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Sunday, October 30, 2011

At the Heart of the Invention: Development of the Holter Monitor

Model 445 Mini-Holter Recorder, illustration from a brochure, 1976
You probably know someone with a heart condition or someone who had a heart attack or even heart surgery.  I know I do. According to the Center for Disease Control, heart disease is the leading cause of death for both men and women in the United States and the most common type of heart disease is coronary artery disease (CAD), which can lead to a heart attack. Sobering data.  There are ways to prevent heart disease such as embracing a healthy lifestyle and there are diagnostic tools to monitor our hearts too, thanks to the work of two creative and persistent men, Norman “Jeff” Holter (1914-1983) and Bruce Del Mar (b. 1913-). Their collaboration, which spanned two decades, produced a commercially viable heart monitor known as the Holter Monitor Test. The Holter Monitor is a portable device for continuously monitoring heart activity for an extended period of time, typically twenty-four hours.  The monitor records electrical signals from the heart that are sent via a series of electrodes attached to the chest. The data is then analyzed for different sorts of heart beats and rhythms.

Norman J. Holter
Holter's business card
Norman “Jeff” Holter, a native of Helena, Montana, was a biophysicist and inventor whose interests were not confined solely to physics. In the 1940s, Holter organized Applied Micro Sciences, a scientific photography business, and began working with Dr. Joseph A. Gengerelli (1905-2000) of UCLA on nerve stimulation in frogs and brain stimulation in rats. Holter’s interest in studying electrical activity in humans during their daily activities without touching them spawned his lifelong pursuit to develop the Holter Monitor.  In 1947, Holter formed the Holter Research Foundation in Montana, with a laboratory to pursue his research interests.  Holter continued his collaboration with Dr. Gengerelli of UCLA in attempting to transmit biological information, primarily brain waves, by radio. Holter turned his attention from the brain to the heart because the heart's greater voltage made the electronics easier, and because heart disease was far more prevalent than brain disease. Holter’s introduction to Dr. Paul Dudley White (1886-1973), a renowned physician and cardiologist, also helped convince him to focus his research on recording electrical activity from the heart.  Holter's goal was to radio broadcast and record the more obvious electrophysiological phenomena occurring in humans while carrying on their normal activities, rather than having them be inactive.

Patient attached to heart monitoring equipment: slide of a cartoon, undated
Holter’s first broadcast of a radioelectrocardiogram (RECG) took place circa 1947 and required 80 to 85 pounds of equipment, which Holter wore on his back while riding a stationary bicycle. This was not practical and in no way could be worn by a patient for a sustained period of time.  The initial transmitter and receiver required that the subject remain in the general area of the laboratory, so a more portable and lighter receiver-recorder had to be developed. Holter responded by creating a briefcase-like device that could be carried by a patient. Holter noted in 1982 that “The 85 lb. RECG, while not practical, represented a major breakthrough since before that time a patient had to lie quietly.  Out greatest contribution was a radical one and was the beginning of an era where one could take ECG’s on skiers, parachute jumpers, runners, and just about any other type of vigorous physical activity.”  Holter’s other contribution was to bring the overall size down to less than cigarette package size to be worn inside a man’s jacket handkerchief pocket.

With the development of transistors, radioelectrocardiography was made obsolete and it became possible for the amplifier, tape recorder, temperature-control circuits, motor speed control circuits, and batteries to be placed in a single unit small enough for a coat pocket or purse. In 1952, Holter succeeded in creating a small unit that weighed approximately 1 kg.  Wilford R. Glassock, a senior engineer working with Holter, traveled to Cedars of Lebanon Hospital (now Cedars-Sinai Hospital of Los Angeles) in 1962 to demonstrate the Holter monitor system and discuss making it more practical. At Cedars, Dr. Eliot Corday, a cardiologist,  observed the practicality of the system, embracing the technology and becoming an early promoter of the technology to both industry and physicians. Holter and Glassock were issued US Patent 3,215,136 on November 2, 1965 for the Electrocardiographic Means. The Holter Research Foundation ultimately sold exclusive rights to the patent to Del Mar Engineering Laboratories, who became the acknowledged leader in Holter monitoring technology for over 40 years.

Advertisement for Del Mar
Engineering Laboratories, 1965
As articles describing the foundation's invention of these devices began to appear in the professional literature, there was considerable demand from doctors and hospitals for the equipment. Dr. Corday was acquainted with Bruce Del Mar from previous work on a flow meter which Corday put in coronary arteries to register the rate of blood flow. Corday introduced Holter to Bruce Del Mar in 1962.  Holter was seeking a partner to manufacture his monitor. In a March 1, 1962 letter to Dr. Wallace Chan, Special Assistant to the Deputy Surgeon General, Department of Health, Education and Welfare, Holter writes why Del Mar Avionics was selected to manufacture the Holter Monitor.

“After visiting several plants and talking with qualified people, we became aware that many interested manufacturers were not qualified to make an adequate instrument. Out of these meetings evolved a set of ground rules to assist me in selecting a manufacturer who could produce our equipment. 1) A dependable high quality trouble free system; 2) as low a sale price as possible to encourage wide distribution; 3) as rapidly as possible through the use of a team of experienced production and marketing people; 4) with adequate financial backing to avoid company failure during the course of bringing the instrument to market; and 5) last, but very important, with youthful aggressive enthusiasm for the technical ideas involved and an understanding of the revolutionary approach to what I consider obsolete present methods for electrocardiography.”  

Del Mar Avionics fit Holter’s “ground rules.” Not surprisingly, Del Mar employed a youthful, aggressive, and enthusiastic staff.  The company strove to create an environment “for creativity and growth opportunities.”  It would be a mutually beneficial partnership for both parties. 

Bruce Del Mar’s role as an innovator and collaborator with Holter is especially important because his work spurred the development of an entire diagnostic industry.  In November of 1963 Holter wrote to Del Mar, “Do not be too discouraged; careful evaluated clinical research goes even slower than R & D. My visits around the country convince me that you will encounter a chain reaction in time. Physicians have to be especially conservative in evaluating the results of such a revolutionary approach but once they have, interest will develop by geometric progression.”

Bruce Del Mar
Holter and Del Mar had their hearts in the right place. The Del Mar Avionics Holter Monitor Records, 1951-2011, held at the Archives Center, document through correspondence, engineering notebooks, operator’s manuals more than just the invention of a heart monitor. The records reflect the successful collaboration of an independent inventor and a manufacturing firm to problem-solve, develop a solution, and bring to market a diagnostic technology.  Del Mar wrote in February of 1965, ”We have continually improved circuitry and mechanical details to obtain greater service  fidelity, accuracy, response and reliability.  The instruments we are now delivering are performing very well in the field. We should, however, be thinking and actively working ahead on new model improvements for 1966.  Can I have your suggestions in this respect? That would be very much appreciated.”  The correspondence reveals a deep level of commitment and investment from both parties to work out technical details, successfully market the monitor, and in general keep the project moving forward.


Corday, Eliot. “Historical Vignette Celebrating the 30th Anniversary of Diagnostic Ambulatory Electrocardiographic Monitoring and Data Reduction Systems,” American Journal of Cardiology. 1991, pp. 286-292.

Del Mar, Bruce Eugene. Ready for Takeoff: An Autobiography. 2010.

Kennedy, Harold L. “The History, Science, and Innovation of Holter Technology,” Annals of Noninvasive Electrocardiology, January 2006, pp. 85-94.

Roberts, William C. and Marc A. Silver. “Norman Jefferis Holter and Ambulatory ECG Monitoring,” American Journal of Cardiology, 1983, pp. 903-906.

By Alison Oswald, Archives Center, National Museum of American History


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