Unipolar vs Bipolar Signals
Every electrogram is a conversation between two points. Depending on where those points are, the signal tells a completely different story.
Fundamentally, an electrogram is nothing more than a difference in voltage between two points. It is a simple subtraction problem drawn over time. But the secret of the EP lab lies entirely in where we place those two points.
If you take one recording electrode and press it against the beating heart, and place the second recording electrode infinitely far away, you get a unipolar signal. It acts as an omnidirectional antenna, sensing the electrical weather of the entire organ as it rolls toward and away from the catheter tip.
If, instead, you place both electrodes millimeters apart right on the tip of your catheter, you get a bipolar signal. It is remarkably nearsighted, blind to the horizon, seeing only the microscopic patch of tissue squeezed directly between its two metal poles.
Each signal sees the exact same depolarization wavefront. But each tells a fundamentally different truth.
The Bipolar Signal
The bipolar electrogram is the workhorse of the modern EP lab. It measures the voltage difference between two closely spaced electrodes—typically the very tip of the catheter (the distal electrode) and a ring just a few millimeters behind it (the proximal electrode).
Why are they spaced so closely together? To achieve something called common mode rejection.
Imagine a massive electrical wavefront, like a normal ventricular contraction, firing far away from your catheter. That distant electrical field is huge. As it washes over your catheter, it hits both the tip electrode and the ring electrode at the exact same millisecond with the exact same voltage.
Because the bipolar signal is simply the difference between the two (Tip minus Ring), and both are experiencing the exact same voltage from that distant storm, the difference is zero. The far-field signal cancels itself out.
The only thing a bipolar pair will "see" is a wavefront small enough and close enough to hit the tip electrode at a slightly different time than the ring electrode. It only registers what is happening directly underneath it. This makes the bipolar signal exceptionally precise for one thing: timing. It gives us a crisp, sharp spike the exact moment the tissue beneath it depolarizes.
The Unipolar Signal
The unipolar signal looks outward. It measures the voltage between the catheter tip inside the heart and a distant "indifferent" electrode—usually a patch placed on the patient's leg or back.
Because the second electrode is so far away, common mode rejection doesn't happen. The unipolar tip sees everything. It sees the distant ventricle contracting, it sees the atria, it sees the whole landscape.
But its true power is in how it behaves as a wavefront moves. Electricity in the heart behaves like a physical wave. When a wavefront of depolarization moves toward the unipolar tip, it pushes positive charge ahead of it, creating a positive deflection on the screen.
As the wavefront slides directly under the catheter tip, the voltage sharply drops from positive to negative. This precipitous drop is the exact moment of local activation.
Finally, as the wavefront moves away from the tip, it leaves a trailing negative tail on the electrogram. The resulting signal looks like an RS wave: a positive mountain as it approaches, a sharp cliff as it passes, and a negative valley as it departs.
The Magic of the QS Pattern
Now, imagine a different scenario. What if the wavefront doesn't approach your catheter from somewhere else? What if the tachycardia originates exactly at the spot where your catheter tip is pressing against the tissue?
If the spark starts exactly beneath your unipolar electrode, there is no wavefront approaching it. The wavefront can only move away from it in all directions.
Because a wavefront moving away creates a negative deflection, a signal originating at the catheter tip will produce an electrogram that plunges straight down. There is no initial positive rise because nothing ever approached. It is entirely negative.
This pure negative unipolar electrogram is called a QS pattern. Finding a beautiful, sharp QS unipolar signal during a focal tachycardia is the holy grail of EP mapping. It means you are standing precisely on ground zero.
To find the precise location to ablate a focal tachycardia, we rely on the marriage of both signals.
The bipolar signal gives us absolute timing. Because it ignores the far-field, its sharpest, fastest deflection marks the exact millisecond the tissue under the catheter fires. We map the chamber looking for the spot where this bipolar signal occurs earlier than anywhere else.
But early timing isn't enough to prove we are at the very origin; we could just be close. That is where the unipolar signal steps in to give us direction. If the unipolar signal on that exact same catheter tip shows a pure QS pattern, it proves the wavefront is moving entirely away from us.
When you have the earliest crisp bipolar signal, perfectly aligned with the precipitous downstroke of a pure QS unipolar signal, you have found the spark. You have found the target.
Key Takeaways
- Bipolar Signals: Measure voltage between two close electrodes. They cancel out distant signals (common mode rejection) and are incredibly precise for local timing.
- Unipolar Signals: Measure voltage between the catheter tip and a distant patch. They see the entire electrical landscape and show the direction of wavefront movement.
- Approaching Wavefront: Creates a positive deflection on the unipolar signal.
- Departing Wavefront: Creates a negative deflection on the unipolar signal.
- The QS Pattern: A purely negative unipolar signal indicates a wavefront originating at the catheter tip and moving exclusively away. Combined with the earliest bipolar timing, it is the hallmark of a focal source.