ECG leads: Einthoven's triangle, augmented and chest leads
- Concept
- Exam priority: medium
Each ECG lead looks at the heart's electricity from one angle. Twelve angles together give a 3D picture of every beat.
Also called: Einthoven's law, Limb leads, Precordial leads, 12-lead ECG
See it in 3D: Left ventricle · Right ventricle · Interventricular septum · Sternum
What it is
A lead is a pair of recording points (or one electrode against a combined reference) that measures voltage along one direction. The six limb leads view the heart in the frontal plane; the six chest leads view it in the horizontal plane. A wave is positive when the current spreading through the heart runs toward the lead's positive pole.
Key facts
- Body fluids conduct well, so current from a partly depolarized heart flows through the whole chest; the base points negative and the apex positive during most of the QRS.
- The limbs act as extensions of the wires: an electrode on the wrist records much the same as one on the shoulder.
- Augmented leads (aVR, aVL, aVF) compare one limb with the average of the other two, which fills the gaps between leads I, II and III every 30°.
- V1–V2 sit over the right ventricle and septum (rS pattern); V5–V6 sit over the left ventricle (qR pattern).
- Depolarization and repolarization both make waves; the sign of each wave depends on the direction of the current and the direction in which the change travels.
How it works, step by step
- Depolarized muscle is negative outside, resting muscle positive outside; the boundary between them is a dipole that drives current through the chest.
- Ventricular depolarization runs mainly from base to apex and from endocardium to epicardium, so the mean current points down and to the left.
- Lead II (right arm to left leg) lies almost along this direction, so it records the tallest normal R and upright P.
- When all muscle is depolarized (ST segment) there is no boundary and no current: the trace returns to baseline.
- Repolarization makes the opposite charge change, so a wave travelling the same way as depolarization would be inverted.
- In the ventricle the epicardium has a shorter action potential and repolarizes first, so repolarization travels epicardium → endocardium, opposite to depolarization.
- Opposite change and opposite direction cancel twice: the T wave is upright in leads where the QRS is mainly upright.
Electrocardiogram: a normal beat in lead II
HR 75 /minRR 0.80 sPR 0.16 sQRS 0.09 sQT 0.37 sQTc 0.41 s
Tap a wave (P, QRS, T, U), an interval (PR, ST, QT) or the RR.
- ECG trace (lead II)
- Small box: 0.04 s · 0.1 mV
- Big box: 0.20 s · 0.5 mV
- Selected part
Normal reference values
| Parameter | Value | Note |
|---|---|---|
| QRS voltage in a limb lead (peak R to trough S) | ≈ 0.5–2.0 mV | lead II usually largest, III smallest |
| R/S transition in chest leads | V3–V4 | |
| Paper speed and gain | 25 mm/s, 10 mm/mV |
Approximate values for a healthy adult at rest; each laboratory sets its own ranges.
The 12 leads and the region of the heart each one faces
| Leads | Plane and angle | Main view |
|---|---|---|
| I, aVL | Frontal, 0° and −30° | High lateral left ventricle |
| II, III, aVF | Frontal, +60°, +120°, +90° | Inferior wall |
| aVR | Frontal, −150° | Cavity from the upper right (normally all negative) |
| V1–V2 | Horizontal, right of sternum to left parasternal | Septum and right ventricle |
| V3–V4 | Horizontal, anterior | Anterior wall and apex |
| V5–V6 | Horizontal, left lateral | Lateral left ventricle |
Clinical notes
- Inferior infarction shows in II, III and aVF; anterior in V1–V4; lateral in I, aVL, V5–V6. Lead groups map to coronary territories.
- Poor R-wave progression (small R up to V3–V4) can follow an old anterior infarction, but also misplaced electrodes, LVH or a large chest.
Test yourself
Think of the answer, then tap the question to see it.
In one beat the R wave measures 0.4 mV in lead I and 0.9 mV in lead III. What R voltage should lead II show?
1.3 mV. Einthoven's law states that lead II equals lead I plus lead III at the same instant.
Why is the normal T wave upright in lead II even though repolarization is the opposite charge change to depolarization?
Ventricular repolarization travels in the opposite direction (epicardium first, because its action potentials are shorter). Opposite sign and opposite direction cancel, so T follows the QRS.
A tracing shows an inverted P, QRS and T in lead I and an upright complex in aVR. What is the most likely cause?
Left and right arm electrodes swapped. Dextrocardia can look similar in the limb leads but also shows loss of R-wave progression across the chest leads.
Which chest leads normally show an rS pattern, and why?
V1–V2. They face the right side: the small r is septal depolarization moving toward them, the deep S is the larger left ventricle depolarizing away from them.
Connections
See it in 3D4
The structures of Durgical's 3D anatomy where it happens.
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: Electrocardiogram: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2025); Myocardial Infarction (StatPearls (NCBI Bookshelf), 2025).
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Representative model, for educational purposes only.