., VVIR, DDDR) use physiologic sensors to adjust pacing rate based on physical activity or metabolic demand, simulating the natural sinus node response. 1
Critical Clinical Considerations
Maintaining atrioventricular synchrony is essential for optimal hemodynamics, particularly in older patients and those with ventricular dysfunction. 1, 3 Loss of AV synchrony can reduce cardiac output by up to 50% because the atrial contribution to ventricular filling is eliminated. 3
Pacemaker Syndrome
Definition and Mechanism: Pacemaker syndrome occurs when AV synchrony is lost, resulting in retrograde ventriculoatrial conduction, cannon A waves, and inadequate cardiac output. 1, 3
Clinical Manifestations: Patients experience fatigue, dyspnea, dizziness, orthostatic hypotension, light-headedness, or syncope; this syndrome affects 30-50% of patients who retain intact atrial function when AV synchrony is lost. 3, 6
Prevention: Dual-chamber pacing modes that preserve AV synchrony should be used instead of ventricular-only pacing when the sinus node functions normally. 3, 6
Common Pitfalls to Avoid
Do not use ventricular-only (VVI) pacing in patients with normal sinus node function, as this eliminates AV synchrony and can precipitate pacemaker syndrome or worsen heart failure, especially in those with dilated ventricles. 3, 6
Do not assume a simple pacing mode is adequate without considering the patient's activity level and chronotropic competence; rate-responsive pacing is generally preferred because it more closely simulates physiologic sinus node function. 1
Do not overlook symptoms of inadequate cardiac output (fatigue, dyspnea, dizziness, orthostatic hypotension) in paced patients, as these warrant reassessment of the pacing strategy and possible mode change. 3