AJP - Heart and Circulatory Physiology, Vol 270, Issue 6 2070-H2080, Copyright © 1996 by American Physiological Society
Characteristics and mechanisms of the effects of heart rate history on transient AV nodal responses
J. Zhao and J. Billette
Department of Physiology, Faculty of Medicine, University of Montreal, Quebec, Canada.
The characteristics and functional origin of the changes in transient
atrioventricular (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 consisting of a ramp increase and decrease were performed starting
from a control basic cycle length, after 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, the
direction, shape, and magnitude of which varied markedly with the nodal
history. That is, the nodal response to a given ramp took a variety of
forms, depending on the initial condition. The effects of the initial
condition also depended on 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.
