ECG criteria
- A pacemaker spike is visible as a narrow vertical artifact in any lead
- Atrial capture is assessed by a P wave associated with the spike, occurring with the expected delay and a morphology characteristic of the pacing site; its absence after the spike indicates loss of capture
- Following a ventricular spike, there is no wide QRS complex with a typical paced morphology
- During effective pacing, the evoked complex begins almost immediately after the spike; the absence of associated depolarization when analyzing multiple leads confirms loss of capture (there is no universal time threshold in ms)
- A single reliably ineffective stimulus is sufficient for diagnosis; recurrence across multiple or all stimuli confirms intermittent or permanent malfunction and makes an artifact or accidental occurrence in the refractory period a less likely explanation
- The pause following an ineffective spike may be terminated by an intrinsic or escape beat
- Sometimes only the spike is visible against a background of bradycardia or asystole, without any intrinsic complexes
What it looks like on ECG
The main sign is a pacemaker spike (a short vertical artifact, usually corresponding to a pulse duration of less than 1–2 ms; its amplitude and visibility on the tracing depend on the lead, gain, and filters), which is not followed by the expected depolarization.
| Pacing mode | Normal (capture present) | Failure to capture |
|---|---|---|
| Atrial | spike → P wave of paced morphology | spike without a P wave or with an altered, unrelated P wave |
| Ventricular | spike → wide QRS (during RV apex pacing — usually LBBB-like morphology; axis may be deviated leftward or superiorly, but is not strictly defined and depends on the pacing site) | spike without QRS, isoelectric line or intrinsic QRS temporally unrelated to the spike |
| Dual-chamber | atrial spike → P wave → after programmed AV delay, ventricular spike → QRS | loss of capture in one or both channels, spikes "dropping into empty space" |
Spikes may occur at equal intervals (base pacing rate), but some fail to capture the myocardium — in such cases, the spike is immediately followed by the next interval, a pause, or an intrinsic beat. It is important to examine multiple leads: sometimes the paced complex is of low amplitude and not visible everywhere, easily mistaken for a lack of capture (a pseudo-problem, not true loss of capture).
Mechanism
A pacemaker delivers an impulse via a lead to the endocardium. To trigger depolarization, the impulse energy must exceed the tissue's capture threshold at the contact point of the lead. If the threshold exceeds the pacemaker's output energy, the impulse dissipates into surrounding tissues without propagating a depolarization wave, and no response P wave or QRS complex occurs. The capture threshold increases with fibrosis and fibrous capsule formation around the lead tip, tissue edema after implantation, electrolyte and metabolic disturbances, lead displacement from the myocardial wall, or perforation. Loss of capture also occurs if output amplitude or pulse duration is programmed below the actual threshold, or if the generator power source is depleted.
Causes
- Lead dislodgement (displacement) from the atrial or ventricular wall
- Lead fracture or insulation breach, mechanical lead damage
- Elevated pacing threshold: in the early postoperative period — edema and inflammatory reaction at the lead fixation site; later — fibrosis and fibrous capsule formation around the lead
- Perforation of the myocardial wall by the lead
- Inappropriately programmed low output amplitude or pulse duration
- Electrolyte disturbances (hyperkalemia) and metabolic disorders that increase the threshold
- Myocardial infarction or scar in the lead contact area
- Class IC antiarrhythmic drugs, which increase the pacing threshold
- Pacemaker battery depletion — primarily upon reaching end of life (EOL) or generator malfunction; modern devices usually provide early warning of this state (ERI indicator) and alter pacing parameters or modes
Differential diagnosis
- Pseudo-failure to capture — a paced complex is present but has low amplitude and is not visible in the given lead; other leads must be checked.
- Sensing failure (undersensing) — the pacemaker fails to sense intrinsic activity and delivers spikes inappropriately, but these spikes successfully capture the myocardium (a complex follows the spike).
- Functional inexcitability (physiological refractory period) — a stimulus falling into the refractory period after an intrinsic or preceding paced complex may fail to evoke additional depolarization, which is physiological given correct device timing. However, if this is the result of inadequate sensing, incorrect settings, or timing abnormalities, the situation is pathological; surface ECG alone cannot reliably distinguish between these variants — programmed mode analysis and device interrogation are required.
- Spike-like artifact (mains interference, muscle tremor) — lacks strict periodicity related to the pacemaker programming.
Clinical significance and management
Failure to capture is dangerous due to the potential development of significant bradycardia, prolonged pauses, syncope, and asystole — especially in pacemaker-dependent patients (complete AV block, absence of intrinsic rhythm). The physician must: document the finding on an ECG and assess clinical status (syncope, weakness, cardiac arrest); in cases of clinical instability and pacing dependence — without waiting for further investigations, ensure monitoring, readiness for external pacing/defibrillation, and temporary pacing as indicated; diagnostic workup should not delay resuscitation measures. The key initial investigation is urgent device interrogation with measurement of the capture threshold, lead impedance, and analysis of stored episodes/electrograms, with correction of output parameters if possible. If lead dislodgement, perforation, or mechanical damage is suspected, a chest X-ray is performed, but a normal radiograph does not rule out perforation: if clinical suspicion of perforation exists, echocardiography (or CT if uncertain) is necessary to assess for pericardial effusion and signs of tamponade. Additionally, electrolytes (potassium) are checked, and ischemia/infarction in the lead contact area is ruled out. If lead dislodgement, fracture, perforation, or uncorrectable programmed threshold elevation is confirmed, cardiac electrophysiologist/surgeon consultation for lead repositioning or replacement is required.
Common interpretation errors
- Mistaking a low-amplitude paced complex in a single lead for a lack of capture without checking other leads
- Confusing failure to capture with normal spike occurrence in the refractory period, without considering that they cannot always be reliably distinguished without device interrogation
- Failing to differentiate from undersensing — confusing the cause (sensing/not sensing) with the effect (capturing/not capturing)
- Ignoring the clinical context — failing to assess whether the patient is pacemaker-dependent
- Considering a normal chest X-ray sufficient evidence against lead perforation
1. Which ECG sign indicates pacemaker failure to capture?
The true sign of loss of capture is a spike not followed by the expected atrial or ventricular depolarization.
2. What is the most characteristic cause of failure to capture in the first weeks after lead implantation?
In the early postoperative period, a transient rise in pacing threshold due to edema and inflammatory reaction is typical; fibrosis and capsule formation around the lead occur later and cause chronic threshold elevation.
3. How does failure to capture fundamentally differ from undersensing (sensing failure)?
These are two different malfunction mechanisms: one is related to stimulus effectiveness (capture), and the other to the device's recognition of intrinsic activity (sensing).
Frequently asked questions
How to distinguish loss of capture from sensing failure?
In loss of capture, a spike is present but is not followed by a P or QRS wave. In undersensing, the pacemaker delivers a spike at the wrong time (failing to sense intrinsic activity), but this spike most frequently captures the myocardium successfully, with a complex following it.
Is pacemaker loss of capture dangerous?
Yes, especially in patients who are entirely pacemaker-dependent (e.g., in complete AV block without an intrinsic rhythm). This can lead to significant bradycardia, a prolonged pause, and asystole with syncope.
Can loss of capture be temporary and not require lead replacement?
Yes, in the first weeks post-implantation, the capture threshold may be elevated due to edema and inflammation at the electrode fixation site and decreases over time; sometimes reprogramming the pacemaker's output parameters without surgical intervention is sufficient.
How to confirm that this is true loss of capture rather than a lead artifact?
Check multiple ECG leads simultaneously: the absence of a visible response in a single lead does not prove loss of capture, as a low-amplitude paced complex may go unnoticed in just one or two leads. Definitive confirmation in case of doubt is device interrogation.
What does a physician do when loss of capture is detected on an ECG?
Immediately order device interrogation to measure the pacing threshold, lead impedance, and adjust output energy; check electrolytes; if lead displacement or perforation is suspected, perform a chest X-ray and, if suspicion of perforation persists, echocardiography. In case of clinical instability and pacing dependency, involve a cardiac electrophysiologist/surgeon and provide temporary pacing.
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Related terms
More in the section "Cardiac Pacing"
Sources
- Goldberger's Clinical Electrocardiography, latest edition
- Surawicz B., Knilans T. Chou's Electrocardiography in Clinical Practice
- AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram, 2009
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.