Limitations of bipolar and unipolar conditioning stimuli for inhibition in the human heart

Abstract

Noncapturing, conditioning electrical stimuli (S c) delivered within the ventricular refractory period can prolong refractoriness and prevent later stimuli from eliciting a propagated response (inhibition). The purpose of this study was (1) to further define the spatial effects of S c, (2) to determine if the effects of S c can be enhanced by the use of unipolar as opposed to bipolar stimulation, and (3) to evaluate the effect of S c on the physiologic spread of excitation during atrioventricular reentry tachycardia. In 23 patients the right ventricular refractory period was determined before and after the introduction of bipolar, unipolar cathodal, and unipolar anodal noncapturing S c with pulse widths of 2 or 9 msec and strengths of twice diastolic threshold and 10 MA. Pacing and conditioning stimuli were delivered at the same site and at sites separated by 3 mm. During ventricular pacing both bipolar and unipolar S c prolonged the ventricular refractory period by ≥10 msec in 22 of 23 patients when both S c and pacing stimuli were delivered to the same site. However, when S c was delivered 3 mm away from the pacing stimuli, the ventricular refractory period increased by ≥10 msec in only 1 of 17 patients who received bipolar S c and in none of 13 patients who received unipolar S c. In seven patients bipolar conditioning stimuli were delivered as close as possible to the atrial insertion of an accessory atrioventricular connection during circus movement tachycardia with a well-localized accessory pathway. S c did not terminate or slow tachycardia in any patient. We conclude that the limited spatial effect of noncapturing single stimuli precludes their use for termination of reentry tachycardias with current catheter techniques.