Atrial Pacing and P Wave Generation
Yes, atrial pacing generates a P wave on the ECG, though the morphology differs from sinus P waves—paced P waves typically have lower amplitude, longer duration, and distinct morphology that varies based on the pacing site. 1
ECG Characteristics of Paced P Waves
When pacing from the right atrial appendage (the most common site), the paced P waves demonstrate specific features:
- Amplitude is reduced compared to sinus rhythm (0.11 ± 0.032 mV vs 0.16 ± 0.05 mV during sinus rhythm) 1
- Duration is prolonged (0.08 ± 0.017 sec vs 0.07 ± 0.009 sec during sinus rhythm) 1
- Morphology is site-dependent: P waves are typically positive in leads I (80%), II (71%), III (80%), and aVF (76%) when pacing from the right atrial appendage 1
- Lead aVL often shows a diphasic pattern with initial negative deflection (36% of cases) 1
Clinical Recognition Challenges
Identifying atrial capture can be difficult on surface ECG, particularly with unipolar pacing systems where the pacing artifact may obscure the P wave. 2
Key strategies for confirming atrial capture:
- Lead III is optimal for unipolar pacing because the pacing spike is smallest in this lead 1
- Lead II is best for bipolar pacing systems 1
- Precordial leads are generally unhelpful due to small amplitude of paced atrial depolarization 1
- Alternative verification methods using telemetered intracardiac atrial electrograms can confirm capture when surface ECG is equivocal 2
Site-Specific P Wave Morphology
The P wave morphology during atrial pacing provides information about the pacing site location along the atrioventricular ring:
- Positive P wave in lead I indicates right atrial origin (predictive value 98.9%) 3
- Negative or isoelectric P wave in lead I indicates left atrial origin (predictive value 94.6%) 3
- Negative P waves in leads II, III, and aVF indicate posterior site of origin (predictive value 91.2%) 3
- Negative or isoelectric P wave in V1 suggests right atrial free wall location (predictive value 87.5%) 3
Technical Considerations
Modern pacemaker technology requires improved algorithms for detecting and displaying low-amplitude pacemaker stimuli without artificial enhancement, as recommended by the American Heart Association. 4 The challenge is preserving the true ECG waveform while making pacing artifacts visible for clinical interpretation 4.
Careful systematic examination of all 12 leads is essential rather than relying on a single rhythm strip, as P wave morphology varies significantly across leads 5. This principle applies equally to paced rhythms and spontaneous atrial activity 5.