DR P. MARAZZI / SCIENCE PHOTO LIBRARY DR P. MARAZZI / SCIENCE PHOTO LIBRARY
Heart pacemaker. Electrocardiogram (ECG) showing the heartbeat of a 72-year-old man fitted with an internal artificial pacemaker. This supplies regular electrical impulses (red) at the beginning of each heart beat (wave complex), which ensure that the heart beats regularly. An ECG measures the pumping of the heart's chambers using 12 external electrodes (I-III, V1-V6 and aVR, aVL and aVF) and records it as a series of electrical waves (blue), one from each electrode. This patient was suffering from heart block. His sinoatrial node, the area of the heart which normally controls the heart rate, failed to conduct impulses to the rest of the heart.
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