When interpreting a 12-lead ECG it is important to use a systematic approach. This involves looking at the rhythm strip on the 12-lead ECG to determine heart rate and rhythm, using the previously recommended 6-step approach. The complex configuration and intervals of the complexes, for example, PR, QRS and QT should be examined and measured to identify any delays or abnormalities in conduction (for example, AV, Bundle Branch block or pre-excitation).
The ECG should be examined to look for evidence of the following:
- R wave progression; normal or abnormal cardiac axis; chamber enlargement; or hypertrophy (seen by complexes bigger than expected)
- ST segment and T waves changes (resulting from acute ischemia; myocardial infarction; inflammation; or drug and electrolyte abnormalities)
- Presence of pathological (abnormal) Q waves i.e. > 0.04 duration, > 1/4 of the following R wave
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Slide 1: Normal ECG
Slide 2: Evolution of an acute myocardial infarction
This demonstrates typical, progressive ST changes throughout the evolution of an acute myocardial infarction.
It is possible to determine the approximate stage of a myocardial infarction by recognising these changes.
Slide 3: The ST segment is measured in small squares / millimetres (mm).
- Demonstrates a normal ST segment, starting on the isoelectric line then deviating towards the T wave.
- ST depression, below the isoelectric line.
- ST elevation above the isoelectric line.
Slide 4: Reciprocal/Ischaemic Changes: If a lead is looking directly at the infarct site it will produce ST segment elevation. When a lead sees the infarct from the opposite perspective, the ST segment may become depressed in that lead.
At the end of the QRS complex is the J point. The ST segment starts at the J point.
Page last reviewed: 30 Jul 2020
