What starts each heartbeat, and what does an ECG actually show?

Your heart starts its own beat. A small patch of special muscle cells in the upper wall of the right atrium, called the sinoatrial node, or SA node, produces an electrical impulse again and again. That impulse travels through the heart's wiring and makes the muscle contract, so the SA node sets the rhythm and is known as the heart's natural pacemaker. An ECG, or electrocardiogram, is a line graph of that electrical activity, recorded by electrodes placed on the skin.

The SA node has a rhythm of its own, about 100 beats a minute. Nerves modify that rate, which is why the average resting heart rate in adults is about 70 beats per minute. From the SA node the signal spreads across the atria and reaches the atrioventricular node, the AV node, which acts as a gate. It holds the signal briefly to coordinate it before passing it on to the ventricles. If the SA node fails or its signal is blocked, a group of cells further down the heart becomes the pacemaker instead.

An ECG trace has a familiar shape. The P wave is the atria activating, the QRS complex, the tall spike, is the ventricles activating, and the T wave is the ventricles recovering. The test is safe and painless, and it picks up only electrical signals through the skin.

The discovery has a charming story. A young medical student named Martin Flack found the node in the heart of a mole, while his mentor, Sir Arthur Keith, was away on a bicycle ride with his wife. The two worked in a makeshift laboratory in a farmhouse in Kent and published the discovery in 1907. A few years earlier, in 1903, Willem Einthoven's string galvanometer had made it possible to record the heart's electrical signals precisely, and he won the 1924 Nobel Prize for it.