Unipolar pacing versus bipolar pacing

ECG 6A: Atrial and ventricular pacing in bipolar mode (small amplitude of atrial and ventricular stimuli which are sometimes unidentifiable);

ECG 6B: Identical tracing but with atrial and ventricular pacing in unipolar mode;

Comments: The vast majority of pacing leads implanted today in the right atrium or right ventricle are bipolar. On a bipolar lead, it is possible to program both pacing and sensing configurations in unipolar or bipolar mode. All pacing circuits are bipolar by definition with a flow of electrons moving from the cathode to the anode. The term unipolar pacing is thus incorrect and reflects the fact that a single electrode is located at the end of the lead in contact with the heart, the pacemaker device representing the other electrode of the circuit. For bipolar pacing, the two electrodes of the circuit are located at the end of the lead in the heart separated by a very small inter-electrode distance.

Certain elements favor the choice of a unipolar pacing configuration. The unipolar pacing threshold is typically very slightly lower than in bipolar pacing due to a slightly lower overall pacing resistance (larger anode). The pacing artifact is more easily identifiable being more prominent (at equal amplitude) during unipolar pacing than during bipolar pacing. The size of the stimulus in unipolar mode facilitates the interpretation of the tracing especially if the physician or nurse is unaware that the patient is wearing a pacemaker.

The limitations of unipolar pacing reside in the fact that the field of pacing being wider and including the can, the risk of pectoral pacing in the pocket and the risk of cross-talk (atrial pacing sensed by the ventricular channel and conversely) are considerably increased.

Take-home message: Unipolar pacing can be easily identified on the electrocardiographic tracing by the high amplitude of the stimuli. It should be noted that pacing is always bipolar in implantable defibrillators.