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The sliders allow the user to change the parameters describing the transmembrane potential: the depolarization time (the fast onset of the action potential), the repolarization time (defined as the moment of maximum down-slope during repolarization), and the magnitude of the action potential. The action potential duration that results is taken to be the difference between the specified repolarization and depolarization times.
The left knob of the slider pair beneath the plot allows the user to specify the onset (dep) of the action potential at the selected node. The right knob affects the depolarization time (rep). When the left slider knob is moved the right one moves along. In this way the action potential duration (apd) remains constant. Moving the right knob sets the repolarization timing (rep) only. This also affects the duration since rep = dep+apd. The numbers shown indicate (from left to right): dep, apd and rep.
The slider on the right is used to change the action potential magnitude, expressed as a fraction of its normal value.
If the user changes the source parameters, the effect on the ECG is immediately visible in the other panes, provided that the appropriate functions have been selected.
The Membrane menu on the menu bar contains the following entries:
The image shown in the membrane pane may be copied to the clipboard by selecting Copy from the Membrane menu, or by pressing Ctrl-C while the mouse is within the membrane pane.
In this dialog window, you may set the depolarization time of the focus, and the speed of the depolarization wave front started from this focus. Furthermore you may choose whether the selected node should be the only focus of depolarization (Replace current activation sequence), or whether the node should be an additional focus (Add to current activation sequence). In the latter case the original depolarization times will be retained for nodes that are reached by the new depolarization wave front at a later time than its original depolarization time.
If a focus of depolarization is defined on the heart surface, the repolarization time rep(n) of each node n is set as follows:
rep(n) = dep(n) + OldDur(n) + 0.4 (OldDep(n)-OldMeanDep) - 0.4 (dep(n)-MeanDep),
where OldDur(n) is the original action potential duration of node n (i.e: before the focus was defined), OldDep(n) is the original depolarization time of node n, and MeanDep and OldMeanDep are, respectively, the new and original mean depolarization times.
This, heuristic, assignment of repolarization times is based on the following arguments. The original distribution of the action potential duration is partially an expression of the intrinsic characteristics of the myocardium. For another part it is the result of the distribution of the activation times, where regions that depolarize early tend to repolarize late. Hence the new action potential duration is set the original one, plus a term that is negative is the new activation time is later than before, and vise versa. The weight factor 0.4 is based on the statistical relation between depolarization and repolarization times described in Genesis of the T-wave as based on an Equivalent Surface Source Model listed in the references section.
The size of the range is controlled by the range dialog window, which may be activated from the membrane menu. In this dialog window you may also choose whether you want to compute the range through the myocardium, or along the myocardial surface. In this way various myocardial malfunctions may be modeled. For instance, a bundle branch block would affect both epicardium and endocardium in roughly the same way. So here one would choose the range through the myocardium. On the other hand, hypertrophy, may be modeled by increasing the activation times at the epicardium only. In such cases one would want to restrict the range to the epicardial surface.
As an alternative, you may also set the size of the range by clicking with the middle mouse button (or, if your mouse doesn't have a middle button, clicking with the right mouse button while holding down the shift key) on the heart surface displayed in the heart pane (see Setting the range for details).
Note: if you first change some parameter(s) of the selected node, and subsequently change the range, the changes will be recomputed for the new range. If you don't want this (e.g, if you want to make new changes on top of the old ones for a different range), first select another node and then come back to this node.
By selecting Undo All, or clicking on the Undo All tool button, all changes made since the start of the program or since the last time a source parameters file was opened are undone. Once you have opened a source parameter file, you may go back to the default source parameters (i.e. the ones with which the program starts) by selecting Default from the File menu.
By selecting Maximum repolarization time from the Membrane menu, a dialog window is opened that allows the user to choose between the values 500 ms and 1000 ms for the maximum repolarization time. All sliders involving depolarization time or action potential duration are adapted to the selected maximum repolarization time.
When reducing the maximum repolarization time from 1000 ms to 500 ms, there may be nodes that have a repolarization time beyond the new maximum. If such is the case, the user is prompted to consider whether these values should be clipped to 500 ms.
If a source parameter set loaded from file (see Open: Source parameters) contains repolarization times beyond 500 ms, the maximum repolarization time is automatically set to 1000 ms.
The dialog window contains a table that displays the mean, standard deviation, minimum, and maximum of the depolarization times, repolarization times, and action potential durations.
The dialog window also contains sliders to scale the means and dispersions (standard deviations) of these timing parameters. The result of these changes are immediately visible in the various panes. Note that while this dialog window is being displayed, all controls outside the dialog window are inactive.
The size of the sliders and slider knobs are automatically adjusted to ensure that no non-valid values of the timing parameters are generated (i.e. depolarization before 0 ms, repolarization beyond 500 ms, or action potential duration less than 50 ms). Also, scaling of the depolarization dispersion will lead to changes in the action potential duration dispersion and vice versa (recall: for each node the repolarization time is equal to the depolarization time plus the action potential duration: rep = dep + apd).
By selecting the Repolarization waveform item from the Membrane menu, the overall waveform is recomputed from the dominant T wave estimated from the current reference ECG (see File, Open: Reference surface potentials). Furthermore, a dialog window appears containing a slider by which the steepness of the down slope may be changed, as percentage of the slope that was computed from the reference ECG.