A comparison of unipolar and bipolar electrograms for cardiac pacemaker sensing

Simultaneous unipolar and bipolar electrograms were recorded and compared from 49 pacemaker patients with bipolar endocardial electrodes. Average bipolar depolarization signal voltage equalled that of unipolar but showed greater variation. Bipolar and unipolar slew rates were equal in both mean and variance. The proximal pole voltage had little effect on the bipolar result in 8% of the cases, tended to cancel the tip voltage in 49% of the cases and augmented the tip voltage in 43% of the electrograms. The average bipolar R wave duration was 28% less, the T wave amplitude 34% less, and the ST-segment elevation 37% less than the unipolar values. By consistently attenuating the undersirable T waves and ST elevations, while leaving the depolarization signal unaffected, the bipolar electrode offered the advantage of a superior signal-to-noise ratio for sensing depolarization. In one case, however, the bipolar signal was so small as to cause a clinical sensing failure.