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Mac's Service Shop: Development of a Modern ECG
January 1974 Popular Electronics

January 1974 Popular Electronics

January 1974 Popular Electronics Cover - RF Cafe Table of Contents

Wax nostalgic about and learn from the history of early electronics. See articles from Popular Electronics, published October 1954 - April 1985. All copyrights are hereby acknowledged.

Hewlett-Packard HP1515B Electrocardiogram (Wellington Hospital photo) - RF Cafe

Hewlett-Packard HP1515B ECG
(Wellington Hospital photo)

Probably the first exposure most of us have had to an electrocardiogram (ECG) is from a television show or a movie. Earlier instances appeared in hospital settings in a patient's room or in the operating room. Around the time this installment of "Mac's Service Shop" appeared in Popular Electronics magazine (1974), the TV show "Emergency" introduced my generation to the field-portable ECG machine as deployed by Emergency Medical Technician (EMT) responders Johnny Gage and Roy DeSoto. I remember some of the first episodes had the emergency room doctors lamenting not having equipment available on the scene that could wirelessly send critical heart status parameters along with the standard fare of the victim's "vital signs." Then, one day a portable ECG unit was finally available, ostensibly revolutionizing the profession. Along with learning about heart arrhythmia and sinus rhythm as the squiggly lines were drawn on the chart paper, we had the phrase "administer __ cc of ringers lactate" indelibly impressed on our minds. As is common with John T. Frye's techno-dramas, he mentions some actual real-world equipment that includes an ECG instrument made by Hewlett-Packard (HP). Here is some information on the "HP ECG Study Course" that appeared in the March 1974 issue of HP's "Measure" publication.

Mac's Service Shop: Development of a Modern ECG

Mac's Service Shop: Development of a Modern ECG, January 1974 Popular Electronics - RF CafeBy John T. Frye, W9EGV, KHD4167

Barney came shivering into the service department, blowing on his cold-numbed fingers to warm them, and found Mac, his employer, examining a long strip of 2 1/2' wide cross-ruled paper with a magnifying glass.

"I'll bite; what's that?" Barney demanded.

"It's an electrocardiogram I ran on my long-suffering guinea pig wife last evening," Mac explained. "As you know, I'm growing increasingly interested in medical electronics and recently signed up for Hewlett-Packard's ECG Study Course, for which I must have access to an electrocardiograph. Dr. Brown was good enough to lend me his Sanborn Model 51 Viso-Cardiette portable EGG that he acquired back when he hung out his shingle. That's it in the mahogany case over there on the end of the bench. While the instrument is more than twenty years old, it is excellently engineered and still functions beautifully."

"How does the heart make this squiggly little line?" Barney asked, picking up the strip of paper and looking at the tracing.

Hewlett-Packard's Model 1515A three-channel ECG machine - RF Cafe

Panel view of Hewlett-Packard's Model 1515A three-channel ECG machine.

"You're looking at an amplitude-vs-time graph of the voltage generated by a beating heart and picked up at various points on the surface of the body. Heart beats are initiated by rhythmic pulses of electrical current generated by a bit of specialized tissue called the S-A Node located in the upper portion of the right atrium. This tiny current spreads over the entire heart, proceeding from cell to cell in a toppling-domino type of action. As the current reaches a cell, that cell 'depolarizes' and contracts. The interior of a resting cell is about 90 millivolts negative with respect to the exterior, but when stimulated by the spreading current from the S-A Node, this polarity reverses and the interior briefly becomes about 20 millivolts positive with respect to the exterior. Then the cell 're-polarizes' to its former resting state.

"As a cell depolarizes, it creates a tiny voltage that triggers the depolarization of its neighbor; and so the electrical action initiated by the S-A Node spreads over the atria, down through the septum between the ventricles, thence to the endocardium, or lining, of the ventricles, and finally out through the muscular ventricular heart walls to the surface. On this journey the depolarization wave of a normal heart follows a certain predictable path at various predictable speeds so that the accompanying muscular contractions of the various chambers of the heart are properly timed for most effective pumping action.

"At any particular moment, the voltage produced by this depolarization wave has both amplitude and direction - in short, it is a vector quantity. As such, it must be 'viewed' from more than one angle to determine its direction and maximum amplitude. This viewing is performed by the electrocardiograph, EGG, or EKG (after the German spelling). The travelling wave of depolarization voltage does not stop when it reaches the surface of the heart but continues on through the body tissue to the surface, where it can be detected through electrodes in contact with the skin going to sensitive electrical instruments.

"The first practical EGG instrument was developed in 1903 by a Dutchman named Einthoven. It consisted of a very thin gold-plated quartz fiber suspended between the poles of a powerful magnet. When the ends of this fiber were connected to electrodes on the surface of the body, the picked-up heart voltages sent current through the conducive 'string' that produced electro-magnetic fields which reacted with the permanent field of the magnet and caused the string to move one way or the other, depending on which way the current was momentarily flowing through the fiber. An optical system magnified these motions and they were reproduced on a moving strip of photographic paper by a special camera. This 'string galvanometer ECG' was very delicate and hard to adjust, and it has been completely superseded by the amplifier type that is portable, rugged, easy to use, and more accurate.

An Amplifier Type ECG. "I assume that one on the bench is an amplifier type."

"Right. It consists essentially of a vacuum tube differential amplifier that boosts up thousands of times the tiny difference in voltages presented to its two inputs by the body electrodes; of a rugged D'Arsonval movement galvanometer similar to the ones used in our meters but of a much heavier, stiffer construction and having, instead of a pointer, an and carrying a heated stylus attached to its moving coil; and of a paper transport system that moves a special coated paper over a knife-edge and beneath the stylus at a speed of exactly 25 mm per second. The greatly amplified voltage is fed to the galvanometer coil that responds by moving the heated stylus up and down over the horizontally moving paper stretched taut over the knife edge. With no voltage present, the stylus rides in the center of the paper. A depolarization wave moving towards a positive electrode moves the stylus up on the paper; a wave going the other way moves it down. Heat from the stylus produces a continuous black marking on the heat-sensitive coating of the moving paper."

"With the paper sliding under the stylus at 25 mm/s, you can tell how much time elapses between any two points on the tracing," Barney observed.

"Yes. Each small square on the paper is a mm on a side; so the distance between two adjacent vertical lines represents 0.04 second.

"How do you measure voltage?"

"You inject a standardizing calibration pulse of exactly 1 mV amplitude into the input of the amplifier by momentarily pushing a button on the ECG panel and adjust the amplifier gain until this square-topped pulse is exactly ten small squares tall; then the distance between two adjacent horizontal lines represents 0.1 mV. All electrocardiograms are properly made with this standardization. Each fifth horizontal and vertical line is made heavier for convenience in measuring."

"Where do you attach the electrodes?"

"Normally one labeled RL goes to the right leg, LL to the left leg, RA to the right arm, and LA to the left arm. A fifth suction-type electrode C is attached to various positions on the chest. Wires from all electrodes connect to the ECG through a plug-in cable. The RL electrode goes to ground, and combinations of the other electrodes are selected by a rotary switch on the panel of the ECG and are connected to the input of the amplifier.

"Each such combination of electrodes is called a 'lead' and is designated by number, letters, or a combination of letters and numbers. Recordings from twelve such leads constitute a standard electrocardiogram, although other combinations and placement of electrodes are occasionally used. Einthoven used three bipolar leads called I, II, and III. Lead I teamed a positive LA with a negative RA. Lead II combined a positive LL with a negative RA. Lead III attached LL to the positive amplifier terminal and LA to the negative terminal.

"Now we also have the augmented unipolar limb leads: aVR, aVL and aVF, in which a single limb electrode is attached to the positive terminal of the ECG amplifier and the remaining two are tied together to form a 'neutral' electrode and attached to the negative terminal. aVR has the RA positive, aVL has LA positive, and aVF has LL positive. Finally we have the V leads in which all three limb electrodes are tied together to form a neutral 'central terminal' and the suction cup electrode C is successively attached to six different positions on the chest, starting to the right of the sternum in the fourth interspace and proceeding across the sternum and then down and to the left around the apex of the heart to the mid-axillary line. Each such position of C constitutes a different lead, and these are called V1, V2, V3, V4, V5, and V6."

"What's this little button above the paper viewing window and this switch marked 'Insto'?" Barney asked, looking at the panel of the ECG.

"The button marks the top of the paper when depressed. You use it to mark each section of the serially recorded electrocardiogram with a code of long and short marks so you can tell, after the strip is cut into twelve pieces for mounting, which piece displays a recording of which lead. The 'Instomatic Switch' is closed when switching leads so that the amplifier adjusts instantly to the different dc potentials existing between electrodes of the different leads. Otherwise, when you switch leads, the stylus is likely to be knocked clear off the paper and stay there for up to thirty seconds before returning to center.

"But let me describe how I make an electrocardiogram on the Viso-Cardiette: I start the paper, close the Insto switch, move the lead selector to I, open the Insto switch, push the 1 mV standard switch, hit the marker button once to make a short dash, close the Insto switch, move the lead selector to II, open the Insto switch, push the 1 mV switch, hit the marker button twice, etc., right on through leads III, a VR, aVL, and aVF to V. From here on I do not have to change the lead selector, but I must move the C electrode to a new position for each different V lead, and I still must operate the Insto switch, the marker button, and the 1 mV switch for each lead. Since I only want about six seconds worth of each lead, I am busier than the performer on one of those keep-the-plates-spinning circus acts."

"Are modern ECG's much different from this one?"

Modern ECG's. "I'll say! Sanborn became a subsidiary of Hewlett-Packard in 1961 and formed the basis of H-P's Medical Electronics Division; so let me tell you about the H-P Model 1515A ECG Phone Terminal to show you how far one company has gone with this equipment in about twenty-five years. The 1515A is, first, a three-channel recorder having three instant-warm-up solid state amplifiers driving three high-fidelity galvanometers and recording three leads simultaneously. Here's a picture of it and a sample of the recording it makes. All twelve standard leads are recorded in four sets of three leads each. Lead switching, operation of the Insto switch, marker identification of lead sets, and standardization are all performed automatically. You simply connect the electrodes (there are separate electrodes for all six V leads), press a button, and in ten seconds you have a complete 12-lead electrocardiogram."

"But there's more. The recording can be simultaneously sent over an ordinary telephone circuit to a computer system that digitalizes the data by taking hundreds of samples a second of the ECG signal. This data is stored in the computer memory, analyzed, and compared with hundreds of thousands of other ECG's. Then, in less than a minute, the computer sends a print-out back to the patient site containing many significant amplitude, duration, and vector axis measurements together with a classification of the electrocardiogram as normal, atypical, borderline, or abnormal. By performing many tedious, necessary measurements for the cardiologist, the computer provides him with guidelines he can combine with information from auscultation, enzyme studies, X-rays, angiograms, cardiac catheterization, and experience to arrive at a diagnosis in 1/5 to 1/3 the time it would otherwise take."

"What's, this about a 'high fidelity galvanometer'?"

"Ideally the stylus position should change in a linear fashion with respect to applied voltage. With the ordinary galvanometer this doesn't quite happen because of frictional drag between stylus and paper and flexing of the mechanical coupling between stylus and rotor. H-P has largely overcome this 'hysteresis distortion' with a rigid coupling and a feedback system that senses the rotor position and forces the stylus to move to where it should be. As a result, H-P's new ECG's reproduce often significant fine detail of heart voltage waveforms with a fidelity approaching that of a scope. Furthermore, these new instruments include features that insure the patient against electrical shock hazard.

"Let me wind up by saying that with 160 million electrocardiograms being run annually world wide-70 million in the U.S.-busy cardiologists need all the help they can get.



Posted April 1, 2020

Mac's Radio Service Shop Episodes on RF Cafe

This series of instructive stories was the brainchild of none other than John T. Frye, creator of the Carl and Jerry series that ran in Popular Electronics for many years. Mac's Radio Service Shop began life in Radio & Television News magazine (which itself started as simply Radio News), and then changed its name to Mac's Service Shop after the magazine became Electronics World. "Mac" is electronics repair shop owner Mac McGregor, and Barney Jameson his his eager, if not somewhat naive, technician assistant. "Lessons" are taught in story format with dialogs between Mac and Barney.

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