A Method of Graphic Registration of Heart Sounds.

Abstract

The method is an adaptation of the “ultramicrometer” described by Dowling. A Hartley circuit is used. A condenser microphone and 2 “pancake” inductance coils of 85 and 100 turns each form the oscillating circuit. Because the electrical changes to be recorded are of relatively short duration, the “zero shunt” described by Dowling is not used. A variable non-inductive resistance is placed in the plate circuit and the “ultramicrometer” is coupled to a string galvanometer through a variable, decade condenser. This does away with the necessity of compensating for the “zero” plate current of the oscillator. In addition it furnishes a means of tuning the recording circuit. Using a very light, tightly stretched string and a rather crude condenser microphone made from an antiquated telephone transmitter, we have been able to record sounds up to 650 double vibrations per second. Harmonics up to the 6th partial of spoken sounds showing a fundamental rate of about 100 are readily recorded. With a well constructed microphone the characteristics of the ordinary galvanometer string would be the limiting factor in recording sounds of high frequency. No attempt has been made as yet to determine the limits imposed by the string that we are using. The microphone is placed directly on the chest wall. A hole drilled in the “mouthpiece” maintains atmospheric pressure in the vibrating column of air. The main source of “noise” seems to be the vibration of the elements of the electron tube. Mechanical shielding of the tube is necessary. The heart sound records obtained resemble those made with Einthoven's “Saitenphonograph,” The 3rd heart sound appears in the records of all of the few subjects examined. So far we have had opportunity to obtain a record from only one case showing a heart murmur.