Multichannel Systems Cardio2D Manual - Manual (Page 71)

Experimental Proceeding
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The sodium current is detected by the rapid change in the field potential, indicated by the rise
time. The rise time refers to the time required for a signal to change from a specified low value to
a specified high value. Typically, these values are 10 % and 90 % of the step height. The output
signal of a system is characterized also by fall time. If the rise time is chosen very short, the slope
of the peak is very steep, if the rise time is longer, the slope will be smooth. You have to try out,
which time is the best to characterize the Na-Peak, you want to detect.
You can either define a fixed value for the amplitude of the field potential, for example, 100 μV.
Using the signal dependent amplitude, the waveform detection is more flexible: The detection
routine detects amplitudes in relationship to the noise level. The amplitude is calculated with
its standard deviation in a range from -1 to -50. It is recommended to use a signal dependent
amplitude. The standard deviation is used to estimate the individual amplitude change for each
channel separately. A time interval of 1 s is used to calculate the standard deviation. You set
the factor, by which the standard deviation is multiplied (-1 to -50). To compensate individual
differences in the noise level of channels, it is recommended to use the signal dependent
amplitude as detection parameter. The data is scanned for a situation, where the minimum
amplitude (either fixed or relative to the standard deviation) is met and the rise time is in the
defined range as well. Limit the rise time range to exclude artifact signals that are in the range
of the defined amplitude. As the sodium peak is generally very fast, small rise times such as
100 to 400 μs can usually be used. The higher you define the FP change parameter, the higher
you need to set the rise time as well, as a higher FP change needs longer than a smaller one.
The rise time and the amplitude of the Na-Peak are used to detect only signals that show the
expected kinetics of the waveform. Slow potential shifts are excluded.
5.3 Automated Waveform Detection
This illustration shows how the Cardio2D routine detects the fast sodium peak by the three
parameters field potential change, minimum and maximum rise time. The software scans
the recorded data points, and it checks whether following data points meet the required criteria.
For example, when the waveform detection routine arrives at the data point marked with the red
line in the figure, the following data points meet the amplitude criterion (shaded in gray). The
signal reaches this field potential change in the preset range of the rise time (shaded in green).
The data point, at which both the FP change and the rise time criteria are met is marked with
a red dot in this figure. Would the signal be smaller than the minimum FP change or faster or
slower, it would not be detected. In other words, the signal needs to cross the bottom border
of the green shaded area.
Any signal component that matches the criteria will be detected. The FP change parameter is the
main parameter that should discriminate biological signals from noise. The rise time parameter
ensures that biological or artefact signals that meet the FP change criterion, but have a shorter
or longer rise time than the sodium peak, are not detected.
Please read the next chapter "HB Detection" in "The Cardio2D+ Analyzer Program: Overview"
about detection parameters in Cardio2D+.