STEVE ALLEN / SCIENCE PHOTO LIBRARY STEVE ALLEN / SCIENCE PHOTO LIBRARY
Electrocardiogram (ECG) trace. Print-out from an ECG machine showing the heartbeat of a patient. An ECG uses external electrodes to measure electrical currents produced by the activity of heart muscle. It is used to diagnose diseases and abnormalities of the heart, and to monitor the condition of patients during and after operations.
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