A multi-channel cathode-ray phonocardiograph.

The difficulties of recording the faint murmurs of early rheumatic valvulitis are well known, and for this reason it was considered advisable to develop a cathode-ray phonocardiograph. This is theoretically the ideal recording device owing to its minimal inertia and high sensitivity. This phonocardiograph is intended to provide objective confirmation of clinical findings on auscultation, recording change in murmurs during rheumatic activity and convalescence and at subsequent examinations. The patients, mainly cbildren, are admitted to hospital shortly after the onset of rheumatic fever, and are not discharged until convalescence is completed. The ear is more sensitive to sounds of high frequency than to those of low frequency, and although the actual intensity of the low-pitched heart sounds is far greater than that of high-frequency murmurs, the logarithmic low-frequency attenuation of the ear may cause them to sound equally loud. For this reason, filtering of the low-frequency sounds must be employed in order to record the murmurs satisfactorily. Mechanical filtering is employed in routine clinical auscultation by the use of the stethoscope bell or diaphragm. Rappaport and Sprague (1941, 1942) employ similar methods to record linear, stethoscopic, or logarithmic phonocardiograms. The first of these corresponds to the apex cardiogram without filtration, and the latter two filter the lower frequencies in increasing degree, so that the logarithmic record is equivalent to the auscultatory findings. In this country more stress has been laid on electrical methods of sound filtration (Leatham, 1949, and Cowen and Parnum, 1949), by attenuation of the lower and amplification of the higher frequencies. Various numbers of filters are employed by different workers, and we use four. The two low-pass filter positions are used for recording heart sounds and mitral diastolic murmurs, whereas the high-pass filter position (with maximal attenuation of low-pitch sounds) is best for recording aortic diastolic murmurs. The over-all gains on these amplifiers are 104 db. on the lowfrequency, and 101 db. on the high-frequency positions at 500 c./s.