Learn EP · Lesson 3 of 6

Intracardiac Electrograms

Core EP skill25–35 minSignals & recording

Intracardiac electrograms are local electrical recordings obtained from catheter electrodes inside the heart. Their value comes from precise timing, anatomical context, and comparison across channels.

Illustrative recording channels Surface IIHRAHisCS proxCS distRV AHV

Common catheter recording positions

High right atrium

Provides a local atrial electrogram and can also be used for atrial pacing.

His region

A carefully positioned recording may contain atrial (A), His (H), and ventricular (V) components.

Coronary sinus

Multipolar recordings show activation along the AV groove and are especially useful for comparing atrial activation sequence.

Right ventricle

Provides ventricular electrograms and a site for programmed ventricular stimulation.

A, H and V signals

On a His-region recording, the A component represents local atrial activation, the H component represents His-bundle activation, and the V component represents ventricular activation. Identifying them requires more than morphology: correlate them with catheter position, other channels and timing.

Coronary sinus activation sequence

CS catheters are usually displayed as a sequence of bipoles. The order in which those bipoles activate provides a spatial clue about atrial activation. A change from one sequence to another can be diagnostically important, but the meaning depends on catheter orientation and rhythm context.

Bipolar vs unipolar recordings

Bipolar

Displays the voltage difference between two nearby electrodes. It tends to emphasize relatively local activation and is widely used for timing and mapping.

Unipolar

Compares a local electrode with a distant reference. It contains different spatial information and can be useful in selected mapping applications.

Near-field vs far-field signals

Near-field components usually arise from tissue close to the recording electrodes and may appear sharper or more prominent. Far-field components arise from more distant electrical activity. Amplitude and sharpness alone are not enough to prove origin.

Filtering, gain and sweep speed

FiltersChange the frequency content that is displayed. Excessive filtering can alter signal appearance, so settings must be appropriate for the recording goal.
GainChanges displayed amplitude. Gain can make small components easier to see but does not change the underlying timing.
Sweep speedChanges the time scale. Faster sweep speeds spread events apart visually and help with fine timing measurements.

Golden rule

Never interpret an intracardiac signal in isolation. Correlate anatomy + catheter position + timing + neighboring channels + pacing state + surface ECG.

Catheter positions: connect anatomy to the channel label

HRA His CS RV Simplified educational anatomy — positions vary with procedure, anatomy, and catheter orientation.

Coronary sinus activation: why sequence matters

Example A: earlier distal activation

CS 1-2CS 3-4CS 5-6CS 7-8CS 9-10 Illustration: earliest signal at CS 1-2, then progressively later proximally.

Example B: earlier proximal activation

CS 1-2CS 3-4CS 5-6CS 7-8CS 9-10 Illustration: earliest signal at CS 9-10, then progressively later distally.
Important: A CS activation sequence is a spatial clue, not a diagnosis by itself. Electrode numbering and what is labeled “proximal” or “distal” depend on the lab’s display convention and catheter orientation.

Bipolar vs unipolar: same activation, different view

Bipolar example

Local voltage difference between two nearby electrodesOften emphasizes relatively local, high-frequency activation.

Unipolar example

Local electrode referenced to a distant electrodeContains different spatial information and often a broader waveform.

Mini tracing quiz: can you identify A, H, and V?

Question 1

On a His-region recording, which component should represent activation of the His bundle?

Question 2

If the earliest atrial electrogram in a CS catheter appears on the most distal displayed bipole, what is the safest interpretation?

Question 3

Which display change spreads electrograms farther apart horizontally so fine timing is easier to compare?

Trusted resources

Heart Rhythm Society — EP Lab Standards
Laboratory processes, protocols, equipment, personnel, safety and quality.
Open HRS resource →
ESC/EHRA — Heart Rhythm learning references
Recommended reading and competency topics for invasive electrophysiology.
Open ESC resource →
Educational scope: This lesson explains foundational concepts. It does not teach independent performance or interpretation of invasive EP procedures. Follow institutional training, current professional guidance, and manufacturer instructions.
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