Reading Intracardiac Electrograms
From satellite images to weather stations on the ground. How we track the electrical storm from the inside.
The surface ECG is a satellite image. It looks at a hurricane from space. You can see the general direction and size of the storm, but the details are a blur.
Intracardiac electrograms are weather stations on the ground.
They don't just tell you a storm is happening. They tell you exactly when the storm hits a specific city block.
Highly Localized Truth
An intracardiac electrogram (EGM) is a highly localized recording. It measures the voltage difference between two very closely spaced metal poles on a catheter inside the heart.
Unlike a surface ECG lead, which looks at the electrical sum of millions of cells across the entire chest, an EGM only "sees" the tissue immediately touching it.
When the wave of depolarization passes exactly under those two poles, the recording needle jumps. If the wave is happening on the other side of the heart, the catheter sees almost nothing.
The Standard Placement
When a patient arrives in the EP lab, we typically slide several multi-electrode catheters through the femoral veins and maneuver them into the right side of the heart.
We place them in specific anatomical landmarks. The High Right Atrium (HRA) catheter sits near the SA node, catching the earliest atrial signals. The Right Ventricular Apex (RVA) catheter sits at the very bottom of the right ventricle.
The Coronary Sinus (CS) catheter is threaded into the venous drainage of the heart, which conveniently wraps around the back of the left atrium and ventricle. It serves as our roadmap to the left side of the heart, without us ever having to puncture the septum.
Timing over Morphology
On a surface ECG, we obsess over the shape of the QRS complex. Is it wide? Is there a delta wave? Is it notched?
On an intracardiac EGM, morphology matters far less. We care about one thing above all else: timing.
We are not asking what the wave looks like. We are asking: "Did the electrical wave hit the Coronary Sinus before it hit the High Right Atrium?"
In the EP lab, whoever activates first is closest to the source of the arrhythmia.
The Rosetta Stone
The His bundle (HIS) catheter sits right across the tricuspid valve, recording the tiny bridge of tissue connecting the upper and lower chambers. It is the Rosetta Stone of electrophysiology.
A properly positioned His catheter records three distinct signals in a single heartbeat: Atrial activation (A), His bundle activation (H), and Ventricular activation (V).
It cleanly separates the "above the His" world from the "below the His" world. If a block occurs between the A and the H, the delay is safely within the AV node. If it occurs between the H and the V, the block is deep in the conduction system—a much more dangerous scenario, as we explored in Volume IV.
The Coronary Sinus (CS) catheter is an extraordinary tool. It typically has 10 metal poles (arranged as 5 recording pairs). Because the coronary sinus is a vein that wraps around the posterior mitral annulus, sliding a catheter into it gives us a direct, internal view of the left atrium.
By convention, CS 9,10 is the proximal pair sitting just inside the opening on the right side of the heart. CS 1,2 is the distal pair sitting far out on the left lateral wall.
Reading the CS catheter from 9,10 to 1,2 shows you exactly how the left atrium activates. If the electrical wave hits CS 9,10 first and sweeps out to 1,2, the impulse is moving normally from right to left. If it hits CS 1,2 first, the storm is originating on the far left side and moving backwards.
Key Takeaways
- Scale: The surface ECG provides a global view of the heart; intracardiac EGMs are highly local weather stations.
- Timing: In the EP lab, analyzing the relative timing of local activations is far more important than the shape (morphology) of the signals.
- Anatomy: Standard catheters include the HRA (right atrium), HIS (AV junction), CS (left atrium), and RVA (right ventricle).
- The Pivot: The His bundle catheter is the only place we can simultaneously record atrial (A), His (H), and ventricular (V) activation.
- The Map: Reading the 10-pole Coronary Sinus catheter from proximal (9,10) to distal (1,2) reveals the exact left-to-right or right-to-left activation sequence of the left atrium.