MEHAU KULYK / SCIENCE PHOTO LIBRARY MEHAU KULYK / SCIENCE PHOTO LIBRARY
Heart trace. Computer illustration of the trace of a normal human pulse, the beat of the heart. The image is based on an electrocardiogram (ECG) trace, a record of the electrical activity within the heart as it beats. The large "spike", called QRS waves in ECG jargon, represents the activity of the ventricles during contraction.
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