His bundle injury current during lead implantation
His bundle pacing has expanded the options for physiologic cardiac stimulation, particularly when right ventricular pacing could worsen ventricular dyssynchrony or when conduction system recruitment is clinically desirable. During implantation, operators may observe a transient current of injury on the unipolar electrogram recorded from the pacing lead. This signal reflects the local electrode–tissue interface as the lead is advanced and fixed near the His bundle.
The finding is clinically interesting because it may provide immediate information when fluoroscopy alone cannot distinguish an optimal His position from a nearby but less effective location. Yet injury current should be interpreted alongside His potentials, paced QRS morphology, capture thresholds, sensing, impedance, and conduction response. It is a marker of tissue interaction, not an independent guarantee of durable conduction system capture.
The available evidence is shaped by differences in lead systems, implantation techniques, recording filters, definitions, and operator experience. A careful assessment therefore requires attention to both the electrogram and the clinical context.
What the signal represents
An injury current is a voltage shift or ST-segment-like deflection on the unipolar electrogram after the lead tip contacts or penetrates tissue. In His bundle pacing, the signal may arise from local myocardial injury at the fixation site and from close contact with tissue surrounding the conduction system. It commonly becomes visible after screwing in a helix lead and may change as the lead is repositioned.
The His bundle lies within a complex anatomical region that includes the membranous septum, atrioventricular conduction tissue, and adjacent ventricular myocardium. A lead can therefore produce a strong local injury signal while remaining slightly superior, inferior, or lateral to the ideal His location. The amplitude and polarity of the current depend on contact, orientation, tissue characteristics, lead design, reference electrode placement, and recording settings.
A transient signal should not be confused with permanent structural damage. Some injury is expected during active fixation, but excessive advancement may increase the risk of perforation, rising thresholds, loss of capture, or damage to nearby conduction tissue. The electrogram is most useful as one component of a multimodal implant assessment.
Predictors of an observable injury current
Lead–tissue contact is the most immediate determinant. A stable lead tip, adequate fixation, and a favorable angle against the septal surface can produce a clearer injury pattern than a lead that is loosely apposed or repeatedly moved. Active fixation may increase the likelihood of seeing a signal, although visibility varies substantially between patients and systems.
Anatomical factors also matter. Septal thickness, the depth of the membranous septum, scarring, calcification, prior valve surgery, and congenital or acquired changes in cardiac structure can alter the electrogram. A prominent His potential may help locate the conduction bundle, but its amplitude is not uniform and may be difficult to record in patients with baseline conduction disease.
The underlying conduction disorder may influence interpretation. Patients with atrioventricular block, bundle branch block, cardiomyopathy, or prior ablation can have altered local signals and different probabilities of successful His capture. Lead type, delivery sheath shape, unipolar versus bipolar recording, filter bandwidth, gain, and the timing of measurement all affect whether a current of injury is recognized.
Operator technique is another important predictor. The number of rotations, the depth of fixation, the use of a backup map, and the sequence of testing can change the signal. These variables make direct comparisons between studies difficult and explain why an absent injury current should not automatically prompt excessive lead advancement.
How clinicians should assess it
Assessment begins with a stable unipolar recording before and after fixation. The operator should document the baseline electrogram, the appearance and amplitude of any injury current, the timing of the His potential, and changes after a short observation period. A signal that rapidly resolves may have a different meaning from one that persists with a marked threshold rise or new conduction abnormality.
Electrical testing remains essential. The capture threshold should be measured at appropriate pulse widths, with attention to selective His bundle capture, nonselective His capture, and myocardial-only capture. QRS duration, paced morphology, stimulus-to-ventricular activation time, and response to changes in output can help establish whether the lead is recruiting the conduction system.
Impedance and sensing provide additional safety information. A sudden impedance change, unstable sensing, diaphragmatic stimulation, or an abrupt threshold increase should prompt a broader review of lead position and possible tissue injury. Fluoroscopy, echocardiography, and chest imaging may be needed when perforation or lead instability is suspected.
A single numerical cutoff is unlikely to be universally reliable. Recording equipment and definitions differ, and the amplitude of injury current may be reported in absolute terms, as a change from baseline, or according to the presence or absence of a visible deflection. Standardized acquisition and reporting would improve the value of this marker in research and clinical practice.
| Finding during implantation | Possible interpretation | Practical response |
|---|---|---|
| Clear transient unipolar injury current with stable low threshold | Good local tissue contact and a potentially favorable implant site | Confirm His capture, sensing, impedance, and lead stability |
| Injury current without His potential or conduction-system response | Nearby septal or ventricular contact rather than definite His capture | Re-map carefully and avoid relying on the signal alone |
| Absent or minimal injury current with acceptable electrical and QRS criteria | Adequate implant may still be present | Use the full pacing assessment rather than advancing the lead solely to create a signal |
| Large persistent signal with rising threshold or unstable impedance | Possible excessive tissue trauma, poor fixation, or perforation | Stop further advancement and reassess lead position and safety |
| Signal that changes after repositioning | Variable contact or orientation | Compare electrograms and pacing responses at each candidate site |
Clinical significance after implantation
The main potential value of His bundle injury current is procedural guidance. When present at a successful site, it may support the impression that the lead has firm contact with tissue close to the target. Some studies have associated a visible injury current with acute His bundle capture, acceptable thresholds, or improved implant success. These associations are useful but should not be treated as proof of long-term lead performance.
The signal may also help explain an unexpected electrical result. A strong injury current with myocardial-only capture could indicate that the lead is close to, but not engaging, the His bundle. Conversely, a modest or absent signal can occur despite successful pacing, particularly when local anatomy or signal quality limits detection. The predictive value is therefore probabilistic rather than absolute.
Long-term follow-up is important because acute success does not eliminate the possibility of threshold rise, loss of His capture, lead dislodgment, or a change from selective to nonselective capture. Device clinics should track thresholds, sensing, impedance, paced QRS morphology, symptoms, and battery implications. Broader rhythm surveillance also matters in patients whose indication includes atrial arrhythmia; practical approaches are discussed in recurrence monitoring.
Safety considerations and differential interpretation
The term injury current can sound alarming, but its presence alone does not establish clinically important tissue damage. The relevant question is whether the signal occurs with acceptable lead performance and without evidence of perforation, progressive threshold elevation, new valve or septal complications, or deterioration in conduction. Interpretation should focus on the complete pattern rather than the visual prominence of the electrogram.
New right bundle branch block, worsening atrioventricular block, or an unexpected change in intrinsic conduction deserves attention, especially when the lead is positioned near the proximal His bundle. Such findings may reflect the underlying disease, mechanical interaction with conduction tissue, edema, or lead-related trauma. They should be documented and reassessed before the procedure ends.
The differential also includes artifact, baseline ST-T abnormalities, far-field ventricular signals, and changes caused by filtering or reference configuration. A high-quality recording with stable connections and a consistent lead setup is needed before comparing measurements. If uncertainty persists, testing from a nearby site and assessing paced morphology can provide more useful information than repeating the same measurement.
Patients should receive routine device follow-up tailored to their pacing dependence and clinical indication. Those who are pacemaker-dependent may require particularly careful threshold surveillance and a programmed safety margin that accounts for the possibility of changing His capture.
Improving procedural decision-making
The most defensible approach is to use injury current as a supportive marker within a structured implant workflow. Operators can first map the His potential, evaluate local anatomy, fix the lead, and then compare electrograms and pacing behavior. If the injury current appears, it should be documented alongside the exact lead location and electrical results rather than used as a standalone endpoint.
Clinical teams can make their records more useful by specifying how the signal was defined, when it was measured, which electrode configuration was used, and whether the current resolved. Consistent documentation will help future studies determine whether amplitude, duration, morphology, or change from baseline has the strongest association with acute and chronic outcomes.
- Confirm conduction-system capture with paced QRS and threshold testing.
- Record injury current measurements using a consistent configuration and time point.
- Reassess lead stability, impedance, and threshold before leaving the laboratory.
- Avoid advancing the lead solely because the injury signal is small or absent.
- Arrange follow-up based on pacing dependence, threshold behavior, and arrhythmia risk.
Prospective research should connect procedural electrograms with imaging, detailed anatomy, long-term capture status, and patient outcomes. Authors preparing work in this area can consult the journal’s author instructions to support transparent reporting of methods, definitions, and follow-up. Better standardization could clarify whether injury current predicts successful His bundle pacing, durable conduction-system recruitment, or simply secure lead contact.
The signal also belongs within the broader science of pacing and rhythm management. Questions about neural modulation, ablation, and pacing may overlap in patients with syncope or complex autonomic disorders; related perspectives are presented in autonomic modulation. Continued collaboration between electrophysiologists, device specialists, and researchers can turn a useful intraoperative observation into a more reliable clinical tool.
For now, His bundle injury current is best viewed as a real-time clue: informative when combined with anatomy and electrical testing, insufficient when interpreted in isolation. Careful measurement, conservative lead handling, and longitudinal follow-up offer the strongest path to safer implantation and more dependable physiologic pacing.