Electrocardiography Interpretation

The 12-lead electrocardiogram (ECG) is the first-line diagnostic test in the emergency evaluation of chest pain and suspected acute coronary syndrome. Interpreting it involves recognising patterns of ischaemia, injury, and infarction, identifying ST-segment elevation that signals the need for urgent reperfusion, and distinguishing true ischaemic change from many non-ischaemic causes of ST-T abnormality.

Definition

Electrocardiography interpretation is the systematic reading of the surface electrocardiogram to identify rhythm, conduction, and morphological abnormalities, including the ST-segment and T-wave changes that indicate myocardial ischaemia, injury, or infarction.

Scope

This topic covers the role and systematic interpretation of the electrocardiogram in acute chest pain, including ST-segment and T-wave changes, the criteria used to define significant ST elevation, and recognised ischaemic patterns. It is a reference and educational entry focused on interpretation principles rather than on procedural or pharmacological management.

Core questions

• How does the electrocardiogram reflect myocardial ischaemia, injury, and infarction?
• What ST-segment elevation thresholds define a STEMI-equivalent pattern?
• How are ischaemic ECG changes distinguished from non-ischaemic ST-T abnormalities?
• What is the value of serial electrocardiograms in evolving chest pain?

Key concepts

• 12-lead electrocardiogram
• ST-segment elevation and depression
• T-wave changes and hyperacute T waves
• Q waves and evolving infarction patterns
• Reciprocal changes
• Serial and continuous ECG monitoring

Mechanisms

Myocardial ischaemia alters the electrical properties of affected myocytes, producing characteristic changes on the surface electrocardiogram. Transmural ischaemia from persistent occlusion typically generates ST-segment elevation in the leads overlying the affected territory, often with reciprocal depression in opposing leads, whereas subendocardial ischaemia tends to produce ST depression or T-wave changes. The universal definition of myocardial infarction specifies lead-specific ST-elevation thresholds for diagnosis, and serial recordings help capture the evolution of these changes over time (Thygesen et al., 2018; Wagner et al., 2009).

Clinical relevance

Because the electrocardiogram identifies patients whose presentation calls for the most time-sensitive responses, accurate interpretation is central to the emergency assessment of chest pain. The interpretive principles described here explain how electrocardiographic findings are weighed alongside symptoms and biomarkers; they are educational and not a substitute for clinical judgement in an individual patient.

Evidence & guidelines

Standardised electrocardiographic criteria for acute ischaemia and infarction are set out in the AHA/ACCF/HRS recommendations (Wagner et al., 2009) and in the Fourth Universal Definition of Myocardial Infarction (Thygesen et al., 2018). Society guidelines on acute coronary syndromes (Byrne et al., 2023) and chest pain (Gulati et al., 2021) embed the electrocardiogram as the initial triage test, including the recommendation to obtain and interpret it rapidly after presentation.

History

The electrocardiogram has been a cornerstone of cardiac diagnosis since the early twentieth century, and its application to acute ischaemia matured as the relationship between ST-segment shifts and coronary occlusion was clarified. Standardisation efforts (Wagner et al., 2009) and successive universal definitions of myocardial infarction (Thygesen et al., 2018) progressively codified the lead-specific criteria now used to identify infarction.

Seminal works

• thygesen-2018
• wagner-2009

Frequently asked questions

Why is the electrocardiogram obtained so early in chest pain evaluation?

It is the fastest test that can identify ST-segment elevation and other ischaemic patterns, which determine the urgency of further assessment; guidelines recommend obtaining and interpreting it promptly after presentation.

Does a normal electrocardiogram exclude acute coronary syndrome?

No. An initial electrocardiogram can be normal or non-diagnostic in acute coronary syndrome, which is why serial recordings and cardiac biomarker testing are used to complement it.