Reading Unipolar Electrograms for Local and Remote Cardiac Signals

Intracardiac electrograms are central to modern electrophysiology, yet their interpretation depends on more than amplitude alone. A signal recorded from a catheter electrode may represent electrical activity immediately beneath the electrode, activity travelling through adjacent tissue, or activation arising in a different chamber. Distinguishing these components is essential for accurate mapping, ablation, pacing, and device troubleshooting.

Unipolar recordings are particularly useful because they preserve information about the direction and timing of activation relative to a reference electrode. Their broad field of view can reveal wavefront propagation and lesion effects that may be less apparent on bipolar recordings. That same sensitivity, however, makes unipolar signals vulnerable to far-field activity, reference effects, pacing artefact, and changes in catheter orientation.

For clinicians and researchers working across Australia, these principles apply in high-volume electrophysiology services in Sydney, Melbourne, Brisbane, Perth, and Adelaide, as well as in regional hospitals supported by telehealth and visiting specialist teams. A consistent approach to signal analysis can improve procedural decisions when access to advanced mapping expertise varies between metropolitan and rural settings.

Why Near-Field And Far-Field Signals Matter

A near-field electrogram is generated predominantly by depolarisation in tissue immediately adjacent to the recording electrode. It usually has a relatively sharp, high-frequency component that coincides closely with local activation. In contrast, a far-field electrogram reflects electrical activity from a more distant structure, such as the opposite atrium, ventricle, His–Purkinje system, or a neighbouring region of scarred myocardium.

The distinction is clinically important because an apparently early electrogram may be remote rather than local. During atrial tachycardia mapping, for example, a ventricular far-field signal can be mistaken for a sharp atrial deflection. In atrial fibrillation, overlapping wavefronts may produce complex fractionated signals that obscure the true local activation time. Misidentifying a remote component can lead to inaccurate activation maps and unnecessary ablation.

Unipolar recordings are especially valuable when the electrode is positioned near a suspected source. The onset of the local deflection, its polarity, and the relationship between the electrogram and the surface QRS or P wave can help establish whether the electrode is detecting local tissue activation. Interpretation should remain integrated with anatomy, catheter position, pacing manoeuvres, and other intracardiac channels.

Unipolar Features That Suggest Local Activation

The most useful feature is often the steepest, fastest voltage change, commonly described as the intrinsic deflection or maximum negative dV/dt. When the depolarisation wavefront travels towards the electrode, a prominent negative deflection may appear. A QS pattern, with an initial positive component absent, can support a site close to the origin or earliest activation front, although it is not definitive in isolation.

Signal timing also matters. A near-field event generally has a stable relationship with neighbouring electrodes and with the expected activation sequence. It may precede the surface P wave during focal atrial tachycardia or occur early relative to the QRS during ventricular tachycardia. A far-field component often remains temporally fixed to activity in another chamber, even when the catheter is moved within the target chamber.

Amplitude and duration provide supporting evidence rather than absolute rules. Local signals are frequently larger and sharper, while far-field signals may be lower amplitude, broader, or smoother. Yet a small near-field electrogram can occur in diseased, fibrotic, or heavily ablated tissue, and a strong remote signal can be recorded when the catheter is close to a large electrically active structure.

Morphology, Polarity, And Reference Effects

Unipolar morphology reflects the three-dimensional relationship between the activation wavefront and the electrode. A predominantly negative signal may indicate movement towards the electrode, while a positive–negative pattern can occur when activation first moves away and then towards it. The precise waveform depends on electrode size, tissue contact, conduction velocity, fibre orientation, filtering, and the location of the indifferent reference.

A distant reference does not make the recording electrically “pure”. It creates a global voltage measurement in which local and remote sources are superimposed. Changes in reference location, grounding, filter settings, or lead configuration can therefore alter the appearance of an electrogram without any biological change. This is particularly relevant when comparing recordings from different mapping systems or laboratories.

Bipolar channels remain important because they suppress common distant activity and emphasise spatial voltage gradients between adjacent electrodes. However, bipolar electrograms can become deceptively small when the wavefront travels parallel to the electrode pair. Reviewing unipolar and bipolar signals together can reveal a local deflection that is hidden by bipolar cancellation or identify a far-field component that has been overinterpreted as local.

Practical Interpretation During Mapping

Signal interpretation should begin with catheter position and anatomy. Fluoroscopy, intracardiac echocardiography, three-dimensional mapping, and impedance trends can establish whether an electrode is in contact with the intended tissue. A sharp unipolar signal recorded from a catheter near the mitral annulus, for instance, may still include atrial, ventricular, or coronary sinus activity depending on exact orientation.

Pacing is a useful diagnostic tool. Stimulus-to-electrogram and stimulus-to-QRS intervals, pace mapping, and changes in activation sequence can help determine whether the recorded signal belongs to the target tissue. Moving the catheter by a few millimetres may cause a true local signal to change substantially, whereas a far-field component may remain relatively unchanged. Care is required because catheter movement can also alter contact and signal quality.

During ventricular tachycardia ablation, a pre-QRS unipolar electrogram with a steep negative deflection can support a site near the exit or origin. In scar-related tachycardia, however, late potentials and fractionated signals may arise from slow-conduction channels while the largest deflection comes from surrounding viable myocardium. For atrial procedures, apparent early activation near the septum may reflect His or ventricular far-field activity, making chamber-specific timing and multipolar comparisons essential.

Australian laboratories often use a mixture of mapping platforms and catheter technologies, influenced by local procurement, hospital contracts, and the availability of specialised operators. Clear documentation of filter settings, reference electrodes, catheter geometry, and electrogram annotations helps make findings reproducible between a tertiary centre such as Westmead Hospital and a regional service linked through a networked programme.

Common Sources Of Misclassification

Far-field ventricular activity is a frequent problem in atrial recordings, especially near the atrioventricular valves, septum, and coronary sinus ostium. It may produce a sharp deflection that coincides with the QRS and can be mistaken for atrial activation. Differential catheter recordings, analysis during bundle branch block or paced rhythms, and comparison with atrial and ventricular reference channels can clarify the source.

Pacing artefact can obscure the onset of local activation and create ringing or saturation in the recording system. Poor contact, excessive catheter motion, respiration, and mechanical interaction with a valve may produce unstable electrograms. Myopotential contamination and electromagnetic interference should also be considered when a waveform changes abruptly without a corresponding anatomical or rhythm change.

Ablation introduces further complexity. Oedema, lesion maturation, oedema resolution, and changing catheter contact can alter amplitude and morphology over minutes or hours. A reduction in unipolar amplitude does not necessarily prove transmural injury, while a persistent local signal does not automatically indicate failed lesion delivery. Energy source, tissue thickness, irrigation, impedance, and lesion location all influence the recording.

Clinical context remains decisive. A useful interpretation asks whether the electrogram fits the activation sequence, the anatomy, the pacing response, and the surface ECG. For clinicians reviewing outcomes after atrial fibrillation procedures, broader procedural and patient factors also matter; evidence on readmission risk factors illustrates why an isolated signal finding should not be treated as a complete measure of procedural success.

Building A Reproducible Signal Analysis Workflow

A practical workflow starts by confirming the recording conditions. Record the rhythm, cycle length, catheter location, electrode configuration, reference, gain, filter range, and relevant pacing state. Mark the earliest reproducible local deflection rather than simply the largest peak. When signals are fractionated, annotate the component that has the clearest anatomical and temporal relationship to the target tissue.

Next, compare multiple views of the same event. Unipolar morphology can suggest wavefront direction, bipolar channels can assess local spatial gradients, and surface ECG leads can provide chamber-level timing. Intracardiac reference catheters help identify whether a suspected component is linked to atrial, ventricular, or His–Purkinje activation. Reviewing stored signals in several beats is preferable to relying on one complex electrogram.

For research, automated algorithms should incorporate more than peak amplitude. Features such as maximum negative slope, onset-to-peak interval, signal duration, polarity, spectral content, stability across beats, and timing relative to reference channels can improve classification. Machine-learning models may assist with large datasets, but their labels must be based on expert anatomical interpretation and validated across different recording systems.

Standardisation is also valuable for training. Fellows and allied health professionals can review annotated cases showing true local activation, chamber far-field signals, pacing artefact, and scar-related fractionation. Australian centres participating in multicentre studies should define signal annotation rules in advance, particularly when data are exported between systems or collected from sites with different catheter platforms. Authors preparing such work can consult the journal’s submission information before finalising reporting methods and figure formats.

The strongest interpretation combines waveform characteristics with anatomy, timing, pacing response, and procedural context. Unipolar electrograms do not provide a single diagnostic sign that separates near-field from far-field activity in every case. They offer a set of complementary clues that become reliable when assessed systematically and checked against independent evidence.

Clinicians, trainees, and investigators can apply this framework to mapping reviews, device recordings, ablation studies, and teaching conferences. Careful signal annotation and transparent reporting will support safer procedures, clearer research findings, and more consistent electrophysiology practice across Australia and the wider Asia-Pacific region.