Patent GB-1025209-A (GB)
Title: Improvements in analyzing electrocardiograms
IPC: A61B5/336
Filed: 1963-06-13
Abstract: 1,025,209. Cathode-ray oscilloscopes. DEL MAR ENGINEERING LABORATORIES. June 13, 1963 [Sept. 10, 1962], No. 23651/63. Heading H4T. [Also in Division G1] An apparatus for analysing a series of irregularly occurring input signals of similar waveform, such as electrocardiograms, comprises arrangements for deriving sampling signals from a characteristic portion of the input signals, two sampling means adapted to receive the input signals and operating in such a way that the amplitude of the input signals is sampled at the instant of occurrence of the sampling signals, the arrangement being such that the phases of the sampling signals at the sampling means are different, whereby the amplitude of each input signal is sampled at two spaced intervals of time to provide a reference sample signal and another sample signal whose value may be compared with that of the reference sample signal. Cardiogram signals EKG occur periodically and normally take the form shown as 14, Fig. 2, but an abnormal one may take the form shown as 16. The displacement of the portion J of the EKG signal waveform relative to a point 90 in the leading part of the curve indicates whether the EKG signal is abnormal. This displacement is determined by sampling the waveform at the points 90 and 92, and feeding the sampled voltages to a differential amplifier whose output is disclosed on an oscilloscope. The EKG signals are fed to a tape recorder 12, Fig. 1, whence they are applied through two offset recording heads 13, 15 to a magnetic tape 11, so that the signals are recorded on two relatively time displaced tracks. The tape signals may then be played back at a later time at a much greater speed than that at which there were recorded by means of a playback unit 18, also having two offset heads 17, 19. To make the sample at the point 90, the earlier signal derived from the playback unit is fed to a trigger circuit 24 where the cardiogram pulse QRS initiates a trigger signal, which is clipped in unit 26 and used to actuate a one-shot multivibrator 58, whose output pulse width may be varied, and whose backward edge is differentiated to produce a trigger pulse. This is applied to actuate a sample gate 60, to which is applied the EKG waveform on the input 64. The gate operates so as to supply the value of the sampled waveform at the sampling time to the differential amplifier 70 continuously until the next sample is taken. A similar arrangement to make the sample at the point 92 is provided, the later EKG signal from the playback unit 18 after amplification and integration being used to actuate a Schmidt trigger 34 when the pulse amplitude rises above a given level. The backward edge of the output from the Schmidt trigger is differentiated and clipped in the unit 73 and fed to the one-shot multivibrator 74, which provides variable delay and operates the sample gate 78. The difference, if any, between the two samples taken at the gates 60 and 78 is determined in the differential amplifier 70 and displayed on the oscilloscope 86. The pulse from unit 26 is also differentiated at 28 and used to trigger a sawtooth generator 30 which energizes the X channel of the oscilloscope 22. The later EKG signal from the playback unit 18 after being amplified at 20 is fed to the Y channel of the oscilloscope. Because of the fast operation of the playback unit, any abnormal cardiogram waveform will be shown up on the oscilloscope since it will overlap the normal waveform, while the point 90, 92 at which the samples are made may be indicated by using the sampling signals to give bright spots. The number of abnormalities may be counted by using the pulse from the Schmidt trigger 34, whose width is related to that of the pulse QRS. The leading edge of this square pulse activates a one-shot multivibrator 36, whose output pulse is set to have a time duration lying between the width of a good and abnormal QRS pulse. This is used to inhibit a gate 38, to the input of which the Schmidt output is directly applied. An output from this gate therefore only appears if the input is related to an abnormal QRS pulse, and this output is fed to a counter 46.