Ventricular Pacing ECG Characteristics
Ventricular pacing produces a characteristic ECG pattern with a pacing spike followed by a wide QRS complex (>120 ms) that typically shows a left bundle branch block (LBBB) morphology with left axis deviation when pacing from the traditional right ventricular apex. 1
Essential ECG Features of Ventricular Pacing
Basic Morphology
- Pacing artifact (spike) appears as a sharp vertical deflection immediately before the QRS complex, representing the electrical impulse delivered by the pacemaker 2, 3
- Wide QRS complex (>120 ms duration) follows each pacing spike when capture is successful, reflecting abnormal ventricular depolarization from the pacing site rather than through the normal conduction system 1, 4
- QRS-T wave discordance is the anticipated pattern, where the ST segment and T wave deflect opposite to the main QRS vector 4
Right Ventricular Apical Pacing (Traditional Site)
- LBBB configuration with predominantly negative QRS in V1 and positive in V6 1
- Left axis deviation in the frontal plane 1
- This pattern occurs because electrical activation spreads from the RV apex across the septum to activate the left ventricle in a delayed, abnormal sequence 1
Left Ventricular Pacing
- Right bundle branch block (RBBB) configuration with positive QRS in V1 3, 1
- Axis depends on lead position within the left ventricle 1
- Absence of positive R-wave in V1 during intended biventricular pacing may indicate LV lead displacement or loss of LV capture 3
Verifying Proper Capture
ECG Confirmation
- Each pacing spike must be followed by a corresponding QRS complex (for ventricular pacing) with appropriate morphology 3
- Absence of QRS after pacing spike indicates failure to capture, meaning the electrical impulse failed to depolarize the myocardium 2
- Optimize ECG lead selection to minimize pacemaker artifact and maximize QRS visualization 3
Mechanical Confirmation (Critical)
- Never rely on ECG alone - always verify both electrical and mechanical capture 2
- Confirm peripheral pulse through arterial line waveform, pulse oximeter plethysmogram, or manual palpation to ensure each pacing stimulus produces mechanical systole 3
- Verify adequate blood pressure and perfusion to confirm effective mechanical capture 3
Common Pacemaker Modes and ECG Patterns
VVI/VVIR Mode
- Paces and senses only in the ventricle 5
- Pacing spike followed by wide QRS when pacing occurs 5
- No atrial activity is tracked or paced 5
DDD/DDDR Mode
- May show atrial pacing spikes followed by P waves and/or ventricular pacing spikes followed by QRS complexes 6
- Maintains AV synchrony when functioning properly 6
Algorithms Minimizing Ventricular Pacing
- May produce "bizarre" ECG appearances that can suggest malfunction but are actually normal algorithm function 6
- Can show longer AV delays for sensed events than paced events, violating traditional AV conduction rules 6
- Mode conversion algorithms switch between AAI(R) and DDD(R) based on AV conduction, creating variable ECG patterns 6
Critical Pitfalls to Avoid
Failure to Capture
- Pacing spike without QRS represents failure to capture and requires immediate intervention, especially in pacemaker-dependent patients 2
- Temporary transvenous pacemakers have higher risk of capture loss due to lead dislodgement, occurring in up to 5.7% of cases with 50% within first 24 hours 3
Distinguishing from Other Malfunctions
- Undersensing: Pacing spikes occur inappropriately when intrinsic QRS complexes are present, but spikes still produce QRS complexes 2
- Oversensing: Inappropriate inhibition of pacing with fewer spikes than expected because the pacemaker misinterprets noise as cardiac activity 2
- Failure to pace: Complete absence of pacing spikes when needed 2
Misinterpreting ST-T Changes
- The altered intraventricular conduction from ventricular pacing changes ST-segment and T-wave configuration 4
- Expected pattern is QRS-ST-T discordance, not concordance 4
- Diagnosing acute myocardial infarction requires recognizing deviations from this expected discordant pattern 4