12 lead ECG: unipolar atrial and ventricular pacing

• permanent atrial and ventricular pacing
• large amplitude of the pacing atrial and ventricular stimuli

12 lead ECG: bipolar atrial and ventricular pacing

• permanent atrial and ventricular pacing
• small amplitude of atrial and ventricular stimuli which are unidentifiable

Comments

• the vast majority of pacing leads implanted today in the right atrium or right ventricle are bipolar; 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 can representing the other electrode of the circuit
• in a bipolar lead, the negative electrode is located at the tip of the catheter and the positive electrode approximately 1 cm proximal to the tip; the current flows from the negative to the positive electrode
• certain elements favor the choice of unipolar pacing
• unipolar pacing thresholds are typically very slightly lower than bipolar pacing thresholds 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 distance between the 2 electrodes being bigger; the magnitude of the stimulus in unipolar mode facilitates the interpretation of the tracing especially if the physician or nurse is unaware that the patient is implanted with a pacemaker
• the limitations of unipolar pacing reside in the fact that the field of pacing being wider (longer distance between the electrodes) and including the can, the risk of pectoral pacing in the pocket and the risk of cross-talk/far-field oversensing (atrial pacing sensed by the ventricular channel and conversely) are considerably increased