ECG criteria
- Failure to capture: no expected QRS complex (in ventricular pacing) or P wave (in atrial pacing) follows the pacemaker spike; the appearance of a different complex after the spike (e.g., an intrinsic QRS conducted via the AV node after an ineffective atrial stimulus) does not rule out failure to capture
- Failure to pace / output failure: absence of a spike during expected and uninhibited pacing at the programmed rate and pause; the absence of a spike due to false oversensing is inappropriate inhibition, not proven output failure—differentiation requires telemetry
- Sensing failure, undersensing: the pacemaker fails to perceive intrinsic cardiac activity and delivers a spike contrary to timing rules—spikes land on intrinsic P or QRS complexes, including the T wave (risk of arrhythmia trigger)
- Oversensing: the expected spike is not delivered on time due to misinterpretation of an artifact (myopotential, T wave, electromagnetic interference) as an intrinsic contraction—manifests as a pathological pause on the ECG
- Evaluate the spike-to-complex interval: in normal capture, atrial spike → P wave after physiological delay, ventricular spike → QRS immediately following it
- Compare the measured pacing rate with the programmed base rate—discrepancy indicates a malfunction, but account for hysteresis, rate response, MVP/AAI-DDD modes, and AV timing, where the absence of a spike is physiological
- Check the morphology of the paced QRS: during right ventricular apex pacing, the morphology often resembles LBBB, the axis is frequently leftward-shifted, but it may vary depending on the pacing site, anatomy, and fusion; a change in morphology without a program change is suspicious for lead dislodgement and requires comparison with a previous ECG, thresholds/impedance, and radiography
- Rule out pseudomalfunction: an intrinsic complex falling within the device's programmed refractory period or blanking period may simulate a sensing abnormality, although this is normal device behavior
What it looks like on ECG
Normal operation of a single-chamber ventricular pacemaker: very narrow pacing artifact (typically about 0.5–2 ms; its visibility depends on the lead and is often maximum in the lead oriented along the pacing vector), immediately followed by a QRS complex, which is wide during traditional RV apex pacing, resembling bundle branch block (most often LBBB morphology). With physiological conduction system pacing (His bundle, left bundle branch area) or significant fusion, the QRS complex may be narrow or only moderately widened. During atrial pacing, the spike precedes the P wave, followed by an intrinsic or paced QRS depending on the mode.
In failure to capture, a spike is present, but the expected depolarization of the corresponding chamber does not occur: no paced QRS after a ventricular spike, no expected P wave after an atrial spike. If a complex of a different nature still appears after such a spike (e.g., an intrinsic ventricular beat conducted via the AV node after an ineffective atrial stimulus), this does not prove atrial capture and does not rule out failure to capture—the depolarization of the specific chamber targeted by the spike is what matters. The failure may be isolated (a single spike) or continuous (every spike fails to capture).
In output failure against the background of expected and uninhibited pacing, the spike simply fails to appear—the tracing shows a prolonged RR interval without an artifact at the required location. It is important to distinguish this from a physiologically inhibited stimulus (intrinsic event correctly recognized) and specific programming features (hysteresis, rate response, AAI-DDD switching modes).
In undersensing, spikes are delivered off schedule: they appear where intrinsic rhythm sensing should have occurred, sometimes directly on the T wave or QRS—it is important to assess whether the spike falls into the vulnerable period (risk of inducing ventricular tachycardia).
In oversensing, conversely, the pacemaker "conceals" spikes that should have occurred, mistaking noise (muscle tremor, electromagnetic field, double-counting of the T wave) for intrinsic activity; the result is an unjustified pause or bradycardia on the tracing.
It is useful to review a long strip in lead II or V1, where the spike and complex are best separated, and compare spike-to-spike intervals with the programmed base rate.
Mechanism
A pacemaker operates in a cycle: sensing (detection of intrinsic electrical activity of the atrium and/or ventricle) → decision on whether pacing is needed → if necessary, output of an impulse via the lead → myocardial capture, i.e., tissue depolarization at the contact site with propagation of excitation.
A malfunction can occur at any of the three stages. Failure to capture—the impulse is present, but energy is insufficient or lead-myocardium contact is impaired (dislodgement, perforation, fibrosis around the tip, battery depletion, lead fracture, increased pacing threshold). Sensing failure (undersensing)—the device fails to see the intrinsic signal due to low amplitude, fibrosis, improperly adjusted sensitivity, and delivers a spike at the wrong time. Oversensing—the device mistakes extraneous potentials (pectoral muscle myopotentials, T wave, electromagnetic interference, lead insulation break) for cardiac signals and inhibits pacing when it was needed.
Causes
- Lead dislodgement or migration (most common in the first weeks post-implantation)
- Myocardial perforation by the lead
- Generator battery depletion (end of life)
- Lead fracture or insulation breach
- Fibrosis/scarring at the lead contact site (elevated pacing threshold)
- Incorrect programming of sensitivity or output energy
- Electromagnetic interference (surgical electrocautery, MRI without compatibility mode, household appliances)
- Hyperkalemia and other metabolic disorders that increase the pacing threshold
- Myocardial infarction in the lead contact area
- Certain antiarrhythmic drugs (e.g., propafenone, flecainide) that increase the capture threshold; the severity of the effect depends on the specific agent and patient
Differential diagnosis
- Pseudomaldetection: an intrinsic complex falls into the programmed refractory period or blanking period of the device and is therefore disregarded for synchronization—this can be normal behavior rather than a malfunction
- Normal fusion complex (fusion of intrinsic and paced QRS)—intermediate morphology, not to be confused with failure to capture
- Pseudofusion—a spike lands on an already initiated intrinsic QRS without entirely altering its morphology
- Magnet mode (response to a magnet): in many, but not all pacemakers, a magnet induces a programmed magnetic response, often asynchronous pacing at a fixed rate; the exact effect depends on the manufacturer, model, battery charge, and settings, and should be interpreted using manufacturer and programmer data. In ICDs, a magnet typically disables tachyarrhythmia therapy rather than switching the device to standard asynchronous pacing—this pacemaker rule cannot be applied to ICDs without qualification
- Pacemaker-mediated tachycardia—not a sensing malfunction, but a pathological response to retrograde conduction in DDD mode during atrial or ventricular premature beats
Clinical significance and management
Failure to capture or output failure in a pacemaker-dependent patient (complete AV block, asystole without intrinsic rhythm) poses a risk of Adams-Stokes attacks, asystole, and cardiac arrest. Undersensing with a spike delivered on the T wave can theoretically induce ventricular tachycardia or ventricular fibrillation, although modern pacemakers deliver low-energy impulses and the risk is low.
For suspected device malfunction, the clinician should: record the finding on a long ECG strip in multiple leads, assess hemodynamics and symptoms (presyncope, syncope, dyspnea), and urgently perform device interrogation using a programmer to check capture thresholds, sensitivity, lead impedance, and battery charge. This is the primary method for evaluating the electrical integrity of the system. Chest radiography is used as an additional method to assess lead position and gross mechanical damage (fracture with visible defect, dislocation), but it does not rule out an electrical fracture or insulation breach and does not replace telemetry. Electrolyte and drug-induced causes must also be ruled out. In case of a clinically significant malfunction, an urgent consultation with a cardiac arrhythmologist/CIED specialist is required for reprogramming or system replacement; in the case of hemodynamically significant bradycardia prior to correction, temporary transcutaneous or transvenous pacing is indicated.
Common interpretation errors
- Mistaking a physiological pseudofusion or fusion complex for failure to capture
- Failing to account for the pacemaker's refractory period or blanking period and misinterpreting normal invisible sensing as undersensing
- Confusing muscle tremor artifact with intrinsic electrical activity and failing to recognize true oversensing
- Assessing pacemaker function based on a single short recording rather than long-term monitoring
- Failing to correlate the finding with the device's programmed parameters (without telemetry, the malfunction diagnosis is preliminary)
1. Which ECG sign is characteristic of pacemaker failure to capture?
Failure to capture means that the electrical impulse was delivered but did not trigger myocardial depolarization, which is why the expected complex is absent after the spike.
2. What occurs during pacemaker oversensing?
During oversensing, an extraneous signal (myopotential, T wave, artifact) is recognized as an intrinsic beat, causing the pacemaker to inhibit the output of an impulse when it was needed—resulting in an unjustified pause visible on the ECG.
3. Which ECG finding in a pacemaker patient is NOT a sign of malfunction?
Pseudofusion is a physiological variant where the spike falls on an already initiated intrinsic depolarization without significant alteration of the QRS morphology; this is not a device malfunction. The other options describe true malfunctions.
Frequently asked questions
How to distinguish failure to capture from a normal pseudofusion complex?
During a pseudofusion, the spike is delivered onto an already initiated intrinsic QRS, and the morphology of the complex barely changes. In true failure to capture, the depolarization of the corresponding chamber induced by the spike is absent: there is no paced QRS after a ventricular spike, and no expected P wave after an atrial one. A subsequent intrinsic complex of a different origin (e.g., conducted via the AV node after an ineffective atrial stimulus) does not rule out failure to capture.
Are pacemaker malfunctions dangerous?
Yes, especially in patients who are entirely pacemaker-dependent (e.g., with complete AV block). Failure to capture or lack of pacing in such patients can lead to asystole and Stokes-Adams attacks. In patients with a preserved intrinsic rhythm, a malfunction is often asymptomatic.
What is undersensing in simple terms?
This is insufficient pacemaker sensitivity: it fails to sense the heart's intrinsic electrical activity and delivers a stimulus off-schedule, sometimes superimposing a spike directly onto an intrinsic complex or T wave.
Can a pacemaker malfunction be diagnosed solely by ECG without a programmer?
The ECG allows suspicion of a malfunction based on characteristic artifacts, but accurate diagnostics (capture thresholds, lead impedance and integrity, battery charge) require device programmer telemetry.
How does oversensing differ from undersensing?
During oversensing, the pacemaker mistakenly interprets interference or a T wave as an intrinsic heart beat and unjustifiably inhibits the stimulus, resulting in a pause on the ECG. During undersensing, conversely, the pacemaker fails to see real intrinsic activity and delivers a redundant or inappropriate spike.
ECG Course
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Related terms
More in the section "Cardiac Pacing"
Sources
- Goldberger's Clinical Electrocardiography, 2021
- AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the ECG, 2009
- Surawicz, Knilans — Chou's Electrocardiography in Clinical Practice
- 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.