• Paced rhythm can be easily identified on ECG by the prominent vertical lines (”pacing spikes”) that mark the time of electrical discharge by the device.
• Pacing spikes are either followed by P waves (if pacing lead is in the atrium) or QRS complexes (if pacing lead is in the ventricle).
• Examples of ECG rhythms generated by different pacing modes are listed elsewhere.
• An atrial-paced beat has a pacing spike (vertical line) at the beginning of P wave.
• All paced P waves have similar morphology (all come from the same pacing output) which look differently than intrinsic P waves.
• Because electrical impulses is initiated above AV junction and delivered via the AV-His bundle-Purkinje system, the resulting conduction is efficient and QRS complexes are narrowed.
• Seen in AOO, AAI, DDI, DDD pacing modes.
• A ventricular-paced beat has a pacing spike (vertical line) at the beginning of QRS complex.
• Because electrical impulses are initiated within the ventricular myocardial cells and not delivered via the His bundle-Purkinje system, the resulting conduction is inefficient and QRS complexes are widened, and depending on the location of pacing lead placement, have either LBBB (if placed in the right ventricle) or RBBB morphology (if placed in the left ventricle).
• Additionally, T waves are discordant to QRS complexes (i.e. inverted T wave and upright QRS and vice versa). Non-paced T waves might also be discordant to QRS complexes due to “cardiac memory” after a long period of pacing (This leads to upright T waves in I and aVL, which if inverted, might suggest anterior myocardial ischemia).
• Depending on the pacemaker type and pacing mode, QRS’s might or might not be preceded by P waves (which come from either intrinsic cardiac electrical activity or additional pacing lead in the atrium).
• Seen in VOO, VVI, DDI, DDD pacing modes.
• An atrioventricular (AV) sequential-paced beat have x2 pacing spikes (vertical line) one at the beginning of P wave and the other at the beginning of QRS complex.
• P waves have similar morphology as in single-lead atrial pacing as described above.
• QRS complexes have similar morphology as in single-lead ventricular pacing as described above.
• Seen in DOO, DDI, DDD pacing modes.
• A biventricular-paced beat has double pacing spikes (2 tightly coupled vertical lines) at the beginning of a QRS complex.
• Because electrical impulses are initiated spontaneously in both ventricles, conduction is more efficient and the resulting QRS complexes are narrower as compared to single-lead ventricular pacing.
• QRS complexes have a fusion morphology between single-lead left and right ventricular pacing, with prominent R waves in V1-2 and Q waves in I, aVL.
• Depending on the pacemaker type and pacing mode, QRS’s might or might not be preceded by P waves (which come from either intrinsic cardiac electrical activity or additional pacing lead in the atrium).
• Biventricular pacing is also referred to as Cardiac Resynchronization Therapy (CRT), which is commonly indicated in heart failure.
