Download Air Leaks as a Source of Distortion in

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