What defines unipolar for a catheter?

• A. Sensor in the heart and electrode on the same catheter but outside the heart.
• B. Sensor in the heart and electrode in the heart.
• C. Sensor outside the heart and electrode inside
• D. Two sensors outside the heart

Answer: (A)

Unipolar recording uses a single sensing electrode inside the heart with a distant reference electrode placed outside the heart (or at a distant site). The electrical activity is measured as the potential difference between the intracardiac sensor and this far-away reference, giving a single-ended, “unipolar” signal. This contrasts with bipolar recording, where two closely spaced intracardiac electrodes provide a differential signal that emphasizes local activation and reduces far-field noise. So the defining setup is a sensor in the heart paired with an electrode outside the heart as the reference.