ECG signal acquisition

Electrical activity in the heart produces changing potential differences at the surface of the body. An electrocardiogram (ECG) records these changes over time.

Electrodes placed on the skin connect the body to an ECG machine. The machine measures the voltage difference between selected electrode positions. Different electrode combinations, called leads, provide different views of the heart’s electrical activity.

The measured voltage is small, typically of the order of millivolts, so the machine amplifies it. Filtering reduces unwanted interference. The resulting signal is displayed or recorded against time as an ECG trace.

The electrodes detect electrical signals. They do not directly measure the heart’s mechanical movement.

Normal ECG waveform

An electrocardiogram (ECG) records changes in potential difference produced by the heart’s electrical activity. A normal ECG waveform shows a repeating pattern as successive cardiac cycles occur. The horizontal axis represents time, and the vertical axis represents the measured potential difference.

The precise appearance of the waveform depends on the lead used: different leads measure the heart’s electrical activity from different perspectives.

Electrical events in the ECG waveform

Depolarisation is electrical activation that precedes contraction. Repolarisation is electrical recovery that precedes readiness for another activation. The ECG waves represent these electrical events, not the contractions themselves.

• The P wave corresponds to depolarisation spreading through the atria.
• The QRS complex corresponds to ventricular depolarisation. Atrial repolarisation occurs around the same time but is normally obscured by the larger QRS complex.
• The T wave corresponds to ventricular repolarisation.

These events recur in the same sequence as the cardiac cycles repeat.