Biomedical Engineering Reference
In-Depth Information
atrial cell with directly applied stimuli at CL 300 ms with
G
C
= 0 nS for the top panel
and
G
C
= 0.3 nS in the lower panel. At this shorter CL, the directly paced atrial cell
''overdrives'' the SAN model cell with 1:1 conduction from the atrial cell to the SAN
model cell.
However, when the directly paced CL of the atrial cell is longer than the intrinsic
SAN model CL, arrhythmias may develop. Figure 6c shows results for the same real
atrial cell as for Fig. 6a, b when we directly paced the real atrial cell at CL 600 ms and
used
G
C
= 0.4 nS. Under these conditions, if a spontaneous activation of the SAN
model cell occurs when the real atrial cell is not refractory, propagation from the
focus model cell to the real atrial cell may occur. The data shown in Fig. 6c are from a
longer recording after a steady state condition had been established. The stimuli to the
real atrial cell are shown as vertical arrows in the lower panel. The upper panel shows
the coupling current, with a positive polarity indicating current flow from the SAN
model cell to the real atrial cell. At time zero there is a stimulus, which directly
activates the real atrial cell and this action potential propagates to the SAN model cell.
The SAN model cell then has a spontaneous depolarization, which leads to an AP in
the SAN model cell (indicated by an asterisk), which then propagates to the real atrial
cell. The second direct stimulus to the real atrial cell (at time 0.6 s) now does not
activate the real atrial cell because it is refractory. There is then another AP
spontaneously generated in the SAN model cell (second asterisk), which also
propagates to the real atrial cell. The third direct stimulus to the real atrial cell (at time
1.2 s) does activate the real atrial cell and this AP propagates to the SAN model cell.
This process then almost exactly repeats for the next three direct stimuli to the real
atrial cell. The APs which occur in the real atrial cell are thus in an arrhythmic
pattern, with a repeating series of 3 cycles (~500, 470, and 230 ms for an average CL
of 400 ms) for each pair of direct stimuli at CL 600 ms. Note that in this simple two-
cell system there is a bidirectional propagation, with the intrinsic automaticity of the
focus cell being modulated by the propagation from the directly stimulated cell and
also able to propagate to the quiescent cell.
5 Interactions Between a Focus Region and Surrounding
Two-Dimensional Tissue
We then used our coupling clamp system to couple together a real spontaneously
active nodal cell (isolated from rabbit AV node) to a two-dimensional sheet of model
cells, which would represent either atrial or ventricular tissue by using specific models
of each tissue type. Since we wanted to use arrays of model cells as surrogates for the
electrical characteristics of two-dimensional arrays of either atrial or ventricular real
cells, we tested the properties of the two models (for atrial cells [6], for ventricular
cells [17]) as to their ability to recreate experimentally recorded differences in
excitability of atrial and ventricular cells. For astimulus frequency of 1 Hz, the atrial
and ventricular cell models we used produce characteristically different action
potential shapes, as expected. The ventricular cell model has a resting potential of -86
mV compared to -80 mV for the atrial cell model. The maximum d
V
/d
t
of the
ventricular and atrial cell models are 379 and 220 V/s, respectively, with the
