ECG criteria
- Pacing spikes are visible on the ECG before the QRS: one, two, or none visible (fusion with intrinsic complex, short VV interval, or specific programming settings)
- QRS is wide (typically ≥ 120–130 ms), morphology differs from isolated RV pacing, but width alone neither proves nor excludes biventricular capture
- In most CRT configurations, the QRS in V1 acquires an RBBB-like, predominantly positive morphology (R or Rs); however, this is not a universal sign—with certain lead positions and fusion variants, V1 may remain negative or biphasic
- Changes in I and aVL (often a negative or biphasic configuration compared with isolated RV pacing) may support the presence of LV pacing, but are not mandatory and depend on the LV lead position and fusion
- The frontal plane QRS axis is variable—it can be normal, left-axis, or right-axis deviated depending on the system configuration; by itself, it does not reliably confirm or exclude biventricular capture without comparison to the baseline tracing
- Fusion complexes are possible—partial fusion of paced and intrinsic conduction, with a QRS that is narrower and closer to the intrinsic morphology
- The QRS often narrows compared to single-chamber pacing of only one ventricle, but this is not mandatory and is not an independent criterion for effective capture; morphological comparison with a reference ECG and device interrogation are required
- In the event of loss of capture on one of the leads, the QRS morphology often changes to that typical of pacing from the remaining ventricle alone—provided the second channel continues to capture effectively and intrinsic conduction does not mask the pattern; other variants are also possible (fusion/pseudofusion, intrinsic conduction, simultaneous loss of capture on both channels), therefore in case of suspicion, malfunction is confirmed via the programmer and electrogram analysis
What it looks like on ECG
During biventricular pacing, one or two pacing spikes are visible on the tracing before the wide QRS complex; their visibility depends on programming (VV delay), paper speed, and the degree of fusion with intrinsic conduction—the absence of a visible spike does not rule out pacing.
The key reference point when comparing with isolated right ventricular pacing is the QRS morphology in V1 and the frontal plane axis. RV-only pacing typically produces an LBBB-like complex (negative V1), but the axis and terminal lead morphology depend on lead localization: an apical position is more frequently associated with a superior/leftward axis, while outflow tract pacing shows a substantially different pattern. In biventricular pacing, the left ventricular lead adds a vector from the lateral/posterolateral wall, which is why in most CRT configurations V1 becomes more positive (R or Rs)—this is a frequent but non-mandatory sign, as the final morphology depends on the position of both leads, the RV pacing site, VV delay, intrinsic conduction, and the degree of fusion.
| Lead | Pure RV pacing (typical) | Biventricular (CRT, typical) |
|---|---|---|
| V1 | negative (deep S, as in LBBB) | often R or Rs, more positive |
| I, aVL | variable, frequently a tall R | often negative/biphasic |
| QRS | wide | wide, frequently somewhat narrower than in single-chamber pacing |
None of these signs individually confirms effective biventricular capture—diagnosis is based on comparing the current ECG with a baseline tracing obtained after implantation/optimization, and the final assessment is complemented by device programmer data (thresholds, impedance, electrograms) and clinical status. It is also necessary to differentiate paced QRS from a spontaneous narrow intrinsic QRS during temporary loss of pacing or inhibition due to sensing.
Mechanism
In heart failure with mechanical dyssynchrony (most commonly in the setting of left bundle branch block), the left ventricle contracts with a delay relative to the right, which reduces pump function. A CRT device features a right ventricular lead and a left ventricular lead—usually transvenous, placed in a lateral or posterolateral branch of the coronary sinus; less commonly, a surgical epicardial approach or a specialized endocardial LV lead is used. Pacing both ventricles with a programmable AV and VV delay alters the sequence of ventricular activation and, in suitable patients, reduces electrical and mechanical dyssynchrony. Clinical and structural response (symptoms, ejection fraction, LV volumes) is variable and not guaranteed even with confirmed biventricular capture. On the ECG, this is reflected by a change in the QRS depolarization vector compared with isolated single-ventricle pacing.
Causes
CRT is indicated in patients with chronic heart failure with reduced ejection fraction (typically, LVEF ≤ 35%):
- Sinus rhythm, left bundle branch block, QRS ≥ 150 ms, and heart failure symptoms despite optimal medical therapy—the strongest indication
- Sinus rhythm, QRS 130–149 ms (with or without LBBB)—benefit is less certain and depends on QRS morphology and clinical presentation
- Non-LBBB QRS morphology in symptomatic patients with LVEF ≤ 35%: with QRS ≥ 150 ms, CRT may be considered in suitable patients; with QRS 130–149 ms, benefit is substantially less certain and decisions are individualized; with QRS < 130 ms, CRT is not recommended per standard indications
- Atrial fibrillation with symptomatic heart failure, typically LVEF ≤ 35% and QRS ≥ 130 ms—considered if near-continuous effective biventricular pacing can be achieved; if the percentage of pacing is insufficient due to rapid intrinsic conduction, rate control and/or AV junction ablation is considered
- Expected requirement for significant/substantial ventricular pacing (prevention of pacing-induced cardiomyopathy), especially with reduced LVEF (e.g., ≤ 35–40%) and high-grade AV block—instead of standard isolated RV pacing in these cases, specific guidelines recommend considering CRT or conduction system pacing, depending on the patient profile; the mere expectation of some percentage of ventricular pacing with an uncertain "borderline" EF is not an independent indication
- Upgrade of a standard pacemaker or ICD to CRT upon the development or progression of heart failure during chronic RV pacing
Mechanical dyssynchrony by echocardiography is not used as an independent criterion for CRT selection.
Differential diagnosis
- Isolated right ventricular pacing only—typically negative V1 (deep S), but the axis and terminal leads depend on lead position (apex vs outflow tract); a sign of LV channel failure is the absence of the LV lead contribution (loss of capture or dislocation)
- Isolated left ventricular pacing only (in case of RV channel malfunction)—morphology is variable, frequently RBBB-like with a positive V1, but without a fixed mandatory pattern; confirmed by checking thresholds, impedance, electrograms, and comparison with a reference ECG rather than a single lead
- Intrinsic rhythm with LBBB without pacing—no spikes preceding the QRS
- Fusion and pseudofusion complexes—intermediate morphology between paced and intrinsic, not a reason to diagnose malfunction
- Pacemaker spike artifact without effective myocardial capture—it is necessary to check whether the spike is followed by a wide QRS of appropriate shape
Clinical significance and management
The physician's task when evaluating an ECG in a patient with CRT is to ensure effective biventricular capture, rather than simply confirming the presence of pacing. The current QRS morphology (duration, configuration in V1, I, II, III, aVL, aVF) is compared with the reference ECG recorded during device setup. A change in shape toward an isolated single-chamber pacing pattern indicates a probable loss of capture in one of the leads (lead displacement, wire fracture, high pacing threshold, device malfunction)—the QRS duration in this case is secondary to morphology and requires device interrogation. The percentage of biventricular pacing according to device programmer data is also evaluated: clinical efficacy requires the highest possible truly effective fraction of BiV pacing—usually aiming for values above 95% and preferably close to 100%; in atrial fibrillation, it is especially important to achieve near-continuous pacing, often targeting a fraction of at least 98%. However, the percentage reported by the device may overestimate the actual capture efficiency due to fusion and pseudofusion, which the counter may register as paced complexes. A low pacing percentage (frequent intrinsic beats, extrasystoles, fast-conduction AF) reduces therapy efficacy; in fast-conduction AF, rate control, pharmacotherapy, and—if stable capture cannot be ensured—consideration of AV node ablation are often required. If device malfunction is suspected, referral to a cardiac pacing specialist is indicated to check device parameters and pacing thresholds.
Common interpretation errors
- Mistaking a normal biventricular pacing pattern for a bundle branch block without accounting for the presence of spikes
- Failing to compare the current ECG with the baseline "reference" recording after implantation/optimization—without this comparison, it is difficult to notice the loss of capture in a single lead
- Considering QRS narrowing or duration as an independent criterion of CRT efficacy without assessing morphology and device data
- Confusing fusion complexes with device malfunction
- Assessing only the presence of a spike without checking whether it is followed by an effective wide QRS (there may be a spike without capture)
- Forgetting to check the percentage of biventricular pacing via the programmer—a single ECG does not reflect 24-hour dynamics
1. Which sign in lead V1 is typical for effective biventricular pacing (CRT) in most lead configurations?
The vector from the LV lead in most CRT configurations makes V1 more positive, but this is a frequent rather than a mandatory sign—it depends on lead position and fusion.
2. What indicates a possible loss of capture in one of the ventricular leads during CRT?
If one of the leads stops effectively pacing the myocardium, the QRS shape shifts to that typical of pacing the remaining ventricle alone; this is confirmed by device interrogation.
3. Which indication for CRT is considered the strongest according to current selection criteria?
The combination of low LVEF, sinus rhythm, LBBB, QRS ≥ 150 ms, and persistent symptoms on optimal medical therapy yields the greatest proven benefit from CRT.
Frequently asked questions
How to distinguish CRT from standard right ventricular pacing on the ECG?
The main landmarks are the morphology in V1 and changes in I, II, III, aVL, aVF. In isolated RV pacing, V1 usually shows a deep S wave (resembling LBBB). In biventricular pacing, V1 becomes more positive in most configurations, but a definitive conclusion can only be made by comparing with the patient's baseline reference ECG and device data.
Is a wide QRS dangerous during CRT?
No, a wide QRS during biventricular pacing is expected and is not pathology in itself—it may remain wide even with effective capture. Width per se is less important than morphology stability and comparison with the baseline ECG after implantation: a shape change may indicate loss of capture.
What does a low biventricular pacing percentage mean?
This means the device paces the rhythm less frequently than required for therapeutic effect—for example, due to frequent intrinsic beats or rapid-conduction atrial fibrillation. For a clinical effect, the goal is the highest truly effective pacing fraction, usually above 95%, and in AF often ≥ 98%; a low percentage requires adjustment of therapy or device settings, and in some cases, AV node ablation.
Can CRT efficacy be assessed from a single ECG?
A single tracing shows the current moment—the presence of spikes and QRS morphology—but does not reflect the fraction of time over 24 hours that biventricular pacing is active. A complete assessment requires device programmer data, clinical presentation, and echocardiography.
What is a fusion complex in CRT?
This is an intermediate QRS morphology that occurs with partial fusion of paced and intrinsic ventricular depolarization. This is a normal phenomenon and not a sign of device malfunction.
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Related terms
More in the section "Cardiac Pacing"
Sources
- Goldberger A. Clinical Electrocardiography
- AHA/ACCF/HRS Recommendations for Standardization of ECG (2009)
- Orlov V.N. Guide to Electrocardiography
Educational material for medical students and physicians. Does not replace clinical guidelines and physical examination; the ECG must always be interpreted in conjunction with the clinical picture.