Principles of Cardiac Pacing 

Cardiac pacing is the process of using electrical impulses to stimulate the heart to contract when the intrinsic (natural) cardiac rhythm is too slow, irregular, or absent. Below are the key principles and concepts: 
 
1. Basic Concept 
Definition: Cardiac pacing involves delivering timed electrical impulses to the myocardium via electrodes, causing depolarization and subsequent contraction. 
Purpose: To maintain adequate heart rate and cardiac output when the heart’s own pacemaker (the sinoatrial node or conduction system) fails. 
 
2. Indications 
Bradyarrhythmias (slow heart rates): 
Sinus node dysfunction (e.g., sick sinus syndrome) 
Atrioventricular (AV) block (Mobitz II or complete heart block) 
Tachyarrhythmias (for overdrive pacing) 
Cardiac resynchronization therapy (CRT) in heart failure with dysynchrony 
Temporary pacing after cardiac surgery, myocardial infarction, or drug overdose 
 
3. Components of a Pacemaker System 
Pulse Generator: 
Contains the battery and electronic circuitry. 
Determines rate, output, and timing of impulses. 
Leads (electrodes): 
Conduct impulses to and from the myocardium. 
Can be endocardial (transvenous) or epicardial. 
Myocardium: 
The heart tissue that receives the pacing stimulus and contracts. 
 
4. Pacing Modes (NBG Code) 
The NASPE/BPEG (NBG) code describes pacemaker function with up to five letters: 



Examples: 
VVI: Ventricle paced and sensed, inhibits pacing if intrinsic beat occurs. 
DDD: Both chambers paced/sensed, coordinates AV synchrony. 
AAI: Atrial pacing and sensing, inhibits pacing when intrinsic P wave present. 
 
5. Electrical Parameters 
Threshold: Minimum energy required to cause depolarization (check by reducing voltage until capture is lost (this is the threshold; note do not reduce past 40 bpm; only to be completed if assessed as competent to do so). 
Output (Amplitude): Usually set at 2–3× threshold to ensure capture. 
Pulse Width: Duration of the pacing stimulus. 
Sensitivity: Ability of the pacemaker to detect intrinsic electrical activity (set in millivolts) (the lower the sensitivity the more of the patients own rhythm can cut through)  
 
6. Types of Pacing 
Temporary pacing: For short-term support (transcutaneous, transvenous, or epicardial). 
Permanent pacing: Implanted system for chronic rhythm support. 
Biventricular pacing (CRT): Stimulates both ventricles to improve synchrony in heart failure. 
Rate-adaptive pacing: Adjusts heart rate based on physiological demand (using sensors). 
 
7. Pacing Timing Cycles 
Lower Rate Limit (LRL): Minimum rate the pacemaker maintains. 
Upper Rate Limit (URL): Maximum pacing rate allowed. 
AV Delay: Time between atrial and ventricular pacing to mimic physiological AV conduction. 
 
8. Complications 
Mechanical: Lead displacement, lead fracture, or perforation (tamponade). 
Electrical: Failure to capture, sense, or pace. 
Infective: Pocket infection, endocarditis. 
Physiological: Pacemaker syndrome (loss of AV synchrony). 
 
9. Follow-Up and Optimization 
Regular pacemaker checks to assess: 
Battery life 
Lead integrity 
Capture and sensing thresholds 
Appropriateness of pacing mode and rate response 
 
10. Special Considerations 
MRI-conditional devices allow safe MRI scanning. 
Pacemaker interference can occur from electromagnetic fields—use caution around strong magnets or electrical devices. 
 
 
Rhythm Strip examples: 
 
Atrial Pacing – Pacing spike before each P wave: 

 
Ventricular Pacing – Pacing spike before each QRS:

Atrial and Ventricular (Dual Chamber) Pacing – pacing spikes before each P wave and QRS complex