ECG Interpretation: Norm, Waves, Intervals, Algorithm

1. Technical Fundamentals

• Recording speed 25 mm/s (50 mm/s can be used for greater detail).
• Calibration: 10 mm = 1 mV (standard).
• On standard grid paper: 1 small square = 1 mm = 0.04 s; 1 large square = 5 mm = 0.20 s.
• Lead placement: limb electrodes (red — right arm, yellow — left arm, green — left leg, black — right leg / ground).
• Chest leads V1–V6: V1 — 4th intercostal space at the right sternal border, V2 — 4th intercostal space at the left sternal border, V4 — 5th intercostal space at the midclavicular line, V3 — midway between V2 and V4, V5 — anterior axillary line at the level of V4, V6 — midaxillary line at the level of V4.

2. Leads and Their Axes

3. Waves and Normal Values

• P wave — atrial depolarization; positive in I, II, aVF, V2–V6; amplitude < 2.5 mm; duration < 0.12 s.
• PR interval — from the onset of P to the onset of QRS; 0.12–0.20 s.
• QRS complex — ventricular depolarization; 0.06–0.10 s.
• R wave — main positive wave; progressive increase in amplitude from V1 to V5–V6.
• S wave — deep in V1–V2, diminishing towards V5–V6.
• Q wave — < 0.03 s and < 1/4 of R wave amplitude (in leads other than aVR, V1); pathological if > 0.04 s or deeper.
• ST segment — isoelectric; elevation/depression > 1 mm is considered abnormal.
• T wave — ventricular repolarization; positive in most leads except aVR and V1 (variable).
• QT interval — from the onset of QRS to the end of T; Bazett's correction formula QTc = QT / √RR.
• U wave — small positive deflection following the T wave; not always visible, associated with hypokalemia.

4. Electrical Axis of the Heart

• Norm: from −30° to +90° (most commonly 0° to +60°).
• Left axis deviation (LVH / LBBB): from −30° to −90°.
• Right axis deviation (RVH / PE / left posterior fascicular block): from +90° to +180°.
• Extreme axis deviation (−90° to ±180°): "northwest quadrant" — rare, indicative of pathology.
• Rapid method: QRS in I and aVF. Both positive — normal; I negative, aVF positive — right axis deviation; I positive, aVF negative — left axis deviation.

5. Heart Rate Calculation

• Regular rhythm: 1500 / (number of small squares between R-R) or 300 / (number of large squares).
• Large squares between R-R: 1 → 300, 2 → 150, 3 → 100, 4 → 75, 5 → 60, 6 → 50.
• Irregular rhythm: count the number of QRS complexes in 6 seconds (30 large squares) and multiply by 10.
• Norm 60–100 bpm; bradycardia < 60, tachycardia > 100.

6. Sinus Rhythm

• Regular P waves preceding each QRS complex, identical in morphology.
• P wave positive in I, II, aVF, negative in aVR.
• PR interval constant and within normal limits.
• HR 60–100 bpm (< 60 — sinus bradycardia; > 100 — sinus tachycardia).
• Sinus (respiratory) arrhythmia in young individuals and athletes — normal variant.

7. Conduction Blocks

• First-degree AV block: PR > 0.20 s.
• Second-degree AV block, Mobitz type I (Wenckebach): progressive PR prolongation until a QRS complex is dropped.
• Mobitz type II: constant PR interval with intermittent dropped QRS complexes without prior prolongation.
• Third-degree AV block (complete heart block): atria and ventricles beat independently.
• RBBB (Right Bundle Branch Block): rSR' in V1–V2, wide S in I, V5–V6; QRS > 0.12 s.
• LBBB (Left Bundle Branch Block): wide mono- or biphasic R in I, aVL, V5–V6; QS or rS in V1–V2; QRS > 0.12 s.
• Incomplete RBBB — QRS 0.10–0.12 s with rSR' in V1.

8. Hypertrophy

• Left ventricular hypertrophy (Sokolow-Lyon): S(V1) + R(V5 or V6) ≥ 35 mm.
• Cornell index: R(aVL) + S(V3) > 28 mm in men, > 20 in women.
• Romhilt-Estes — point score system.
• Right ventricular hypertrophy: R/S > 1 in V1, deep S in V5–V6, right axis deviation.
• Right atrial hypertrophy: "P-pulmonale" — tall peaked P ≥ 2.5 mm.
• Left atrial hypertrophy: "P-mitrale" — notched wide P in I, II; negative phase in V1 deeper than 1 mm and wider than 0.04 s.

9. Ischemia and Infarction

• Subendocardial ischemia — ST depression ≥ 1 mm, horizontal/downsloping.
• Subepicardial ischemia — ST elevation ≥ 1 mm in two contiguous leads (in V2–V3 — ≥ 2 mm in men, 1.5 mm in women).
• MI zones by leads: inferior (II, III, aVF → RCA), anterior (V1–V4 → LAD), lateral (I, aVL, V5–V6 → LCx), posterior (depression in V1–V3, reciprocal picture; V7–V9 — elevation), RV (V3R, V4R).
• MI dynamics: ↑ ST (minutes-hours) → pathological Q (hours-days) → inverted T (weeks-months) → normalization.
• Reciprocal depression — confirms STEMI.

10. Interpretation algorithm

• 1. Sinus rhythm? (regular P before each QRS, positive in II).
• 2. Heart rate (60–100?).
• 3. Electrical axis.
• 4. Intervals: PR, QRS, QT/QTc.
• 5. P wave (amplitude, duration, morphology).
• 6. QRS complex (amplitude, morphology, presence of pathological Q waves).
• 7. ST segment (elevation, depression).
• 8. T wave (amplitude, inversion, symmetry).
• 9. Conduction abnormalities.
• 10. Comparison with previous ECGs.

11. Electrolyte disorders on ECG

• Hyperkalemia: tall peaked T waves → PR prolongation → QRS widening → sine wave pattern → asystole.
• Hypokalemia: flat T wave, prominent U wave, ST depression, QT prolongation.
• Hypercalcemia: shortened QT.
• Hypocalcemia: QT prolongation, without T wave changes.
• Hypothermia: Osborn wave (J wave) — positive at the J point, most commonly in V3–V6.

12. FAQ

Why does aVR show ventricular processes in a mirror image?
The axis of aVR is directed from the heart to the right shoulder (−150°), i.e., opposite to the main LV depolarization vector. Therefore, all main waves are inverted: P is negative, R is small, Q is deep, T is normally negative.

What is QTc and why is it corrected?
QT duration depends on heart rate — the faster the rhythm, the shorter the QT. For standard comparison, QTc is used (Bazett: QT/√RR; Fridericia for rapid rhythms). Normal is < 0.44 s (M), < 0.46 (F); > 0.50 indicates a high risk of Torsades de Pointes.

When are additional leads V7–V9 and V3R–V4R needed?
V7–V9 — when posterior MI is suspected (with ST depression in V1–V3 and high R). V3R–V4R — during inferior MI to rule out right ventricular involvement (ST elevation in V4R is a marker).

How to quickly distinguish sinus tachycardia from AFib?
Sinus tachycardia is regular, with a P wave before each QRS. Atrial fibrillation is irregular, lacking P waves, with chaotic "f" waves and varying R-R intervals.

Почему важна сравнительная ЭКГ?
Many changes can only be interpreted in dynamics. New ST elevation or the appearance of Q waves indicates an MI; chronic changes indicate prior MI. An isolated "reciprocal" picture without old ECGs may be mistaken for normal.

Смотрите также: Myocardial infarction · Arrhythmias · ECG blocks · Hyperkalemia · Cardiology catalog