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Electrocardiography (EKG or ECG) consists of making a recording of heart activity. This is done by placing suction cups or disk electrodes upon the skin, with the main electrodes called Leads 2 and 3. Additional electrodes — named aVR, aVL, aVF, and V1-V6 — are also used. Each lead records the signals generated by different areas of the heart, which are interpreted in the form of a graph produced by the EKG machine. The large equipment used for this monitoring was the only method available until a portable EKG unit was developed that could record heart activity by using miniature electronic sensors.
After being invented in the early 1900s, EKG machines became standardized over time to the 12-lead unit used in medical facilities. Precursor testing for portable EKGs was conducted in 1964 by the U.S. Public Health Service when nurses obtained four-electrode EKGs from patients in their homes and sent these by phone to a medical computer. In 1991, EKG readings were computerized, based on an analytical program called a Louvaine algorithm. This led to the development of portable EKG diagnostic units using the standard 12 leads that could be connected to personal computers (PCs) and wheeled from room to room in hospitals and clinics.
A variation of portable EKG units was the Holter monitor, which had sensing leads that were attached to the patient's chest and connected to a small recording unit carried on the belt. This allowed monitoring of conditions such as irregular heartbeats during daily activities outside clinical offices. Holter monitors were an expensive and tedious means of monitoring, however, and smaller handheld, battery-powered portable EKG monitoring units were developed to monitor cardiac activity more flexibly. Some of these were even capable of 12-lead recording; complete graphs could be stored on the unit and later printed with a PC printer.
A typical handheld portable EKG records heart activity by simply pressing it against the chest or palm of the hand to activate it. An actual graph of heart activity is not shown on the display screen. It typically shows a one-channel graphic tracing of heart activity, which can be switched to show only a digital display of the heart rate.
Heart activity can be recorded in various durations, and up to 400 “snapshot” waveforms can be recorded before the memory card fills up. The portable EKG can then be plugged into a computer and software that comes with the unit can be used to either print or save the snapshots as a computer file.
The less expensive handheld units have become a popular way to make a noninvasive recording of heart rhythms. Even with the optionally attached cables, however, they cannot match the high quality graphs that can be obtained from standard hospital machines. There are also warnings against using them on anyone who is pregnant or who has a pacemaker.