"Restitution Properties and Stability of Response
Patterns in Cardiac Muscle"
Soma Sau, Wanda Krassowska (Department of Biomedical Engineering),
Daniel Gauthier (Department of Physics, Duke University)
Abstract
We will present experimental evidence demonstrating that the restitution
properties of cardiac muscle do not
determine the stability of the response pattern to periodic stimuli
as previously believed. Bullfrog ventricular
tissue is periodically paced for a fixed time interval. This is repeated
as the basic cycle length (BCL) is
decrementally swept from longer to shorter values. Action potential
duration (APD) and preceding diastolic
interval (DI) are measured at each BCL. It is known that the tissue
initially responds to such pacing with a 1:1
pattern, but as the BCL becomes shorter, the 1:1 response becomes unstable
and action potentials alternate
in amplitude and duration. From theory based on 1-d return maps, it
is believed that this bifurcation occurs at
the point where the slope of the associated restitution curve (APD
vs. DI) exceeds one. In our in-vitro
experiments, the tissue responded with a stable 1:1 pattern even as
the slope of the restitution curve
exceeded one, indicating that tissue dynamics are not governed solely
by the restitution properties of the
tissue.