ECG criteria
- A narrow, vertical pacing spike immediately preceding the QRS; best visualized in leads whose axis is most parallel to the vector between the cathode and anode of the lead, with spike polarity depending on the direction of this vector
- In conventional right ventricular myocardial pacing, the QRS is widened, often ≥ 120 ms; initial morphology, including the presence of q/Q waves, is variable and depends on the pacing site and the lead. With conduction system pacing (His bundle, left bundle branch), the QRS may be narrow or only moderately widened
- Typical morphology during right ventricular apical pacing resembles left bundle branch block (wide QRS in V1 with a predominantly negative complex)
- Frontal plane electrical axis is frequently deviated to the left or markedly left-axis with an apical lead position
- Discordant ST segment and T wave (directed oppositely to the main QRS deflection)—usually represents secondary repolarization changes but does not exclude ischemia; the entire clinical and ECG picture must be evaluated
- With right ventricular outflow tract or septal pacing, the QRS may be somewhat narrower and exhibit a different transition and axis, but it generally retains a LBBB-like morphology with a predominantly negative complex in V1; a positive QRS in V1 is atypical and points more toward left ventricular or conduction system pacing
- Intrinsic P waves may be seen independently of the QRS in the absence of atrial synchronization (VVI mode), with the PR interval being irregular relative to the spike
What it looks like on ECG
Each paced ventricular contraction is usually preceded by a sharp vertical pacing spike—a short, high-frequency artifact lasting a few milliseconds. It is best seen in leads whose axis is most parallel to the vector between the cathode and anode of the lead; the polarity of the spike depends on the direction of this vector rather than whether the lead is 'opposite' to the pacing axis. The spike may be of low amplitude, visible only in some leads, or completely invisible on a surface ECG, especially with a bipolar lead system—the absence of a visible spike does not rule out pacing.
During conventional right ventricular myocardial pacing, the spike is followed by a wide, bizarre QRS complex (usually ≥ 120 ms) because depolarization spreads slowly through the myocardium from the pacing site rather than via the specialized conduction system. With a typical right ventricular apex lead position, the QRS morphology resembles left bundle branch block: a wide negative (QS or rS) complex in V1, and a wide R wave in I and aVL. The initial QRS morphology (presence or absence of q/Q waves) is variable and depends on the pacing site and the specific lead. The frontal plane electrical axis is typically deviated to the left, often markedly (northwest axis is also possible).
The ST segment and T wave are generally discordant with the QRS, meaning they are directed oppositely to the main deflection of the complex—this is an expected secondary repolarization change, but it does not exclude concomitant ischemia. In VVI mode (ventricular pacing and sensing only, without atrial synchronization), intrinsic P waves may appear asynchronously without a fixed relationship to the paced QRS—the PR interval will vary from complex to complex. During ventricular pacing in DDD/VDD modes, a native or paced P wave is typically visible before the ventricular spike with a fixed PR-like interval. In biventricular pacing (CRT) and conduction system pacing (His bundle, left bundle branch), QRS morphology and duration differ from the classic picture and may be narrow or intermediate.
Mechanism
In conventional myocardial pacing, the lead is situated in the right ventricle (most commonly at the apex or outflow tract septum). The pacemaker impulse depolarizes the myocardium at the contact point of the lead, and excitation spreads slowly through the ventricle, cell by cell, bypassing the specialized conduction system (Purkinje fibers). This produces a wide and aberrant QRS similar to a functional bundle branch block. Repolarization also proceeds in an altered sequence, hence the discordant ST-T changes.
With conduction system pacing (His-pacing, left bundle branch pacing), the impulse enters the conduction fibers directly, and excitation propagates faster and more physiologically; therefore, the QRS may be narrow or moderately widened. In biventricular pacing (CRT), simultaneous activation from the right and left ventricles produces a different, often narrower QRS morphology compared to isolated right ventricular pacing.
Causes
- Complete AV block (third-degree) and symptomatic second-degree AV block
- Sick sinus syndrome with pacing indications
- Bradyarrhythmias following cardiac surgery, ablation, or myocardial infarction
- Atrial fibrillation with symptomatic or clinically significant bradycardia/pauses, high-grade AV block, or when required pharmacotherapy causes marked bradycardia (not based on a single long pause alone)
- Implantation of a resynchronization device (biventricular or left ventricular pacing) or ICD with ventricular pacing function
- Conduction system pacing (His bundle, left bundle branch) as an alternative to right ventricular myocardial pacing
- VVI mode as a back-up mode in permanent atrial fibrillation/flutter when atrial synchronization is unnecessary
Differential diagnosis
- Intrinsic left bundle branch block: no pacemaker spike precedes the QRS, morphology is similar but without the artifact
- Ventricular premature complex: no preceding spike, the complex usually has a different morphology, no fixed coupling to the programmed rate
- Antidromic AV reciprocating tachycardia in WPW syndrome: wide tachycardic QRS without a pacemaker spike; typical delta waves and a measurable short PR interval during the tachycardia may be absent because the ventricles are activated predominantly via the accessory pathway—diagnosis relies on clinical context, baseline ECG, and electrophysiological data
- Hyperkalemia with marked QRS widening: no spike, accompanied by characteristic T wave changes and the disappearance of P waves
- Fusion complex: a QRS of intermediate morphology between an intrinsic and a paced complex, occurring during simultaneous activation from an intrinsic pacemaker and the device
Clinical significance and management
Ventricular pacing per se is not a pathology, but the expected result of device function when indicated. The physician's task during interpretation is to ensure that the stimulus is effective (every spike is followed by a QRS—this is 'capture') and that the device senses intrinsic rhythm correctly (no competition with native complexes). One must evaluate the underlying rhythm hidden beneath pacing and look for signs of complications: loss of capture, undersensing, oversensing, pacemaker syndrome (symptoms in VVI mode without preservation of atrioventricular synchrony: fatigue, dyspnea, neck pulsations due to atrial contraction against closed AV valves).
A separate issue is the consequences of long-term right ventricular pacing. A high percentage of RV pacing (especially from the apex) creates ventricular electrical and mechanical dyssynchrony, similar to left bundle branch block, and over time leads to a decrease in left ventricular contractility in some patients — pacing-induced cardiomyopathy. The risk is higher with baseline LV dysfunction and a high pacing burden; therefore, follow-up examinations assess the percentage of ventricular pacing from device data and LV function by echocardiography, and when necessary, discuss minimizing unnecessary ventricular pacing, switching to conduction system pacing, or cardiac resynchronization therapy. Potential complications of the system itself (lead dislodgement, lead fracture) and infectious complications of the device pocket and leads are considered separately.
Assessing ischemia against the background of paced rhythm is also difficult: conventional ST elevation/depression criteria do not work as reliably as with intrinsic QRS without bundle branch block. Modified Sgarbossa (Smith) criteria, adapted inter alia for paced rhythm, are used: concordant ST elevation ≥ 1 mm in any lead, concordant ST depression ≥ 1 mm in V1–V3, and excessive ST discordance — the ratio of ST amplitude to the amplitude of the preceding S wave (or deep wave with predominantly negative QRS) ≤ −0.25 (the original Sgarbossa criterion used a fixed threshold of discordant ST elevation ≥ 5 mm, while the modified one replaces it with a proportional ratio). If acute coronary syndrome, let alone coronary artery occlusion, is suspected in a patient with a pacemaker, immediate expert evaluation of the ECG and clinical picture is required; reperfusion pathway activation should not be delayed waiting for troponin dynamics or searching for an old ECG. Serial ECG recording, comparison with pre-implantation recordings (if available), and troponin are complementary rather than exclusionary tests.
Common interpretation errors
- Mistaking a wide QRS with a spike for intrinsic bundle branch block and failing to mention the presence of a pacemaker
- Failing to distinguish a fusion beat from a sensing abnormality
- Interpreting any discordant ST-T waves as unconditionally benign and failing to consider ischemia in the appropriate clinical context
- Missing a failure to capture (a spike is present, but not followed by a QRS)
- Ruling out pacing solely due to the absence of a visible spike on the tracing
- Failing to notice asynchronous intrinsic P waves in VVI mode and mistaking this for atrial pathology
1. Which ECG sign directly indicates the presence of ventricular pacing rather than just altered QRS morphology?
QRS width, ST discordance, and axis deviation also occur in intrinsic bundle branch block; a specific sign of pacing is the spike preceding the paced complex, although it is not always visible.
2. Typical QRS morphology during right ventricular apical pacing resembles:
Activation spreads from the right ventricle to the left with a delay, bypassing the conduction system, which produces a picture similar to left bundle branch block.
3. What could the absence of a visible QRS complex immediately following a pacemaker spike mean?
The absence of a timely ventricular response after a spike, when intrinsic or fusion complexes are excluded, indicates a possible failure to capture; the device, output energy and pacing threshold, leads and connections, as well as reversible causes (hyperkalemia, ischemia, battery depletion) should be checked.
Frequently asked questions
How to distinguish ventricular pacing from left bundle branch block?
If the pacemaker spike is visible before the QRS, it supports the diagnosis of pacing; its absence on the tracing does not rule out pacing, as the spike may be of low amplitude or not visualized on the surface ECG. QRS morphology may resemble LBBB, so the issue is ultimately resolved by clinical data regarding the implanted device and, if necessary, analysis of its programming and telemetry.
Is ventricular pacing dangerous in itself?
Pacing itself is an expected function of the device when indicated, not a pathology. However, prolonged and high-burden right ventricular pacing can cause electromechanical dyssynchrony and pacing-induced cardiomyopathy with decreased LV function, so such patients require periodic clinical and echocardiographic monitoring, as well as evaluation for capture abnormalities, sensing issues, and pacemaker syndrome.
Can myocardial infarction be seen during ventricular pacing?
Yes, but standard STEMI criteria do not work reliably with a wide paced QRS. Modified Sgarbossa (Smith) criteria with numerical thresholds are used, clinical presentation and dynamics are evaluated, and if coronary artery occlusion is suspected, action is taken immediately without waiting for troponin dynamics or an old ECG.
Why is the T wave often directed oppositely to the QRS during pacing?
This is ST-T discordance — usually a secondary repolarization change due to an altered sequence of ventricular excitation propagation. Such a pattern is typical and in itself does not indicate ischemia, but does not rule it out: with a corresponding clinical picture, concordant or excessively discordant changes must be taken into account.
What does VVI mode mean on an ECG, and is ventricular pacing always VVI?
VVI means ventricular pacing, ventricular sensing, and stimulus inhibition upon detection of an intrinsic contraction. This is only one of the ventricular pacing modes: the ventricle can also be paced in DDD, VDD modes, during biventricular pacing (CRT), or conduction system pacing, and in these cases the ECG picture differs from classic VVI.
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Related terms
More in the section "Cardiac Pacing"
Sources
- Goldberger A.L. Clinical Electrocardiography: A Simplified Approach
- Surawicz B., Knilans T.K. Chou's Electrocardiography in Clinical Practice
- AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the 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.