The mythology of threshold variations as a function of electrode surface area.

It has been established that the chronic thresholds of cardiac pacing leads vary as a function of the (spherical) electrode's radius or (geometric) surface area and the thickness of fibrotic encapsulation. Where the radius of the electrode is equal to the thickness of the fibrous capsule (about 0.7 to 1 mm for polished surfaces), threshold should be at a minimum. Where the radius of the electrode is larger or smaller than the thickness of the fibrous capsule, then thresholds should increase since the electric field strength required to stimulate decreases as the square of the distance between the electrode's surface and stimulatable tissue. In addition, it has become (incorrectly) accepted that small electrodes do not sense well. About 8-mm electrodes, therefore, became the "standard" surface area, providing the best tradeoffs between pacing and sensing. Analysis of 18 years of canine data in our laboratory, however, suggest that these relationships may be overemphasized for the surface areas of clinical interest. In fact, new small porous and steroid-eluting electrodes do not have high thresholds, are efficient, and their sensing is excellent.