12-lead ECG electrode placement: limb leads and precordial leads
Limb lead electrode placement: (1) RA — right arm or right wrist. (2) LA — left arm or left wrist. (3) RL (ground) — right leg or right ankle. (4) LL — left leg or left ankle. Place electrodes on the limb itself (wrist, inner forearm, or ankle) rather than the torso — torso placement changes the electrical axis and affects lead morphology.
Precordial (chest) lead placement (critical for diagnostic accuracy): V1 — 4th intercostal space (ICS), right sternal border. V2 — 4th ICS, left sternal border. V3 — between V2 and V4 (diagonal placement). V4 — 5th ICS, midclavicular line. V5 — anterior axillary line, same horizontal level as V4. V6 — midaxillary line, same horizontal level as V4–V5.
Finding the 4th ICS: palpate the sternal angle (Angle of Louis — the bony horizontal ridge on the sternum where the manubrium meets the body). The rib attached here is the 2nd rib. Count down to the 2nd ICS (space below 2nd rib), 3rd rib, 3rd ICS, 4th rib, 4th ICS. V1 and V2 are placed at the 4th ICS.
Common placement errors and their ECG effects
V1–V2 too high (3rd ICS instead of 4th): produces falsely elevated P waves in V1–V2, poor R-wave progression, may create false RBBB morphology, and changes ST-segment appearance — misdiagnosis risk.
V4 too lateral (placing V4 at axillary line instead of midclavicular): makes V5 and V6 appear on the posterolateral wall, losing anterior transition and creating false poor R-wave progression.
Limb leads on torso: produces significant axis changes and altered waveform morphology. Particularly problematic in patients with amputations or casts — document the electrode placement location if non-standard.
RA/LA reversal: produces inverted P waves and QRS in lead I, negative aVR (normally negative in I, aVR positive after reversal), mirror-image changes. Lead II and III swap appearances.
Right-sided leads (V3R, V4R) for RV assessment: mirror-position of standard leads on the right chest. V4R is the most clinically valuable — 4th ICS, right midclavicular line (mirror of V4).