Electrocardiography

Electrocardiography is the recording of the heart's electrical activity from electrodes placed on the body surface. The summed electrical currents generated as cardiac cells depolarize and repolarize produce small voltage changes at the skin, which are displayed as the electrocardiogram (ECG): a sequence of waves and intervals that mirror the timing of atrial and ventricular electrical events.

Definition

Electrocardiography is the technique of recording, from electrodes on the body surface, the time-varying electrical potentials produced by the depolarization and repolarization of cardiac cells, yielding the electrocardiogram as a graphical record of cardiac electrical events.

Scope

This entry covers the physiological origin of the ECG, the meaning of its principal waves and intervals, the concept of leads as views of the heart's electrical activity, and the relationship between the surface tracing and the underlying conduction sequence. It explains how the ECG reflects cardiac physiology and is not a guide to clinical interpretation or diagnosis.

Core questions

• Where do the voltages recorded on an ECG come from?
• What do the P wave, QRS complex, and T wave represent?
• What is a lead and why are multiple leads recorded?
• How does the surface tracing relate to the sequence of cardiac activation?

Key concepts

• P wave (atrial depolarization)
• QRS complex (ventricular depolarization)
• T wave (ventricular repolarization)
• PR interval and QT interval
• Leads and the cardiac electrical axis
• Volume conductor and body-surface potentials
• Standardized waveform nomenclature

Mechanisms

As the wave of depolarization and subsequent repolarization sweeps through the heart, it creates moving boundaries of charge that act as electrical sources within the body, which behaves as a volume conductor. Electrodes on the skin detect the resulting potential differences. Atrial depolarization appears as the P wave, ventricular depolarization as the QRS complex (atrial repolarization is normally hidden within it), and ventricular repolarization as the T wave. Intervals between these deflections reflect conduction times, the PR interval indexing atrioventricular conduction and the QT interval indexing the duration of ventricular depolarization and repolarization. Each lead records the projection of the heart's net electrical activity along a particular direction, so multiple leads provide complementary views of the same electrical sequence.

Clinical relevance

The ECG is the principal non-invasive window onto cardiac electrical activity, and its waves and intervals are interpreted against the physiological sequence of activation. This entry describes the physiological basis of the recording and is educational background, not instruction in clinical ECG interpretation or diagnosis.

Evidence & guidelines

Professional societies have published standardized recommendations for recording and describing the electrocardiogram, including consistent nomenclature for waves, intervals, and conduction patterns; these standards underpin uniform reporting of the surface tracing.

History

Willem Einthoven developed the string galvanometer in the early twentieth century and defined the standard limb leads, work recognized with the 1924 Nobel Prize; his terminology for the P, QRS, and T deflections remains in use. The lead system was later expanded, and professional bodies have since issued standardized recommendations for recording and interpreting the electrocardiogram.

Key figures

• Willem Einthoven
• Frank Norman Wilson
• Borys Surawicz
• Andre Kleber
• Yoram Rudy

Seminal works

• einthoven-1924
• surawicz-2009

Frequently asked questions

What do the main waves of the ECG represent?

The P wave reflects atrial depolarization, the QRS complex reflects ventricular depolarization, and the T wave reflects ventricular repolarization; the intervals between them index conduction times.

Why are several leads recorded?

Each lead captures the heart's net electrical activity along a different direction, so recording multiple leads provides complementary views of the same activation sequence from several angles.