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Air Leaks as a Source of Distortion in Apexcardiography John A. Kmtor, M.D.,' Saul Aronow, Ph.D.,O0 Robert E . Nagle, M.B., M.R.C.P.,) Thomas Garber, S . M . ( E . E . ) , $ and Harriet Walker$ Small air leaks have been observed to be fkequentiy present in a poprrlar crg, tal transducer and to cause s@%cant distortion of the apexcardiogram. Such leaks introduce a short time constant thereby differentiating the wave form and producing: (1) peaked and amplied A waves; (2) early fall off of the qetolic wave; (3) increase in dope and relative duration of the rapid filling wave. Leaking was found to develop at the seal between the traneducer housing and a gasket and was easily eliminated with the application of silicone lubricant. A test "square wave" was produced by quickly clamping the tube connection from bell to transducer, and a short time constant thereby observed as a clue to the presence of leaking. The possibility of such leaks and of certain other physical limitations discussed in this paper support the need for a new device which can simultaneo~lslyreproduce the phonocardiogram and apexcardiogram. E x t e r n a l recordings of apical movements have been used increasingly in recent years for timing of cardiac events and for the diagnosis of abnormalities of cardiac function. Among several techniques available for these purposes ;he apexcardiogram is popular because of its simplicity in use. In recent years investigators including Benchimol J. S. Fleming and Dimond,l Coulshed and Ep~tein,~ Tafur and co-workand Hamer,3s4 J. W. Fle~ning,~ e r ~ Tavel , ~ and associate^?^^ and others have described the usefulness of apexcardiography in many clinical states. These authors have outlined the method of obtaining reproducible records and have From the departments of Medicine, Haward Medical School and Massachusetts General Hospital (Cardiac Unit), Boston, Massachusetts. This study was supported in part by USPHS grants HE-5198 and HEPP-06684. Portions of this study were presented at the annual m e e t i n c f the Laennec Society, Bal Harbour, Florida, Novem r 20, 1968. "Instructor in Medicine, Haward Medical School; Assistant in Medicine, Massachusetts General Hospital, Boston. Massachusetts. ''Research Associate in Medicine, Harvard Medical School. Associate Applied Physicist, Massachusetts General Hospital, Boston. f Queen Elizabeth Hospital, Birmingham, England. $Senior en 'neer, Waltham Division, Hewlett-Packard Com, pany, ~ Z t h a mMassachusetts. PPhonocardiographic Technician, Massachusetts General Hospital, Boston. shown the artifacts which improper technique can induce.6,"12 However, although warnings have been raised, little specific attention has been paid to the problem of time-constant variation induced by small air leaks in the recording equipment. We have observed this technical imperfection quite commonly and have devised methods to detect its presence so as to eliminate a significant source of signal distortion. In this report we shall describe the nature of the problem, discuss a method for its detection and finally consider the construction of an ideal apexcardiographic recording device. METHODS AND RESULTS Equipment Many apexcardiograms are recorded with a crystal microphone which converts pressure changes into electrical signals. The microphone frequently employed for this purpose is manufactured by Sanbom Company, now a division of Hewlett-Packard Company, Waltham, Massachusetts (Fig 1). This instrument was first described in 1941 by Miller and Whitel" and early clinical records of use of the technique were published by Rappaport and Spraguel4 ( A d dendum 1). The transducing element of the microphone is a piezoelectric crystal which has the property of generating an electric voltage when twisted or squeezed; the magnitude of the voltage is proportional to the applied force. The electrical energy is led off the crystal by thin metal foil electrodes Downloaded From: http://journal.publications.chestnet.org/pdfaccess.ashx?url=/data/journals/chest/21488/ on 06/18/2017 KASTOR ET AL Air-Tight Coupling Circuit FIGURE 1. Sanborn (Hewlett-Packard) No. 374 crystal transducer (top right) and associated box containing resistor and capacitor (Fig 9). The transducer is connected by a flexible tube to a two-inch cup at the front of a Sanborn sowld microphone (top left). This familiar arrangement has been used for over 25 years to record simultaneous phonocardiograms and apexcardiograms. cemented to opposite faces of the crystal. The wires from this crystal lead to a box which contains a resistor and capacitor, the function of which will be explained below. The electrical output of the crystal is then amplified and either photographically or mechanically recorded. The tracings for this study were amplified and recorded on a Sanborn (Hewlett-Packard) series 580 unit which includes 350-3200A preamplifiers for apex and electrocardiography and 350-17000(= preamplifiers for phonocardiography. Top cover I I Goskel VOLUME Protective cover - --- Conical dlophrogm Piezocrlectric transducer-- spew - At the "front end" of the recording assembly is an airtight coupling between the skin of the patient's chest and the microphone assembly. The mechanical movement of the apex is transmittted as a pressure wave in air, first through the cone which is applied to the chest then via a rubber or plastic tube to the piezoelectric sensor itself. It is clear that an air leak in this circuit could distort the signal before it has been converted into electrical information. We were surprised to find that such air leaks are commonly and easily produced and that their distortion may be great though their magnitude be small. Our attention was directed to this problem when a crystal assembly was inspected to learn why the reproduced signal decreased rapidly when a steady "square wave" of pressure was introduced into the microphone. A small leak was noted at the seal between the crystal unit itself and its housing (Fig 2). When the assembly was made air-tight, signal reproducibility returned to satisfactory levels. Artif;cial Production of Leaks In order to study the nature of this artifact, a controlled leak was introduced into the pneumatic circuit of a previously nonleaking unit. The rubber tubing between the pick-up cone on the patient's chest and the crystal microphone was cut and a thick-walled metal tube inserted. Into this metal tube was drilled a small hole 5 mm long and 0.325 mm in diameter (Fig 3). By uncovering the hole, a leak with known properties could be introduced at will. Apex recordings were then made on patients with various forms of heart disease in end expiration in the left lateral position with and without leaking. The artifacts induced by leaks can be clearly seen (Fig 4-6). Testing for Leaks A convenient method was devised to test whether the Sanborn No. 374 crystal microphone assembly was leaking. A satisfactory test wave was introduced by quickly compressing with a clamp the rubber tubing attached to the microphone. A small amount of air was displaced and the crystal perceived this as a square wave. When a leak was present the reproduced wave form rapidly decreased and upon release of the clamp, air rushed back into the circuit and a negative deflection was produced. If no leak was present between the clamp and the microphone, the deflection decreased more slowly at a rate determined by the overall time constant of the electronic circuit (Fig 7). Piezoelectric crptol Palpation of the precordial cardiac impulse is a -5% -- . --... _I_ I- ... ~~uminum foil rln Bottom cover ARTIFICIAL LEAK FIGURE 2. Exploded diagram of the Sanborn (HewlettPackad) No. 374 crystal transducer. Small air leaks have been found to develop where the gasket joins the d e d unit enclosing the crystal itself to the top cover. FIGURE3. Metal device constructed to introduce small leaks into the pneumatic circuit flexible tubing is attached to both ends connecting the device to the pick-up h e d and the transducer. By unmvering the hole (0.325 mm diameter) a predictable leak can be produced. CHEST, VOL. 57, NO. 2, FEBRUARY 1970 Downloaded From: http://journal.publications.chestnet.org/pdfaccess.ashx?url=/data/journals/chest/21488/ on 06/18/2017 DISTORTION IN APEXCARDIOGRAPHY These variations in apical records are caused by several factors, some deliberately introduced and others present by accident and not by design. In order to analyze these differences, we may refer to a schematic diagram which shows the steps through which the heart's motion is processed in such studies. This chain of biological, physical and electromechanical components is illustrated in the block diagram (Fig 8). Tramission Through Body to Chest Wall FIGURE4. Simultaneous apex and phonocardiogram (low frequency filter) on a 16-year-old boy who had had rheumatic fever four years previously but was now without symptoms of heart disease. A grade II/VI systolic murmur and a loud third heart sound were recorded at the apex. At paint "X" a hole of 0.325 mm was opened in the h.ansducer pneumatic circuit thus introducing a small leak. Notice the decrease in systolic wave form and relatively increased duration of the rapid filling wave. (Time lines are 40 msec apart). traditional part of physical examination. Its scope has been widened in recent years by the introduction of devices which make a visible record of what the hand feels just as the phonocardiogram visually represents the sounds heard by the ear. In the case of phonocardiography a fairly standardized technique has resulted in tracings which are comparable when taken by machines of different make, in different hospitals or even in different continents. Such standardization has not been achieved in the case of cardiac impulse recorders. According to the descriptions given in many papers both the transducer, which converts mechanical movement into electrical energy, and the way in which the transducer is applied to the patient differ and, to take an extreme case, a tracing which may be normal to one author may be diagnostic of heart disease according to the criteria of another. The transmission of the apex movement from the heart to the surface of the skin is a complex process. The intervening tissues including the pericardium and its fluid contents, muscle, fat, connective tissue and bone all affect the passage of the characteristic low-frequency pulses that constitute the apex displacement. The anatomical changes produced by thoracotomy without removal of a rib can also change the impulse. Inability to make a good recording is also all too frequently due to abnormalities of the chest wall. FIGURE6. Apexcardiogram from a 45-year-old man with idiopathic hypertrophic subaortic stenosis. The leak, introduced at "X", increases the relative height of the A wave, changes the form of the systolic wave and affects the contour of the rapid filling wave. Because of the variables introduced by these interposed tissues absolute criteria for the amplitude of the apical displacement curve has been difficult to establish from patient to patient. Standard deflections with apexcardiograms have been introduced recently, however, by Sutton and Craige.15 To be valid such standardization must be performed on the same type of equipment. Coupling from Body to Transducer FIGURE5. Apexcardiogram and electrocardiogram (lead AVR) from a 58-year-old man with coronary artery disease. The leak was introduced during the downslope of the second beat (at 'X"). Changes include: marked increase in the slope and duration of the rapid filling wave, heightened A wave and particularly striking distortion of the systolic wave. Motion of the body surface at the cardiac apex can be defined in two general ways. If the sensing device is secured to the bed or to a stand, the tracings obtained reflect heart motion within the chest, plus any movement of the chest wall itself, with respect to the fixed external point. The kinetocardiogram of Eddleman and his associates16 and the CHEST, VOL. 57, NO. 2, FEBRUARY 1970 Downloaded From: http://journal.publications.chestnet.org/pdfaccess.ashx?url=/data/journals/chest/21488/ on 06/18/2017 166 KASTOR ET AL -- 2.1 SEC. I 2 -L L i-1 -4 0.09 SEC - I - --I* --I* 0.04 0.04 SEC SEC FIGURE 7. The effect of air leaks on the time constant of a transducer. These illustrations have been taken directly from the photographic record produced by the transducer and amplifier. Time lines are automatically inserted by the recorder and vary appropriately according to the paper speed. Top: A "square wave" of pressure has been introduced by compressing the flexible tubing attached to a nonleaking transducer with a clamp. The recorded signal falls off gradually reaching its time constant (l/e=37 percent) in 2.1 see. Most of this decay is a function of the AC amplifier conventionally used with such instruments. Bottom Left:The same test was applied to a crystal transducer with a small air leak. Note the short time constant of only 90 rnsec. Such distortion of the low frequency signals produces characteristic changes in the apexcardiogram. Bottom Right: The small leak introduced by the metal tube (Fig 3) has a similar slightly shorter time constant. The variations in the form of the apexcardiogram produced I)y the leaking transducer or by the metal tube are virtually identical. impulse cardiogram of Mounsey and his coworkers17 provide such records. In apexcardiography, on the other hand, the sensing device is either secured to the chest wall with a strap or held in place by hand. The device moves with the chest wall, so that the movement of apex with respect to the adjacent chest areas is recorded. Size of pick-up: Difference in the form of the recording is also introduced by the size of the pick-up applied to the chest. Nixonls and others have mentioned and Bancroft and Eddleman16 have ilustrated the changes in pulse contour produced when the contact area is changed. For example, a record made from nearby but not on the apex may be inverted.O Hence recordings centered at the apex but including surrounding areas will be different from the impulses taken at the apex alone. One of the most useful features of the Sanborn assembly is its ability to permit recording of sirnultaneous sound and apex displacement. However, the cup which fits against the chest wall has a twoinch diameter (Fig 1) and consequently movement surrounding the apex is incorporated into the record. Two considerations make this arrangement less than ideal. First, the apex record is not a true reflection of what the observer perceives in palpating the apex itself; and second, the use of instruments with different size pick-ups causes confusion in developing standards for precordial displacement. (Our further studies of this problem will be reported separately.) Air leaks: The investigation reported here may also be considered under the heading of bodytransducer coupling. For the air leaks observed distort the apical record before conversion into electrical information has been accomplished. The effect of leaks is to shorten the time constant of the system, thereby differentiating the wave form. The concept of time constant is not difEcult to understand as it applies to the reproduction of pulse waves and apical impulse recording^.^,^^ A - ------+MECHANICAL TRANSMISSION E L E C T R I C A L TRANSMISSION FIGURE 8. Schematic diagram illustrating some of the processes employed in the conversion of cardiac apical movement into the apexcardiogram (for details see Discussion). CHEST, VOL. 57, NO. 2, FEBRUARY 1970 Downloaded From: http://journal.publications.chestnet.org/pdfaccess.ashx?url=/data/journals/chest/21488/ on 06/18/2017 DISTORTION I N APEXCARDIOGRAPHY 167 FIGURE9. Interior of the "gray box" which connects the crystal transducer to the amplifier input. The 1 microfarad condenser (top) and 22 megohm resistor form an R-C circuit which prolongs the intrinsically short time constant of the crystal itself. This arrangement, however, markedly decreases the output from the crystal and a high-gain AC amplifier is needed for further processing of the signal. Such AC amplifiers shorten the time constant produced by the transducer and its R-C circuit as illustrated in Figure 7, but the result is satisfactory for most clinical purposes in recording of precordial movements. condenser and a resistor in series have a characteristic time constant which expressed in seconds is equal to the product of the capacitance of the condenser (in farads) and the resistance of the resistor (in ohms). The time constant in effect indicates the fidelity with which a resistor-capacitor (RC) circuit will transmit a long duration pulse of low frequency. By definition, in a circuit with a time constant of one second the reproduced square wave will have fallen l / e or .37 of its original amplitude in one second. If the time constant is two seconds, the wave form will take twice as long to decrease to this level, etc. ACOU S T l C COUPLING p - pes pulse at sk~n surface PIEZOELECTRIC TRANSDUCER 8 = mechono-electre axlpl~ngcoeff~c~ent Although time constant measurements are usually applied to electrical circuits, the same mathematical expressions can be derived for an acoustical system such as the pneumatic coupling arrangement discussed above (Addendum 2). A short time constant circuit, either electrical or pneumatic, may also be considered as a differentiating circuit. Differentiation accentuates the rapidly changing segments of a pulse as in the conversion of a square wave into a pair of biphasic spikes. Whether this differentiation is produced electrically by a short RC circuit or by an air leak in a pneumatic circuit, the results are quite similar. The air leak we introduce is very small; nevertheless, its effects are significant and predictable: (1) the A wave becomes more peaked and higher relative to th E - 0 vertical distance; ( 2 ) the systolic wave falls off shortly after the E point and does not maintain its expected upward or level excursion; ( 3 ) the slope of the rapid filling wave is more steep and its vertical interval relatively longer. Perhaps such leaks contribute to the different records obtained by Dimondlo from the Sanborn and from other types of transducers. He observed that the Sanborn No. 374 instrument "did not consistently give a flat response in the very low frequency range (0.1 to 4 cps)," and it is within this range that most of the useful information in apexcardiography is produced. An air leak would distort in particular these low frequency components. COUPLING BOX Cb ; loadlng copoc~tor Rb = bleeder res~stor AMPLlFl E R RECORDER CQ = effectwe capoc~tonce Rg = effect~veres~stance Ca= ocoust~ccap of cup E = voltage pulse Cd = ocoust~cc o p d dlophrogm I: current output RL = leokcqe resstonce Cx= copac~tonce of v = volume flow crystal FIGURE 10. Equivalent electrical circuit for the acoustic and mechanical elements of an apex cardiograph recording system. CHEST, VOL. 57, NO. 2, FEBRUARY 1970 Downloaded From: http://journal.publications.chestnet.org/pdfaccess.ashx?url=/data/journals/chest/21488/ on 06/18/2017 KASTOR ET AL Pressure Pulse Transducer Even with an intact nonleaking pneumatic circuit, the form of the apex record may be significantly affected by the time constant of the transducer itself. A pulse wave transducer, such as the Sanborn No. 374 consists of a piezoelectric crystal element which generates an electric potential under mechanical stimulation." The voltage is briefly stored on the metal foil electrodes attached to the crystal and then discharges across a resistor located in the small box attached to the plug which fits into the amplifier (Fig 9). The crystal and its foil electrodes constitute a capacitor and together with the resistor form an RC circuit whose time constant is veiy short. In order to lengthen it another capacito; is introduced in the coupling box which increases the inherent capacitance of the crystal. The time constant of this circuit is in excess of 20 seconds. Since systole seldom occupies more then 0.3 second, for all practical purposes a reliable wave form will be reproduced. Another method of increasing the effective transducer time constant is to add a preamplifier with a very high input resistance. Such an arrangement has been described by Roberts and Sherwood Jones12 who employed an electrometer tube for this purpose. Such circuits, however, are not commonly available in commercial equipment. Actually, transducers are available which will generate an impulse with an infinite time constant, although the advantage of this feature for clinical purposes is minimal. Such instruments include: (1) Phillips inductive displacement transducer type PR9310"",'~1s~20(2) modified Statham transducer UC3;21 and (3) Hewlett-Packard displacement transducer APT16. Another type of "leak should be mentioned at this point. It has been suggested that the entrance of moisture into the crystal assembly will cause an "electrical l e a k and thus shorten the time constant The presence of this of the transducer it~elf.~JO type of ' l e a k may also be detected by the method described for air leaks. Electrical Amplification In Figure 7 we see that despite the use of a nonleaking crystal transducer with a time constant 'The Sanborn model No. 374 pulse wave pick-up has been superseded by Hewlett-Packard model No. 2105A pulse wave transducer. This new unit is essentially unchanged from the No. 374 model except that the Rochelle salt crystal has been replaced by a ceramic piezoelectric element. This modification has the advantages of better temperature stability and increased output. "Available in the United States from Korfund Dynamics Corp., Westbury, Long Island, N. Y. 11590. of over 20 seconds, the input square wave regresses to about 37 percent of the initial amplitude in 2.1 seconds. This shortened time constant is produced for the most part by the electrocardiographic amplifier commonly used for pulse recording. Such amplifiers are designed for the relatively low signal input of an ECG and the time constant is quite adequate for reproduction of the higher frequency electrocardiographic signal. However, even a 2.1-second time constant is satisfactory for apex recording since this is approximately eight times the duration of systole. Some distortion is present, but it is minimal. Even this limitation can be eliminated, for amplifiers as well as transducers with infinite time constants are available. However, DC (infinite time constant) amplifiers are not commonly used. In providing extended frequency range such circuits introduce certain undesirable features such as signal drift and low frequency interference. Hence at this time, crystal transducers are conventionally fed into relatively high gain AC amplifiers with limited time constants. Recorder The final block (Fig 8) shows that after amplification the signal is converted into an optical image or recorded directly by a stylus onto paper. The principal disadvantage of direct-writing, commonly employed in electrocardiography is that the mechanical movement of the writer is limited by inertia, thus distorting the higher frequency components. However, for the frequency range used with apexcardiography, direct-writing instruments are quite adequate in technical performance and more convenient in clincal use.5 Their limitation is more evident with phonocardiography. "Ideal" System The popular air-coupled crystal transducer still has a place in apexcardiography despite the limitations discussed above. It is the most familiar device used for recording in conjunction with a sound microphone. However, we believe that a new device is needed which will permit simultaneous apex and phono records to be made without the distortion introduced by leaks, time constant limitations, and such factors as the effect of the surrounding chest movement. Nixon, Hepburn and Ikramls have constructed such a unit which consists of a DC transducer for the apex impulse combined in a plastic housing with a sound microphone. This arrangement is an ingenious step on the way toward a single pick-up device which can serve both needs. CHEST, VOL. 57, NO. 2, FEBRUARY 1970 Downloaded From: http://journal.publications.chestnet.org/pdfaccess.ashx?url=/data/journals/chest/21488/ on 06/18/2017 DISTORTION IN APEXCARDIOGRAPHY The requirements for such an ideal instrument are fairly simple to outline. The contact area must be sdciently small to eliminate the influence of movements near but not at the apex; the time constant must be sdciently long to reproduce the apex pulse without distortion; and the upper frequency limit must be sufficiently high so that the phonocardiogram will be accurate. Theoretically, it is possible to construct such a pick-up, the output of which can be fed into two appropriate amplifiers, one tuned for the low frequency impulse record, and the other for the higher frequency phonocardiogram. It seems preferable that a pneumatic coupling be avoided if possible for the reasons discussed previously. The air-circuit devices have provided much useful information since their initial development more than 25 years ago, and with proper service continue to have wide applicability. But a single sensor with direct mechanical coupling from skin to transducer has both theoretical and aesthetic appeal. Such devices are now being tested and their production is awaited with interest. ACKNOWLEDGEMENTS: The authors are much obliged to Miss Marion G. Bagley for assistance in preparation of the manuscript. The illustrations were desi ed by the Medical Art Department, M a r s a c h w ~Genera$~ospital. Figure 1 was photographed by Hutchins Photography, Inc., Belmont, Massachnsetts. 1 DIMOND,E.G., DUENAS, A., AND BENCHIMOL, A.: Apex cardiography, Amer. Heart J., 72:124, 1966. 2 COULSHED, N., AND EPSTEIN,E.J.: The apex cardiogram: its normal features explained by those found in heart disease, Brit. Heart J., 25:697, 1963. 3 FLEMING,J.S.: The assessment of failure in aortic stenosis from the diastolic movements of the left ventricle, Amer. Heart J., 76:235, 1968. 4 FLEMING,J., AND HAMER,J.: Effect of propranolol on left atrial systole in ischaemic and hypertensive heart disease, Brit. Heart J., 29:257, 1967. 5 FLEMING,J.W.: Carotid artery and precordial pulsation recordin@ with the standard directwriter electrocardiograph: Technical consideration, Amer. J. CardwL, 17:707, 1966. 6 T A ~ E., , COHEN, L.S., AND LEVINE, H.D.: The normal apexcardiogram, Circulation, 30:381, 1964. 7 TAVEL,M.E., CAMPBELL, R.W., FEIGENBAUM, H.,AND STEINMFIZ,E.F.: The apex cardiogram and its relationship to haernodynamic events within the left heart, Brit. Heart J., 27:829, 1965. 8 T a v n , M.E.: Clinical Phonocardiograpy and Erternal Pulse Recording, Year Book Medical Publishers, Inc. Chicago, 1967. 9 BENCHIMOL. A., AND DIMOND,E.G.: The normal and abnormal apexcardiogram. Its physiologic variation and its relation to intracardiac events, Amer. J. Cardiol., 12:368, 1963. 10 DIMOND,E.G.: Precordial vibrations. Clinical clues from palpation. Circulation., 30:284, 1984. 11 JOHNSTON, F.D., AND OVERY,D.C.: Vibrations of low frequency over the precordium, Circulation, 3:519, 1951. D.V.,AND SHERWOOD JONES,E.: A new system 12 ROBERTS, for recording the apex-beat, Lancet, 1: 1193, 1963. 13 MILLLEA,A., AND WHITE,P.D.: Crystal microphone for pulse wave recording, Amer. Heart I., 21:504, 1941. 14 F~APPAWRT, M.B., AND SPRAGUE, H.B.: The graphic registration of the normal heart sounds, Amer. Heart I., 23:591, 1942. 15 S ~ O NC.C., , AND CRAIGE,E.: Quantitation of precordial movement. I. Normal subjects. Circulation, 35: 476, 1967. 16 B A N C R OW.H., ~, JR., AND EDDLEMAN, E.E., JR.: Methods and physical characteristics of the kinetocardiographic and apexcardiographic systems for recording low-frequency precordial motion, Amer. Heart 1.. 73: 756, 1967 P.: The left ventricular im17 BEILIN,L., AND MOUNSEY, pulse in hypertensive heart disease, Brit. Heart I., 24: 409, 1962. 18 NEON, P.G.F., HEPBURN,F., AND IKRAM,H.: Simultaneous recording of heart pulses and sounds, Brit. Med. I., 1: 1169, 1984. 19 NAGLE,R.E., AND TAMARA, F.A.: Left parasternal impulse in pulmonary stenosis and atrial septa1 defect, Brit. Heart I., 29:735, 1967. 20 SCHNEIDER, H., AND KLUNHMR, E.W.J.M.: Precordial low-frequency displacements of the thoracic wall. Method of recording and registration, Amer. Heart I., 61 :670, 1961. 21 WATSON,B.W., GAY, P., AND FLEMING,J.S.: A new instrument for apexcardiography, Cardiwmc. Res., 2: 108, 1968. Dr. Arthur Miller has written for us the following historical sketch concerning the development of the Sanborn 374 crystal microphone: "Early in 1939 I was approached by either Doctor Paul Dudley White or Doctor W. Trevor Cooke about the possibility of recording pulse waves simultaneously with the electrocardiograph using the two-string galvanometer system which was then the heart of the electrocardiograph apparatus in the cardiac laboratory of the Massachusetts General Hospital. "I realized that the pressure pulsations to be recorded, in terms of the air pressure which would reach a pick-up device such as a microphone, would be relatively enormous in comparison to the sound waves which constituted the usual input to the microphone. Since the output of a simple crystal microphone in response to sound waves is measured in millivolts, this would imply that the signal generated by the pulse waves would be measured in hundreds of millivolts. 'We had been building vacuum tube amplifier type electrocardiographs since 1935 and so we were CHEST, VOL. 57, NO. 2, FEBRUARY 1970 Downloaded From: http://journal.publications.chestnet.org/pdfaccess.ashx?url=/data/journals/chest/21488/ on 06/18/2017