The normal values below apply to adults in sinus rhythm, recorded at a paper speed of 50 mm/s with 1 mV = 10 mm. At 50 mm/s a small square (1 mm) corresponds to 20 ms and a large square (5 mm) to 100 ms.
Intervals and durations
QTc — decision thresholds
At a QTc above 500 ms: stop QT-prolonging drugs, correct the potassium to > 4.0 mmol/L and the magnesium to > 0.9 mmol/L, and consider telemetry. The QT interval is measured in the lead in which it is longest, usually II or V5, and the U wave must not be included.
Electrical axis
| Electrical axis | Interpretation |
|---|---|
| −30° to +90° | Normal |
| −30° to −90° | Left axis deviation — left anterior fascicular block, inferior infarction, hypertrophy |
| +90° to +180° | Right axis deviation — right ventricular strain, lateral infarction, left posterior fascicular block |
| −90° to ±180° | Extreme axis deviation |
Amplitude criteria
| Criterion | Threshold | Suggests |
|---|---|---|
| Sokolow–Lyon | S in V1 + R in V5/V6 > 35 mm | Left ventricular hypertrophy |
| Cornell, men | R in aVL + S in V3 > 28 mm | Left ventricular hypertrophy |
| Cornell, women | R in aVL + S in V3 > 20 mm | Left ventricular hypertrophy |
| R in V1 | > 7 mm or R/S > 1 | Right ventricular hypertrophy |
| P wave in II | > 2.5 mm tall | Right atrial enlargement |
| P wave in V1 | Terminal negative component > 1 mm × 40 ms | Left atrial enlargement |
| Low voltage | QRS < 5 mm in the limb leads or < 10 mm in the chest leads | Pericardial effusion, obesity, emphysema, amyloidosis |
ST elevation criteria in suspected infarction
New ST elevation at the J point in two contiguous leads:
| Leads | Threshold |
|---|---|
| V2–V3, men ≥ 40 years | ≥ 2.0 mm |
| V2–V3, men < 40 years | ≥ 2.5 mm |
| V2–V3, women | ≥ 1.5 mm |
| Other leads | ≥ 1.0 mm |
| V7–V9 (posterior) | ≥ 0.5 mm |
| V3R–V4R (right ventricle) | ≥ 0.5 mm (≥ 1.0 mm in men < 30 years) |
Other thresholds
- Sinus bradycardia: < 50/min. A rate < 40 while awake should be investigated.
- First-degree AV block: PR > 220 ms. A PR interval > 300 ms may become haemodynamically significant.
- Pre-excitation: PR < 120 ms with a delta wave and QRS > 110 ms.
- Brugada pattern type 1: ST elevation ≥ 2 mm of coved type with a negative T wave in V1–V2.
- Early repolarisation: J point elevation ≥ 1 mm with notching or slurring in the inferolateral leads.
See also the Calculators section for QTc formulas and risk scores.
