Unipolar & Bipolar signals
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# Bipolar vs Unipolar Signals in EP Mapping
## Key Definitions
Q: What is a bipolar electrogram?
A: A bipolar electrogram records the voltage difference between two nearby catheter electrodes.
Q: What is a unipolar electrogram?
A: A unipolar electrogram records from one catheter electrode compared to a distant reference electrode.
Q: Which signal is more local: bipolar or unipolar?
A: Bipolar is more local.
Q: Which signal has a larger field of view?
A: Unipolar has a larger field of view.
## Bipolar Signals
Q: What are bipolar signals best used for?
A: Activation mapping, voltage mapping, scar identification, and identifying fractionated signals.
Q: Why are bipolar signals usually sharp?
A: Because they mostly show local near-field electrical activity.
Q: What does a high-amplitude bipolar signal usually suggest?
A: Healthy tissue.
Q: What does a low-amplitude bipolar signal usually suggest?
A: Scar, diseased tissue, poor contact, or catheter orientation issues.
Q: Why can bipolar amplitude change with catheter direction?
A: Because bipolar signals depend on the direction of the wavefront relative to the two electrodes.
Q: When is a bipolar signal usually largest?
A: When activation travels parallel to the electrode pair.
Q: When can a bipolar signal become small or misleading?
A: When activation travels perpendicular to the electrode pair.
## Unipolar Signals
Q: What are unipolar signals best used for?
A: Determining wavefront direction and confirming whether the catheter is near the site of origin.
Q: Why do unipolar signals show more far-field activity?
A: Because the reference electrode is distant, so the signal captures a larger area of electrical activity.
Q: What does a pure QS unipolar signal suggest?
A: The activation is moving away from the catheter, suggesting the catheter may be at or very near the site of origin.
Q: Why does a QS pattern suggest the origin is under the catheter?
A: Because the wavefront begins near the electrode and spreads away from it.
Q: What does an rS unipolar pattern suggest?
A: The catheter is close to the source, but activation likely reached the catheter from another nearby site first.
Q: What does an RS unipolar pattern suggest?
A: The wavefront is passing by the catheter rather than starting directly underneath it.
## Near-Field vs Far-Field
Q: What is near-field activity?
A: Electrical activity from tissue directly near the catheter.
Q: What does near-field activity look like?
A: Sharp, narrow, and high-frequency.
Q: What is far-field activity?
A: Electrical activity from tissue farther away from the catheter.
Q: What does far-field activity look like?
A: Broader, smoother, and lower-frequency.
Q: Why is it important to separate near-field from far-field signals?
A: Because far-field activity can make a site look earlier or more important than it actually is.
## Earliest Activation
Q: In PVC mapping, what does “-30 ms” mean?
A: The local electrogram occurs 30 milliseconds before the surface QRS.
Q: In atrial tachycardia mapping, what timing are you looking for?
A: The earliest atrial electrogram before the surface P wave.
Q: What three findings make a strong ablation target for a focal PVC?
A: Earliest bipolar activation, QS unipolar morphology, and a high pace-map match.
Q: What does “good bipolar timing but no perfect QS” suggest?
A: The catheter may be close to the exit site but not directly over the true origin.
## Clinical Examples
Q: What findings support a good RVOT PVC ablation target?
A: Early bipolar timing, pure QS unipolar signal, and a high pace-map match.
Q: Why can papillary muscle PVCs be difficult to map?
A: Because catheter stability is harder, exits can shift, and the unipolar signal may not show a perfect QS.
Q: Why might RF lesions be placed around a papillary muscle region instead of one exact point?
A: Because papillary PVCs may have variable exits, catheter contact changes, or preferential conduction.
Q: What does preferential conduction mean?
A: The true origin may be in one location, but the arrhythmia exits to the surface from a different nearby location.
## Comparison
Q: Bipolar signals are best for what?
A: Local timing, voltage mapping, scar, and fractionation.
Q: Unipolar signals are best for what?
A: Direction of activation and confirming whether the catheter is near the origin.
Q: Which signal is better for voltage mapping?
A: Bipolar.
Q: Which signal is better for confirming a focal source?
A: Unipolar.
Q: Which signal is more affected by catheter orientation?
A: Bipolar.
Q: Which signal is more affected by far-field activity?
A: Unipolar.
## Memory Tricks
Q: How can you remember bipolar signals?
A: Bipolar means “between two electrodes.”
Q: How can you remember unipolar signals?
A: Unipolar means “one electrode looking out at the bigger field.”
Q: What should you think when you hear “-30 with a QS”?
A: The local signal is 30 ms early and activation is moving away from the tip, which supports a likely source site.
Q: What should you think when the bipolar is early but the unipolar is not QS?
A: The catheter may be near the source but not exactly on it, or the origin may be intramural, epicardial, papillary, or using preferential conduction.
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