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An ECG (or Electrocardiograph ) is a tracing of the electrical activity of the heart. (In some countries it may be described as an EKG) Multiple sensors are placed on the chest and/or limbs. These are referred to as the leads. When the heart beats, the beat initiates at a point on the upper chambers of the heart and then spreads over the rest of the heart. The sensors pick up this beat and relay them to the read out of the machine. Different sensors have a different view angle on the spread of the beat and so give a different shaped waveform on the readout.