Limb leads: I, II, III, aVR, aVL, aVF
The six limb leads record electrical activity in the frontal plane — looking at the heart from the front. Leads I, II, and III are bipolar leads using two electrode sites each. Lead I records activity from right arm (negative) to left arm (positive). Lead II records from right arm (negative) to left leg (positive) — the most commonly used rhythm strip lead because it best captures P wave and QRS morphology. Lead III records from left arm (negative) to left leg (positive).
The augmented limb leads (aVR, aVL, aVF) are unipolar leads. aVR looks from the right shoulder toward the heart — normally negative in most of its deflections. aVL looks from the left shoulder — faces the high lateral wall of the left ventricle. aVF looks upward from the left foot — faces the inferior wall of the left ventricle. ST elevation in aVF (with II and III) indicates inferior STEMI; reciprocal depression in aVL is a key confirmatory finding.
Axis deviation is assessed using leads I and aVF: both positive indicates normal axis, positive I with negative aVF indicates left axis deviation (associated with left anterior fascicular block and inferior MI), negative I with positive aVF indicates right axis deviation.
Precordial leads: V1 through V6 — placement and what each records
The six precordial leads record electrical activity in the horizontal plane — looking at the heart in cross-section from below. Each lead is unipolar, placed on the chest wall in a standardized location.
V1 is placed in the fourth intercostal space at the right sternal border. V2 is in the fourth intercostal space at the left sternal border. V3 is between V2 and V4. V4 is at the fifth intercostal space, midclavicular line. V5 is at the same level as V4, anterior axillary line. V6 is at the same level, midaxillary line.
Territory by lead: V1–V2 reflect the right ventricle and septal wall. V3–V4 reflect the anterior wall (LAD territory). V5–V6 reflect the lateral wall (diagonal branches, circumflex). ST elevation in V1–V4 indicates anterior STEMI; in V4–V6 indicates anterolateral STEMI.
Normal R-wave progression: R waves should increase in amplitude from V1 to V4–V5 (where the transition from rS to Rs morphology occurs). Poor R-wave progression — small R waves persisting through V4 — suggests anterior MI, LBBB, or COPD.
Right-sided and posterior leads for complete assessment
Standard 12-lead ECG has diagnostic blind spots. Posterior STEMI (circumflex territory) produces ST depression in V1–V3 on the standard ECG — the actual ST elevation is on the posterior surface, visible only on leads V7, V8, V9 placed further left on the back. Any patient with ST depression in V1–V3 and a clinical presentation consistent with ACS should have posterior leads placed.
Right ventricular MI complicates inferior STEMI in approximately 30–40% of cases. Right-sided leads (V3R, V4R — mirror positions on the right chest) detect right ventricular ST elevation. V4R is the most sensitive: ST elevation ≥1 mm in V4R in the setting of inferior STEMI indicates right ventricular involvement. This diagnosis changes fluid management significantly: RV infarction requires volume loading rather than the nitrates used in standard inferior STEMI, and nitrates can cause profound hypotension by reducing RV preload.