March 1948 Radio-Craft
People old and young enjoy waxing nostalgic about and learning some of the history of early electronics.
Radio-Craft was published from 1929 through 1953. All copyrights are hereby acknowledged. See all articles
Every time I see one of these articles on 'modern' medial electronics
it makes me think of the Star Trek IV movie titled, "The
Voyage Home," wherein Dr. McCoy (aka
'Bones') intervenes as a 20th century brain surgeon is about
to operate on Chekov - "My God man, drilling holes in his head is
not the answer!" The 1948-vintage electrocardiograph featured in
this piece looks like it was built from parts salvaged from World
War II field gear. Having a doctor attach wires to you is scary
enough, but back when the probes were powered by instruments using
circuits with 200-300 volts of plate bias in them would add
an extra level of anxiety.
BTW, have you ever wondered how 'star
dates' in Star Trek were determined? As it turns out, the system
has not been consistent throughout the series from television and
the movies then back to television. It began as a random number
to avoid needing to specify a particular century and ended with
a system that included which season the TV season was. Now you know.
Electronics in Medicine - The Electronic Cardiograph
Part I - The electronic cardiograph, its fundamental theory and
notes on application methods
By Eugene Thompson
Courtesy Sanborn Co., Cambridge, Mass.
A direct-writing type of electrocardiograph.
Fig. 1 - Heart, in radio-style block
Fig. 2 - Hookup of early equipment with
Einthoven string galvanometer.
Fig. 3 - Audio amplifier designed for
the electrocardiograph's very low-frequency output.
Fig. 4 - Recorder with a D'Arsonval galvanometer.
How the cardiograph is used. Third electrode
is over the heart.
Medical electronics embraces all these electronic devices and
techniques which are employed in the diagnosis and treatment of
disease. Among these techniques are electrocardiography, bleed pressure
and pulse recording, photoelectric plethysmography, and photoelectric
colorimetry. Electrocardiography equipment serves as a basic component
for a number of the techniques.
Fig. 1 is a diagrammatic sketch of the heart and the flow of
bleed through it. It is essentially a four-chambered mechanical
force pump. Its function is to pump deoxygenated blood, which is
returned to it from the body via the veins, through the lungs, where
it picks up a fresh supply of oxygen, and thence back to the body
by way of bleed vessels known as arteries. The chambers of the healthy
heart contract in a definite, orderly, and rhythmic sequence known
as the cardiac cycle.
The blood circuit
Referring to. Figure 1, the cycle starts as a quantity of deoxygenated
bleed empties into the right auricle. At this time this chamber
is in its resting phase, or diastole, which lasts for 0.7 second.
At the end of this filling period the right auricle contracts (systole),
which lasts 0.1 second and forces the blood into the right ventricle.
After doing this the auricle returns to its diastolic phase (0.7
second) to collect some mere deoxygenated bleed. Immediately after
it receives the bleed from the right auricle, the right ventricle
which has been in diastole for the past 0.5 second, undergoes systole.
Ventricular systole lasts for 0.3 second and propels the deoxygenated
blood through the lungs, where it becomes oxygenated, and back to
the left auricle. This chamber in turn squeezes the blood into the
left ventricle from whence it is pumped back to the body again.
The time relationships for diastole and systole of the left auricle
and ventricle are the same as these given for the right auricle
and ventricle. The halves of the heart work together.
The two auricles contract simultaneously, and then the two ventricles
do the same. Each of course is ejecting a different type of blood.
By far the most striking thing about the cardiac cycle is its
rhythmicity. We now knew that the contractions of the heart are
timed and controlled by nerve impulses which arise within the heart
itself. It has been demonstrated by cathode ray oscillography that
nerve impulses are electrical in nature. Consequently, as these
impulses stream through the heart they leave the tissue through
which they pass momentarily electronegative with respect to the
rest of the heart and body. The resultant shifting of this electronegative
area with the passage of the nerve impulse constitutes a minute
electrical current which can be detected with the aid of sufficiently
sensitive recording apparatus.
The first practical electrocardiographic recorder was the Einthoven
string galvanometer. The basic arrangement of this device is shown
in Fig. 2. Although this type of recorder is still widely used,
the modern trend is away from this design and toward the more versatile
electronic recording system.
Fig. 3 is a schematic diagram of a typical electrocardiograph
amplifier. Although all such amplifiers are not of push-pull design,
this type is capable of doing everything that non-push-pull amplifiers
can do, and has several additional advantages. Among the more important
of these are: push-pull can handle signals of greater amplitude
than single-channel amplifiers under the same operating conditions;
the power output is greater; extraneous noises, such as those produced
by x-ray or diathermy apparatus, feed into the amplifier 180 degrees
out of phase and hence are bucked out to a large extent; second
and all even-number harmonic distortion is reduced.
Because the amplitude of the action potentials produced by the
heart are 1 millivolt or less under normal conditions, an electrocardiograph
amplifier must have high gain. In the unit shown in Fig. 3 this
is accomplished by the 2 stages of push-pull amplification. Employment
of pentodes rather than triodes results in a much higher over-all
gain per stage. Furthermore, using the 6SJ7 is an excellent pentode
in that it is possible to obtain a gain in the neighborhood of 80
to. 100 with relatively low operating voltages (plate supply voltage
Another important characteristic of the heart's action potentials
is their low frequency. This imposes the necessity for a long time
constant in the amplifier and accounts for the higher than usual
values of the interstage coupling condensers and grid lead resisters.
One further complication is added because of the amplifier's
low pass characteristics. An a.c. power supply cannot be used, because
of two reasons. First, the a.c. on the filaments would appear on
the record. Second, the d.c. plate and screen voltage would produce
the same effect, unless the power supply were of such exceptional
design that the ripple content would be of very negligible proportions.
These difficulties are easily solved by using batteries for the
filament supply, grid bias, and the plate and screen voltages.
Two additional components are necessary to adapt the amplifier
in Fig. 3 to the recording of electrocardiograms. These are: a means
for picking up the heart's action potentials and feeding them to
the amplifier, and a device for making a visual record of the amplifier's
The method by which the heart potentials are detected is simple.
Flat metal electrodes about 1 1/2 inches wide and 2 1/4 inches long
are attached at various places on the surface of the body. These
points are: (1) the left wrist; (2) the right wrist; (3) the left
leg just above the ankle, and (4) any other point on the body. To
make an electrocardiogram it is necessary to use at least two of
the first three electrodes. The fourth electrode is also usually
required as a ground connection to bypass extraneous noises.
Any combination of two electrodes is known as a lead. Thus, the
combination consisting of the left and right wrists is called Lead
1. Lead II is composed of the right wrist and the left leg, and
the left wrist and the left leg comprise Lead III. A number of other
leads are sometimes used for special purposes, but the 3 described
here are the ones most commonly used.
Each lead requires a separate amplifier. All three leads must
be recorded to permit accurate diagnosis of cardiac irregularities.
In clinical practice this is accomplished in one of two ways. Either
three amplifiers are employed, thus recording all three leads simultaneously,
or only one amplifier is used together with a switching arrangement
which permits the selection of any desired lead, and the three leads
are recorded in succession. The former method, although more costly
in terms of equipment required is preferable because the effect
of a single given irregularity in the cardiac cycle can be observed
in all three leads.
The electrodes are attached to the body by first preparing the
desired area of skin by rubbing it with a paste containing an abrasive
and salt, to enhance the electrical contact. The abrasive breaks
the tough outer non-conducting layer of skin, reducing the skin
resistance and minimizing polarization and other undesirable effects.
The salt increases conductivity at the contact. The electrode is
placed on the treated area and held in place with a rubber strap.
A wire connected to the electrode goes to one of the input grids
or, in the case of the ground electrode, to the ground terminal
on the amplifier. This arrangement is satisfactory for detecting
the minute potential differences between any two electrodes.
After these potential differences are passed through the amplifier,
a suitable recording device must be placed at the output terminals
of the amplifier to produce a visual record of them. Present-day
equipment uses one of two techniques to obtain this recording of
electrocardiograms, either photographic or direct writing.
In the photographic method, a small moving coil galvanometer
with a tiny circular focusing mirror cemented to the suspension
is employed as the recording unit. As the output signal from the
amplifier is applied to the moving coil, the latter oscillates from
side to side causing the mirror to move in step with it. A beam
of light may thus be projected on a moving photographic surface
which travels past it, making a permanent record. This system is
illustrated in Fig. 4.
Although it is widely employed at present this method has one
great disadvantage. The film must be developed before the cardiologist
can analyze it. In some cases, such as surgical operations, it is
desirable that a visual electrocardiographic record be always available
at the moment the heart produces it. A recent innovation, the direct-recording
electrocardiograph, makes this possible. In place of the moving
coil galvanometer, a light-weight, electromagnetically actuated
recording arm is used. This arm moves back and forth much like the
voice coil in a radio loud speaker. At its end is a small self-feeding
inkwriter which produces a record on moving paper tape. An even
more recent improvement is a heated wiring stylus which records
on a specially prepared plastic surface.
The interpretation of electrocardiograms is a task for a highly
trained expert, an exhaustive discussion of this subject is obviously
beyond the scope of this article. However, the foregoing will give
the reader some idea as to the equipment employed in cardiac diagnosis.
Other electromedical equipment will be considered in later articles.
Courtesy Sanborn Co., Cambridge, Mass.
Posted March 22, 2015