Electrocardiography

Roentgen's discovery of x-rays grew out of the detailed investigation of electricity that was a core scientific concern of the nineteenth century, and it is little surprise that investigators also took a keen interests in the electricity generated by the human body itself. Foremost among these was Willem Einthoven. Before his day, although it was known that the body produced electrical currents, the technology was inadequate to measure or record them with any sort of accuracy. Starting in 1901, Einthoven, a professor at the University of Leiden, conducted a series of experiments using a string galvanometer. In his device, electric currents picked up from electrodes on the patient' s skin passed through a thin filament running between very strong electromagnets. The interaction of the electric and magnetic fields caused the filament or "string" to move, and this was detected by using a light to cast a shadow of the moving string onto a moving roll of photographic paper.

It was not, at first, an easy technique. The apparatus weighed 600 lb, including the water circulation system essential for cooling the electromagnets, and was operated by a team of five technicians. Over the next two decades Einthoven gradually refined his machine and used it to establish the electrocardiographic (ECG) features of many different heart conditions, work that was eventually recognized with a Nobel prize in 1924.

As the ECG became a routine part of medical investigations it was realized that a system that gave only a "snapshot" of a few seconds of the heart's activity could be unhelpful or even misleading in the investigation of intermittent conditions such as arrhythmias. This problem was addressed by Norman Holter, an American biophysicist, who created his first suitcase-sized "ambulatory" monitor as early as 1949, but whose technique is dated in many sources to the major paper that he published on the subject in 1957, and other authors cite an even later, 1961 publication.

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