Further work

If you would like to make this program work better, here are a few ideas:

• Extending it to make measurements on multiple signals is an easy project. Allow the user to set measurement reference points on each signal separately.
• It's also not too difficult to copy the input annotation file, writing the measurements into the num fields of the annotations, so that WAVE can display them as a signal. If you try this, keep in mind that num values must be integers in the -128 to 127 range, so don't record the deviations in V. If you want to write the measurements into a signal file (as we did with tach earlier in this chapter), this can get complicated--see the source for tach for hints about doing this.
• The measurement point should move closer to the J point at high heart rates; see any reference on the ECG for a discussion of this issue.
• The measurements as made above are very easily contaminated by small amounts of noise in the signals. You should be able to improve them significantly by averaging the amplitudes of several samples in the neighborhoods of the isoelectric point and the ST level measurement point.
• The program should not produce measurements for ventricular ectopic beats, or for beats that are extremely noisy. Noise detection is an interesting research problem in itself.
• You may wish to examine using beat averaging (or median filtering) to reduce noise in the measurements further; the ECG Database Programmer's Guide describes a beat averager that may be a useful starting point.
• Another source of measurement error is baseline wander in the ECG, visible as slopes in the intervals between beats. You can reduce this error by modelling the baseline drift and subtracting it from the signal. Simple linear models, or quadratic or cubic splines, can be used.