An electronic phonocardiograph employing a double-beam cathode-ray oscillograph as the recording device.

A phonocardiograph employing a cathode-ray tube is the theoretical ideal since the natural frequency of the instrument is sufficiently high to avoid distortion of the vibrations to be recorded, the amplitude of the deflections is directly proportional to the movements which excite them, and the cathode-ray beam has minimal inertia and is fully damped. With any of the popular recording devices it is possible to obtain some sort of record of the vibrations produced in the front of the chest by the beat of the heart. Such a. record has little resemblance to the sounds heard on cardiac auscultation unless the low-frequency elements are filtered off so that the remaining deflections on the record correspond to vibrations of audio-frequency. A faithful record of vibrations of all frequencies is called a linear phonocardiogram, one in which the low frequency vibrations are attenuated as by the introduction of an ordinary stethoscope is called a stethoscdpic phonocardiogram, and a record in which the low-frequency attenuation of the stethoscope as well as the poor low-frequency response of the human auditory mechanism have been imitated is called a logarithmic phonocardiogram. Such logarithmic attenuation of the low-frequency vibrations in imitation of that inherent in auscultation is most conveniently achieved by electrical filters. An electronic phonocardiograph using a single-beam cathode-ray oscillograph was employed by Boone (1940). This instrument merely recorded the linear phonocardiogram without any reference tracing. Kountz et al. (1940) recorded linear phonocardiograms with a cathode-ray oscillograph; the electrocardiogram (lead II) was recorded as a reference tracing at the same time. Donovan (1944) employed a double-beam cathode-ray oscillograph as the recording device. Electrical filters were incorporated in the amplifier system to give sufficient attenuation of the low frequencies for the recording of stethoscopic phonocardiograms, and the reference tracing recorded on the other beam was either the electrocardiogram or the jugular venous pulse. Later, in 1948, Donovan preferred the logarithmic phonocardiogram because it is the only one comparable to auditory findings, and the phlebogram was preferred as the reference tracing because the electrical activity of the heart as indicated by the electrocardiogram does not bear a constant time-relationship to mechanical events such as heart sounds and pulse waves.