Electrocardiographic Appearance of V-Paced Rhythm
A ventricular paced rhythm typically appears on ECG as a wide QRS complex with left bundle branch block (LBBB) morphology, characterized by a predominantly negative QRS complex in lead V1 (QS or rS pattern) and an upright, notched or slurred R wave in leads I and V6.
Characteristic ECG Features of V-Paced Rhythm
QRS Complex Morphology
- Wide QRS complex (≥120 ms) due to abnormal ventricular activation sequence 1
- LBBB-like pattern is the expected morphology during right ventricular pacing 2
- QS or rS pattern in lead V1 (predominantly negative QRS complex) 1
- Upright, notched or slurred R wave in leads I and V6 1
Other ECG Features
- Pacing spike (vertical deflection) preceding the QRS complex
- Secondary ST-segment and T-wave changes (discordant to QRS direction) 1
- QRS-T wave discordance (ST segment and T wave directed opposite to the main QRS vector) 3
- AV dissociation may be present in ventricular pacing without atrial tracking
Special Considerations
RBBB-Like Pattern During RV Pacing
- While LBBB morphology is expected, approximately 8.1% of patients with right ventricular pacing may show a right bundle branch block (RBBB) pattern 4
- RBBB pattern during RV pacing may raise concern for lead perforation or malposition
- However, this can occur with proper lead placement and doesn't necessarily indicate a complication 2
- Displacement of ECG leads V1-V2 to the sixth intercostal space can help confirm RV lead position by converting the RBBB pattern to LBBB pattern 4
Biventricular Pacing (CRT)
- In biventricular pacing, the QRS morphology depends on the timing of RV and LV stimulation 5
- When simultaneous biventricular pacing is performed, a dominant R wave in V1 typically indicates proper LV capture 5
- If a QS complex appears in V1 during biventricular pacing, it may indicate:
- Prolonged LV latency (delay between pacing stimulus and myocardial activation)
- Fusion with intrinsic rhythm
- RV contribution dominating the ECG appearance 5
Potential Pitfalls in Interpretation
- Paced rhythms can mask underlying ECG changes of various conditions 6
- Hyperkalemia can still cause typical ECG changes (QRS widening, sine waves) even during paced rhythm 6
- Diagnosis of acute myocardial infarction is challenging during ventricular pacing 3
- Comparison with previous ECGs is essential when evaluating for acute changes in patients with pacemakers 3
Clinical Implications
- Recognition of proper pacing morphology helps confirm appropriate lead placement
- Unexpected RBBB pattern during RV pacing warrants investigation but may be normal 2, 4
- Understanding normal paced ECG appearance is crucial for identifying superimposed pathology such as ischemia or electrolyte abnormalities 6, 3
Remember that the ECG appearance of ventricular pacing primarily reflects the altered sequence of ventricular activation, with the ventricles being depolarized from the endocardium (where the lead tip is located) to the epicardium, rather than through the normal His-Purkinje system.