Telemed Corporation was formed for the purpose of providing remote computer reading of Electrocardiograms (ECG) taken at hospitals across the country. The 12-lead ECG was transmitted over telephone lines to the Telemed offices in the Chicago area.
The transmission of non-voice signals was not always so routine as it is now. Early tests referenced the sending of teletype signals over long-distance lines, with concerns about phase jitter and frequency shifts. We did some test of our own using prototype ECG machines, and the results were satisfactory.
The 12-lead Electrocardiogram is taken with electrical contacts made on each of the arms and legs of the patient, and a number of locations on the patient’s chest. These leads are connected to an ECG cart that is wheeled to a patient’s room. The technology of the transmission of the signals from the leads divided the signal into four groups of three leads each. Each group was transmitted for about 10 seconds, making for about a 40-second phone call. Patient identification was sent using touch-tones at the beginning of each call. A light on the front of the unit was be controlled by the computer to send a crude signal indicating whether or not the traces were received without error.
Receiving and decoding the ECG
At the offices, a bank of telephones were connected to a set of demodulators that converted the three-tone ECG signal to three separate voltages, which were then fed to an A/D converter. This converter sampled the voltages each 2 milliseconds, yielding a 10-bit value placed within a 16-bit word.
These samples were written to disk in buffers as the buffers filled up, and when the call was finished, all the buffers for that call pulled together and sent to an pattern-recognition program. This program measured wave heights, durations, slope, and classified the waves into the recognized categories (R, P, Q, T, etc.). These measurements and classifications were fed to a decision table that ultimately generated an English-language report or “reading” describing any abnormal conditions found in the ECG.
Sending the report
At first, this report was written to paper tape which was then fed by hand to one of a bank of ASR-33 teletypes that would dial a printer at the hospital and send the report.
Later, the report was sent directly over outgoing phone lines. The service ultimately had 20 incoming phone lines and processed an astonishing number of ECGs each day.
The computers were 32-bit SDS (ultimately XDS) real-time computers. We had two of these machines, one for backup/development and one for production. If one failed, we could quickly manually switch to the other. All of the above software components ran in one computer, which had 32 k words. Your watch likely has more memory these days. All but the decision process was written in assembly language. This effort was started in 1969.
I was the first employee and wrote most of the system that took in the data on from the A/D converters, wrote it to disk, and wrote it to tape for backup and archival purposes. I also assisted in building the system that sent the data out over the phone lines.