ECG criteria
- The pacing spike is delivered untimely relative to the intrinsic P wave or QRS that should have been detected — meaning the actual sequence of events does not match the programmed temporal logic of the device (base interval, AV/VV delay, post-ventricular atrial refractory period, blanking periods)
- When correlated with device programming: the interval from the intrinsic event to the next spike does not fit the expected pattern of the given mode — this must be evaluated based on specific parameters rather than a single 'base interval'
- The spike may land on the T wave of the preceding intrinsic QRS (risk of pacing during the vulnerable period of repolarization) — a concerning finding, but not mandatory
- Fusion beats (intermediate QRS morphology between intrinsic and paced) and pseudofusion beats (spike superimposed on an already initiated intrinsic complex while QRS morphology remains purely intrinsic) may be observed; both phenomena reflect rhythm interaction but do not prove undersensing on their own, as they also occur during normal demand pacing
- In atrial undersensing — missed P wave: the spike is delivered without regard to preceding atrial activity, visible in leads II, V1
- In ventricular undersensing — missed intrinsic QRS: the ventricular spike is delivered without regard to the preceding complex when it should have been inhibited or reset per device programming
- Absence of expected inhibition or resetting of the pacing timer in the presence of a clear intrinsic P/QRS — confirmed by correlating with device interrogation data
What it looks like on ECG
In undersensing, the pacemaker fails to respond to the patient's intrinsic P wave or QRS complex as programmed, and delivers a pulse that would normally be inhibited or delayed.
Atrial undersensing: against the background of an intrinsic sinus rhythm with normal P waves in II, V1, an atrial spike appears, delivered independently of preceding atrial activity — the sequence does not match the programmed temporal logic of the device, meaning the pacemaker failed to recognize this P wave.
Ventricular undersensing: following an intrinsic QRS that should have inhibited or reset the ventricular pacing output per device programming, the device still delivers a spike. Sometimes it falls on the T wave — a concerning, but neither mandatory nor specific finding. A picture of 'competition' between intrinsic and paced rhythms arises.
Indirect findings requiring cautious interpretation include fusion and pseudofusion complexes. In fusion, intrinsic and paced depolarization waves simultaneously activate different myocardial areas, resulting in a mixed, intermediate QRS morphology. In pseudofusion, the stimulus lands on an already initiated intrinsic depolarization, but no additional myocardial capture occurs — the spike simply superimposes on the QRS, while the complex morphology remains entirely intrinsic. Both phenomena reflect the interaction of two rhythms, but neither confirms undersensing by itself — they are also possible during normally functioning demand pacing. The diagnosis of undersensing requires a separate analysis of pacing conformity to the programmed mode.
Differentiate from normal function: normally, pacing corresponds to the programmed temporal logic of a specific mode (base interval, AV delay, hysteresis, rate adaptation, refractory and blanking periods), and a detected intrinsic event typically inhibits or resets the corresponding output pulse. In undersensing, this logic is disrupted: the expected response to an intrinsic signal is absent.
| Sign | Normal sensing | Undersensing |
|---|---|---|
| Response to intrinsic P/QRS | inhibition or timer reset | no expected response |
| Compliance with device program | pacing fits the set mode logic | mismatch between actual sequence and program |
| Spike position | determined by mode timers | may fall on T or P wave, but not necessarily |
Mechanism
Sensing in a pacemaker is determined by the intracardiac electrical signal recorded by the lead via an endocardial electrogram, and the algorithms of the specific generator: signal amplitude and characteristics, device temporal windows and filters, refractory and blanking periods are taken into account. The surface amplitude of the P or R wave on an ECG is not equal to the intracardiac amplitude 'seen' by the device.
If the actual intracardiac signal is lower than the programmed sensitivity threshold or has an inappropriate morphology, or falls within a period when the channel is temporarily 'blind' (refractory or blanking period), the device does not register it.
As a result, the device's electronic circuitry receives no signal regarding intrinsic activity and delivers a spike according to the set program — regardless of whether the heart has already depolarized independently. This is undersensing: the sensitivity threshold is set too high relative to the actual signal amplitude, or the signal is physically attenuated.
Causes
- Lead displacement (dislodgement) from the implantation site
- Decreased recorded signal amplitude and deteriorated lead-tissue contact, including due to fibrosis around the lead tip (note that fibrosis classically primarily raises the pacing threshold — capture threshold — rather than directly the 'sensing threshold'; the mechanism of undersensing is decreased intracardiac signal amplitude)
- Lead fracture or insulation breach
- Low-amplitude intrinsic signal — e.g., small P wave in atrial pathology, myocardial scar with low-amplitude QRS
- Incorrectly programmed sensitivity threshold (too coarse setting — high value in mV)
- Pacemaker battery depletion (sensing may also be impaired in the terminal stage)
- Myocardial infarction in the lead location area
- Hyperkalemia and other metabolic disturbances, antiarrhythmic drugs — possible, but less specific factors: they affect conduction and signal amplitude differently and may cause not only undersensing, but also oversensing or capture failure; the definitive cause is established during device interrogation with evaluation of electrolytes and medications
Differential diagnosis
Capture failure (failure to capture) — a spike is present, but not followed by depolarization (no P wave or QRS complex after the stimulus); in undersensing, by contrast, an intrinsic complex is present, but the pacemaker fails to notice it.
Oversensing — the reverse situation: the device recognizes extra signals (myopotentials, T waves, artifact) as intrinsic activity and inappropriately inhibits pacing, resulting in pauses. In undersensing, spikes may appear untimely; pauses are more characteristic of oversensing, but the clinical presentation generally depends on the mode and specific device.
Normal function with intermittent intrinsic activity — sometimes rare intrinsic complexes against a paced rhythm are expected behavior of 'demand pacing' rather than a sign of defect; the distinguishing criterion is the conformity of intervals to the device program.
Recording artifacts and interference — may mimic spike irregularity, but do not alter the true relationship between the intrinsic complex and the nearest spike.
Clinical significance and management
Undersensing is dangerous due to competition between intrinsic and paced rhythms: a ventricular spike delivered during the vulnerable period of repolarization (on the T wave) can theoretically provoke ventricular tachycardia or ventricular fibrillation, although this is a rare event with modern pacemakers.
Atrial undersensing is hemodynamically usually less dangerous, but disrupts AV synchrony and the correct operation of the programmed mode (e.g., DDD algorithms): the spike is delivered without accounting for real atrial activity. In some cases, when the spike falls into an inappropriate phase of the atrial cycle, it may contribute to the induction of atrial tachyarrhythmia, but this is neither a typical nor mandatory consequence — the risk depends on the pacing mode and the timing of the stimulus delivery.
When undersensing is suspected on a surface ECG, the physician should refer the patient for pacemaker interrogation: they check the measured amplitude of the intrinsic signal (P-wave and R-wave sensing) and compare it with the programmed sensitivity threshold, assess lead integrity (impedance), and check for signs of lead dislodgement on a chest X-ray. Sensitivity adjustment (decreasing the threshold in mV, i.e., increasing device sensitivity) is performed only after interrogation and determination of the cause, ensuring that this does not provoke oversensing (especially in the presence of noise, myopotentials, or lead insulation failure), and taking into account current blanking/refractory periods. In case of a mechanical lead defect, lead repositioning or replacement is performed.
Common interpretation errors
- Mistaking physiological intermittent appearance of intrinsic complexes (demand mode) for undersensing without analyzing programmed intervals
- Interpreting fusion and pseudofusion complexes as definitive proof of undersensing, although they can also occur during normal device operation
- Confusing undersensing with failure to capture — examining not only the spike, but also what happens before and after it
- Failing to take into account the pacing type (atrial/ventricular/dual-chamber) and specific timing parameters of the mode when assessing which specific channel is losing detection
- Drawing conclusions without access to the device programming parameters — a surface ECG only provides a suspicion, and confirmation requires device interrogation
1. Which ECG sign is most characteristic of pacemaker undersensing?
Undersensing is the failure of the device to detect intrinsic activity, causing the sequence of stimuli to deviate from the programmed mode.
2. How does undersensing differ from failure to capture?
These are two different types of malfunction: one is related to signal detection, and the other to stimulus effectiveness.
3. Which cause is most characteristic of the development of undersensing?
A decrease in the amplitude of the true intracardiac signal due to lead dislodgement or contact alteration prevents the signal from reaching the detection threshold.
Frequently asked questions
How to distinguish undersensing from failure to capture on an ECG?
Undersensing is a detection failure: the device fails to recognize the intrinsic P wave or QRS complex and delivers an untimely stimulus, contrary to the programmed logic. Capture following such an erroneous stimulus is assessed separately—it may be normal, absent (failure to capture), or result in fusion/pseudofusion, as undersensing and failure to capture can coexist. In isolated failure to capture, the spike is delivered on time, but is followed by neither a P wave nor a QRS complex.
Is pacemaker undersensing dangerous?
Yes, especially ventricular — a spike may fall into the vulnerable period of repolarization (on the T wave) and theoretically provoke ventricular tachyarrhythmia, although this is a rare event. Atrial undersensing is usually hemodynamically less dangerous, but disrupts AV synchrony and the function of the programmed mode.
Can undersensing be seen on a standard surface ECG?
It can be suspected based on the discrepancy between the sequence of stimuli and intrinsic complexes relative to the device's programmed logic, but the exact diagnosis and cause are established during pacemaker programming (interrogation), where measured signal amplitudes are visible.
How is undersensing treated?
Treatment is non-pharmacological — after interrogation and clarification of the cause, the physician reprograms the device's sensitivity threshold (monitoring to ensure this does not cause oversensing) or, if the cause is mechanical (dislodgement, lead fracture), performs lead repositioning or replacement.
How does undersensing differ from oversensing?
These are opposite problems: in undersensing, the device does not see the true intrinsic signal and delivers extra stimuli; in hypersensing (oversensing), it mistakes extraneous signals for intrinsic activity and inappropriately inhibits the required stimulus, causing pauses.
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Related terms
More in the section "Cardiac Pacing"
Sources
- Goldberger's Clinical Electrocardiography, 9th ed.
- Hampton. The ECG in Practice
- Surawicz, Knilans. Chou's Electrocardiography in Clinical Practice, 6th ed.
- 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.