There are 2 varieties of stimulating electrodes: unipolar and bipolar:
The unipolar electrode has one pole (cathode or negative stimulating pole) in contact with cardiac tissue, and the other (anode or positive pole) outside of the heart, either in subcutaneous tissue or on the surface of the body
The bipolar electrode has both the cathode (sometimes called its distal or tip pole) and the anode (proximal or ring pole) at the cardiac tissue being stimulated
Figure 1 Unipolar vs. Bipolar pacing circuit
A, Unipolar pacing circuit, with an intracardiac cathode located on the lead tip in the right ventricle. The circuit is completed by the active pulse generator, which forms the anode. B, In a bipolar circuit, both cathode and anode are located on the lead, with the anode generally formed by a ring electrode proximal to the tip cathode.
1)Unipolar pacing - clearly visible large spikes on the surface ECG coincident with each pacing output
2) Bipolar pacing - very small stimulus artefacts which are hardly visible on the ECG due to the proximity of both electrodes
ECG 1 AV sequential stimulation - bipolar ventricular pacing (small stimulus before each QRS complex) vs. atrial pacing switched to unipolar pacing due to lead malfunction (large spikes before each p wave)
ECG 2 Atrial sensed right ventricular bipolar pacing (small stimulus before each QRS complex)
DeCaprio V, Hurzeler P, Furman S. A comparison of unipolar and bipolar electrograms for cardiac pacemaker sensing. Circulation. 1977 Nov;56(5):750-5. doi: 10.1161/01.cir.56.5.750. PMID: 912833.
Kersschot I. (1994) Unipolar versus bipolar leads. In: Aubert A.E., Ector H., Stroobandt R. (eds) Cardiac Pacing and Electrophysiology. Springer, Dordrecht. https://doi.org/10.1007/978-94-011-0872-0_20