Standard 12-Lead ECG Interpretation
For comprehensive ECG interpretation, you must evaluate all 12 standard leads: the 6 limb leads (I, II, III, aVR, aVL, aVF) and the 6 precordial leads (V1, V2, V3, V4, V5, V6). 1
Lead Organization and Display
The standard 12-lead ECG is displayed in four columns, with each column representing 2.5 seconds of continuous recording at 25 mm/s paper speed: 2
- First column: Leads I, II, III (standard limb leads) 1
- Second column: Leads aVR, aVL, aVF (augmented limb leads) 1
- Third column: Leads V1, V2, V3 (right precordial leads) 1
- Fourth column: Leads V4, V5, V6 (left precordial leads) 1
Understanding Lead Information Content
The 12-lead ECG actually contains only 8 independent pieces of electrical information, not 12. 1, 2
Limb Leads (Frontal Plane)
- The 6 limb leads contain only 2 independent measured signals 1
- Modern electrocardiographs measure 2 pairs of limb electrodes and mathematically derive the remaining 4 limb leads 1, 2
- Despite this redundancy, multiple limb leads provide different spatial perspectives that aid clinical interpretation 1, 2
Precordial Leads (Horizontal Plane)
- Each of the 6 precordial leads (V1-V6) provides uniquely measured, independent electrical information 1, 2
- These leads cannot be calculated from other ECG information 1
- V1-V3 represent the right ventricle and septum 2
- V4-V6 represent the left ventricle 2
Anatomic Correlation by Lead Groups
When interpreting for ischemia or infarction, group leads by anatomic territory:
- Inferior wall: II, III, aVF 3
- Anterolateral wall: I, aVL, V5, V6 3
- Anterior/septal wall: V1, V2, V3, V4 2
Critical Technical Requirements
All leads must be temporally aligned to within 10 milliseconds (ideally less) for accurate spatial-temporal interpretation. 1, 2
- Precise temporal alignment allows diagnosis of fascicular blocks with infarction 1, 2
- Simultaneous lead viewing improves arrhythmia interpretation and myocardial infarction diagnosis 1, 2
Common Pitfalls to Avoid
- Do not call augmented or precordial leads "unipolar" - they are derived from electrode pairs, not true unipolar recordings 1, 2
- Do not assume one-to-one correspondence between lead location and myocardial region - each lead reflects electrical activity from all cardiac sources active at that moment 1, 2, 3
- Always compare current ECGs with previous tracings to detect new changes 4
Alternative Display Format
The Cabrera sequence is highly recommended by the American College of Cardiology as an alternative presentation that arranges frontal plane leads in anatomic order (aVL, I, -aVR, II, aVF, III from left to right), which improves spatial quantification and diagnostic accuracy for acute infarction. 2