How do pacemakers work?

How Cardiac Pacemakers Work

Cardiac pacemakers are electronic devices that deliver electrical impulses to the heart muscle through electrodes to regulate heart rhythm when the heart's natural pacemaker is not functioning properly. 1

Basic Functionality

• Pacemakers deliver intermittent electrical impulses to the heart muscle, stimulating it to contract and ensuring a steady heart rate 2
• The primary purpose is to maintain adequate heart rate when the natural pacemaker is too slow or there is a block in the heart's electrical conduction system 1
• Modern pacemakers are externally programmable, allowing cardiologists to select optimum pacing modes for individual patients 1

Types of Pacemakers

Based on Duration:

• Temporary pacemakers: Used to treat short-term heart problems such as slow heartbeat caused by heart attack, heart surgery, or medication overdose 1
• Permanent pacemakers: Used to control long-term heart rhythm problems 1

Based on Chambers Paced:

• Single chamber pacemakers: For use in either atrium or ventricle 3
• Dual chamber pacemakers: For use in both atrial and ventricular chambers (usually programmable to single chamber modes as well) 3

Pacing Modes

Pacemakers use a standardized code system to describe their functionality:

Atrial Pacing (AAI):

• Atrial pacing inhibited by sensed atrial activity 3
• Used for symptomatic sinus node dysfunction when AV conduction is adequate 3

Ventricular Pacing (VVI):

• Ventricular pacing inhibited by sensed spontaneous ventricular activity 3
• Appropriate for symptomatic bradyarrhythmias, especially with no significant atrial hemodynamic contribution 3

VDD Mode:

• Uses a single-lead system with ventricular pacing electrodes at the tip and atrial sensing electrodes on the lead body 4
• Only stimulates the ventricle but detects both atrial and ventricular activity 4
• Maintains AV synchrony by tracking natural atrial activity and pacing the ventricle after a programmed AV interval 4

Dual-Chamber Pacing (DDD):

• Provides synchronous atrial-ventricular contraction in symptomatic bradycardia 3
• Maintains AV synchrony which is particularly important in older patients and those with impaired ventricular function 3

Adaptive Rate Pacing

• Adaptive rate (rate-responsive) pacemakers utilize sensors that respond to physical, chemical, or electrical signals to adjust heart rate 3
• These devices permit heart rate increases in the absence of appropriate spontaneous rate increases, such as in chronotropic incompetence 3
• Sensors may detect various parameters:
  • Activity/motion through vibration sensing
  • Respiratory rate or minute ventilation
  • Right ventricular stroke volume
  • Temperature changes
  • Oxygen saturation
  • Electrical phenomena (impedance variations, intracardiac potentials) 3

Clinical Benefits

• Pacemakers can relieve arrhythmia symptoms such as fatigue and fainting 1
• They allow patients with abnormal heart rhythms to resume more active lifestyles 1
• Long-term absence of AV synchrony increases the incidence of atrial fibrillation and stroke and may reduce life expectancy, particularly in patients with impaired ventricular function 3
• Rate-responsive devices are generally preferred because they more closely simulate the physiologic function of the sinus node 3

Advanced Features

• Modern pacemakers provide telemetry of stored data that can provide information about pacemaker function and clinical performance 3
• Many incorporate additional adaptive functions:
  • Differing AV intervals after paced or sensed P waves
  • Shortening of AV interval with increasing heart rate
  • Extension of atrial refractory period after ventricular premature beats 3
• Some specialized pacemakers can be utilized for noninvasive electrophysiologic and antiarrhythmic studies 3

Important Considerations

• Proper device selection should consider the cardiac conduction abnormality, comorbidities, presence of coronary heart disease, degree of left ventricular dysfunction, and anticipated activity level 3
• In older patients, devices that maintain synchrony between atria and ventricles are preferred as they maintain the increased contribution of atrial contraction to ventricular filling 3
• Pacemakers should be implanted by experts who can select the device offering the best clinical advantage at reasonable cost 3