This historical paper describes the development of a vacuum tube amplifier-based electrocardiograph that provided superior frequency response and record quality compared to the traditional Einthoven string galvanometer.
An electrocardiograph is described which has been designed to secure a more nearly uniform response characteristic, as a function of frequency, over the desired frequency range, than is obtainable with the Einthoven string galvanometer ordinarily used for this work. It is shown by harmonic analysis that relatively high frequency components are present to a large degree in the ordinary electrocardiogram, and due to their magnitudes, have a large influence on the shape of the wave. More accurate interpretation of the electrocardiograph record is desirable, and is made possible by using an amplifier in conjunction with a standard type of oscillograph to produce a larger record, without reducing the accuracy at higher frequencies. The vacuum tube amplifier was specifically designed for the purpose. It has a high input impedance so that the effect of body resistance is negligible. A standardizing circuit is built into the amplifier so that calibrations can be obtained readily. The tests on this apparatus were carried out both at the Massachusetts Institute of Technology and at the Evans Memorial of Boston. Illustrations from these tests are included, showing comparisons between records obtained from a standard type of string galvanometer electrocardiograph and from the new apparatus.
Caldwell et al. (Wed,) studied this question.