SHEILA TERRY / SCIENCE PHOTO LIBRARY SHEILA TERRY / SCIENCE PHOTO LIBRARY
Composite image of a fading electrocardiograph (ECG) trace overlayed onto a false-coloured chest X-ray. The size of the heart (the pear-shaped white area just below and right of centre) shows it to be enlarged, a symptom of hypertension. ECGs provide information about the heart's electrical activity.
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