ECG Basics · Lesson 8 · Leads, axis and conduction
The 12 leads and what they see
Each lead is a viewpoint on the same heartbeat. Knowing where each one looks from turns a 12-lead ECG into a map.
Ten electrodes
Four limb electrodes (right arm, left arm, left leg, and right leg as ground) and six chest electrodes produce twelve leads.
Limb leads: the frontal plane
- Bipolar: I (left arm − right arm, 0°), II (left leg − right arm, +60°), III (left leg − left arm, +120°). Einthoven's law: II = I + III.
- Augmented: aVR (−150°), aVL (−30°), aVF (+90°), each comparing one limb with the average of the other two.
Chest leads: the horizontal plane
| Lead | Position |
|---|---|
| V1 | 4th intercostal space, right sternal border |
| V2 | 4th intercostal space, left sternal border |
| V3 | Midway between V2 and V4 |
| V4 | 5th intercostal space, mid-clavicular line |
| V5 | Anterior axillary line, level with V4 |
| V6 | Mid-axillary line, level with V4 |
Which leads look at which wall
| Territory | Leads |
|---|---|
| Inferior | II, III, aVF |
| Lateral | I, aVL, V5, V6 |
| Septal | V1, V2 |
| Anterior | V3, V4 |
| Cavity view | aVR |
Notice the normal pattern: aVR mostly negative, and the R wave growing from V1 to V5 or V6 while the S wave shrinks (R-wave progression). The transition, where R and S are about equal, is usually V3 or V4.
Swapped arm electrodes invert lead I and swap aVR with aVL. A negative P and QRS in lead I with normal chest leads should prompt a lead check before a diagnosis.
See it move in the live simulatorOpen 12 leads in the simulator
References
- Kligfield P, et al. Recommendations for the standardization and interpretation of the electrocardiogram, Part I: the electrocardiogram and its technology. Circulation 2007;115:1306–1324.