Electrocardiography

Electrocardiography (ECG) records the electrical activity of the heart from electrodes placed on the body surface, producing a time-voltage tracing of each cardiac cycle. As the oldest and most widely used cardiac test, it is fast, inexpensive, and noninvasive, and it underlies the diagnosis of arrhythmias, conduction disorders, and acute myocardial ischemia.

Definition

Electrocardiography is the recording of the heart's electrical depolarisation and repolarisation as voltage changes over time, conventionally displayed as the P wave, QRS complex, and T wave across a set of standardised leads.

Scope

This topic covers the surface electrocardiogram as a diagnostic modality: how the cardiac electrical signal is captured by the standard 12-lead system, the meaning of its principal waves and intervals, and its central roles in detecting ischemia and rhythm disturbances. It treats the ECG as a reference topic and does not provide interpretation rules for individual tracings or patient-specific management.

Core questions

• How does the body-surface signal reflect the sequence of cardiac depolarisation and repolarisation?
• What distinguishes a normal tracing from one indicating ischemia, infarction, or a conduction or rhythm disorder?
• Why is the ECG the first-line test in the evaluation of acute chest pain?

Key concepts

• 12-lead system
• P wave, QRS complex, and T wave
• PR, QRS, and QT intervals
• ST-segment elevation and depression
• Cardiac axis
• Standardisation of recording and interpretation

Mechanisms

Each heartbeat begins with depolarisation of the atria (the P wave), propagates through the atrioventricular node and His-Purkinje system to depolarise the ventricles (the QRS complex), and ends with ventricular repolarisation (the T wave). Electrodes on the limbs and chest sample the resulting dipole from multiple angles, so the standard 12-lead recording reconstructs the spatial and temporal sequence of electrical activation. Deviations - prolonged intervals, abnormal axis, or ST-segment shifts - localise conduction block, ischemia, or infarction. Consistent electrode placement, calibration, and interpretation conventions are essential for comparable tracings (Kligfield, 2007).

Clinical relevance

The ECG is the first test obtained in suspected acute coronary syndrome, where ST-segment elevation versus its absence separates major management pathways, and it is fundamental to identifying arrhythmias and conduction disease (Byrne, 2023; Collet, 2021). The entry explains the modality's role and is not a guide to interpreting any specific patient's tracing.

Evidence & guidelines

Recording and interpretation conventions are standardised in joint AHA/ACC/HRS recommendations (Kligfield, 2007). The ECG's diagnostic role in acute coronary syndromes is defined by ESC guidelines for both ST-elevation and non-ST-elevation presentations (Byrne, 2023; Collet, 2021).

History

Willem Einthoven developed the string galvanometer and the systematic recording of the human electrocardiogram in the early twentieth century, naming the deflections P, Q, R, S, and T and laying the foundation for clinical electrocardiography (Einthoven, 1903). The lead system and interpretation conventions were progressively standardised over the following century (Kligfield, 2007).

Key figures

• Willem Einthoven

Seminal works

• einthoven-1903
• kligfield-2007

Frequently asked questions

What do the P wave, QRS complex, and T wave represent?

The P wave reflects atrial depolarisation, the QRS complex reflects ventricular depolarisation, and the T wave reflects ventricular repolarisation - together tracing one electrical cardiac cycle.

Why is an ECG done first in chest pain?

It is immediate, noninvasive, and can rapidly reveal ST-segment elevation, which signals a major artery occlusion requiring urgent reperfusion, thereby directing the initial management pathway.