Characteristics and mechanisms of the effects of heart rate history
on transient responses in rabbit atrioventricular node.
Zhao, Jie, and Jacques Billette.
Department of Physiology, Faculty of Medicine, University of
Montreal, Montreal, Canada
APStracts 2:0470H, 1995.
The characteristics and functional origin of the changes in transient
AV nodal responses with heart rate history were studied in isolated
rabbit heart preparations. For this purpose, ramp stimulation
sequences were applied to the atrium from different initial
conditions. A ramp decrease and increase in the His-stimulus
interval, and a reverse sequence made of a ramp increase and decrease
were performed starting from a control basic cycle length, following
5-min of fast rate, or with 5-min of fast rate inserted between the
two ramps. The nodal conduction times (NCT) obtained during the ramp
stimulations formed hysteresis loops whose direction, shape and
magnitude varied markedly with the nodal history. That is the nodal
response to a given ramp took a variety of forms depending upon the
initial condition. The effects of the initial condition also depended
upon ramp direction and sequence. A paradoxical NCT-recovery
relationship (decrease in NCT with shortening His-atrial interval)
was consistently observed at the onset of any ramp decrease performed
after 5 min of fast rate. These effects also varied with the rate
used to change the nodal history. The insertion of a control cycle at
every 20th beat during repeated ramp protocols allowed the
determination of the contribution of the nodal property of fatigue to
these effects. Fatigue was found to account for all observed
hysteresis patterns. In conclusion, heart rate history can, by
modulating beat to beat changes in fatigue, transform transient nodal
responses and hysteresis observed during stimulation ramps.
Interpretation of transient nodal responses thus requires exact
knowledge of previous nodal history.
Received 12 April 1995; accepted in final form 9 October 1995.
APS Manuscript Number H360-5.
Article publication pending Am. J. Physiol. (Heart Circ. Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 6 November 95