Digital processing of equine exercise electrocardiograms

We have developed and used a system for recording and analyzing the electrocardiogram (ECG) of the horse during exercise. The system consists of a commercial ECG transmitter telemetering a Z lead (base-apex) ECG from an exercising horse. The received data are then remodulated at an audio frequency and stored on a audio cassette recorder. Exercise protocols of up to 10 minutes are digitized using a Macintosh II computer. For rhythm analysis, a computer program to identify the various waves of the ECG uses a modification and refinement of the integrated-squared-derivative (ISD) technique. This technique consists of frequency selective digital filtering and differentiation followed by integration of the squared derivative. The waves of the ECG are identified using adaptive thresholding of the ISD and the derivative of the ECG. Thus far, we have been able to successfully identify the Q, R, and the end of T in ECGs severely contaminated by exercise artifact. The system can place the absolute time of occurrence of the Q, R, and end of T in a text file for export to other analysis programs such as spreadsheets and statistics packages.