AJP - Heart and Circulatory Physiology, Vol 269, Issue 4 1258-H1267, Copyright © 1995 by American Physiological Society
Mechanisms of conduction time hysteresis in rabbit atrioventricular node
J. Billette, J. Zhao and A. Shrier
Department of Physiology, Faculty of Medicine, University of Montreal, Quebec, Canada.
The functional origin of atrioventricular nodal hysteresis was studied in
isolated rabbit heart preparations. This hysteresis is characterized by
asymmetric changes in nodal conduction time (NCT) occurring for symmetric
changes in cycle length. The respective contribution of the nodal
properties of recovery, facilitation, and fatigue to the beat-to-beat
changes in NCT observed during paired symmetric ramps of decreasing and
increasing cycle length was determined with specifically design stimulation
protocols. Nodal hysteresis was found to be entirely accounted for by
variations in the contribution of nodal recovery and fatigue properties
observed at corresponding cycle lengths. The study establishes how this
contribution varies on a beat-to-beat basis as a result of cycle length
history. This holds true for the numerous changes in hysteresis observed in
response to changes in the sequence and slope of the ramps. Facilitation
clearly affected NCT during these responses but did not contribute to the
hysteresis. Moreover, the study demonstrates that there is no inherent
change in the characteristics of nodal function with the direction of the
ramp that could account for the hysteresis. Thus nodal hysteresis arises
from nodal functional properties of recovery and fatigue but does not
constitute a distinct independent intrinsic property of the node.
