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Multiple-site pacing in congestive heart failure using
cathode and anode as widely-spaced bipole suggested that anodal stimulation might differ
from cathodal in more than just threshold. Experiments in Langendorff-perfused rabbits
then measured speeds of conduction, action potentials, and contractility in single-site
pacing using varying polarity, amplitude, and phases. Arrival times were measured at
plaque array of bipolar extra-cellular electrodes, action potentials by floating
intra-cellular plunge electrodes, and intra-ventricular pressures by fluid filled balloons
attached to pressure transducers. Gated MRI images of differential regional wall tension
were also obtained in 6 intact canines paced similarly. Up to 6 volt amplitudes, 6 msec
durations, and biphasic in addition to monophasic pulses were used. Anodally induced beats
were characterized (in addition to higher threshold) by faster upstroke of phase 0 of
intra-cellular action potential, faster conduction in both transverse and longitudinal
direction (of almost 100% in some cases), and increased contraction strength. Effects were
greater at either polarity with increasing voltage amplitudes, and pulse durations. Speeds
of driven beats seemed to increase to a steady state over the approximately 1 cm distance
studied. Faster and stronger beats were associated with lower and more balanced wall
stresses. Biphasic pulses retained changes characteristic of anodal and higher amplitude
and duration pacing compared to standard cathodal pulse, but in addition, thresholds were
reduced to levels comparable to standard cathodal pulses. We suspect changes are due to
varying initial levels of intra-cellular negativity just prior to myocardial stimulation.
Later activation cascade effects in regard to calcium ions which could explain
contractility changes may also exist. Results suggest that pulse morphology changes may be
useful for novel pacing therapies. Thus, anodal or biphasic pulses may find use in pacing
that seeks to augment electrical conduction (as in single-site congestive heart failure
therapy), and balancing of wall stresses (as for myopathies of hypertrophic and dilated
nature). Further attempts to optimize these phenomena may prove fruitful.
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