March 1948 Radio-Craft[Table of Contents]
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 from Radio-Craft.
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.
Part I - The electronic cardiograph, its fundamental theory and notes on application methods
By Eugene Thompson
Courtesy Sanborn Co., Cambridge, Mass.
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 90 volts).
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 output.
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