Using the Scope window

Figure 3.4: Scope
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Some types of signals are characterized by changes in waveform shape (morphology), which may occur gradually or suddenly. For example, ischemic ST changes in the ECG are usually visible as quasi-continuous changes over a period of at least 30 seconds (often much longer). It can be helpful to view such signals on an oscilloscope, where the trigger can be adjusted so that the (approximately) periodic waveforms are superimposed. Such displays, and their digital counterparts, are very widely used in analysis of long-term ECG recordings. WAVE 's Scope window offers some of the features of this type of display, with a few unique capabilities and limitations of its own.

To open the Scope window, first open the Analyze window, then click left on \ovalbox{\rule[-.3mm]{0cm}{2.5mm}\small\sf ~Show scope window~}. The Scope window simulates an oscilloscope by using color map animation (on hardware with an X server that supports this technique) or stipple masks (otherwise). The display is ``triggered'' by beat (QRS) annotations (other annotations, such as rhythm changes, are ignored). As each waveform is drawn, the previously drawn waveform is made to fade progressively into the background, disappearing completely after four new waveforms have been drawn. The appearance is similar to that of an analog scope with moderately long-persistence phosphor. The x-position in the window of the trigger point (the sample that the annotation points to for each waveform) is adjustable using the dt: setting (the value is expressed in seconds, and may be negative). It is also possible to resize the window, so that several waveforms can be viewed side-by-side as well as superimposed. As shown in the figure at left, WAVE shifts waveforms to the bottom of the window if they have been annotated as ventricular ectopic beats. (This behavior was chosen to be most useful for studying ECGs, but other signals can be annotated in the same way if desired.)

The navigation controls are similar to those in the Log window. \ovalbox{\rule[-.3mm]{0cm}{2.5mm}\small\sf ~\tt <~} and \ovalbox{\rule[-.3mm]{0cm}{2.5mm}\small\sf ~\tt >~} may be used to move backward and forward through the record one beat at a time. \ovalbox{\rule[-.3mm]{0cm}{2.5mm}\small\sf ~\tt <<~} and \ovalbox{\rule[-.3mm]{0cm}{2.5mm}\small\sf ~\tt >>~} provide a continuous ``movie'' (at a speed controlled by the Speed slider) that can be interrupted by \ovalbox{\rule[-.3mm]{0cm}{2.5mm}\small\sf ~Pause~}, which also recenters the signal window on the current beat. Whenever the current beat is visible in the signal window, it is marked with the selection rectangle.

Only one signal at a time can be displayed in the Scope window. Select that signal using the Signal field in the Analyze window, or press and hold the \Ovalbox{\small\sf Shift} key while clicking left in the signal window near the desired signal.

George B. Moody (george@mit.edu)
2014-03-13