ECG (Electrocardiogram) — Detailed Explanation

What is an ECG?

The ECG Grid

The Waveform Components

The 12 Leads — What Each Sees

Limb Leads (Frontal Plane)

Precordial (Chest) Leads — Horizontal Plane

Systematic ECG Interpretation (Step-by-Step)

1. Rate

• Regular rhythm: 300 ÷ (number of large squares between R–R peaks)
• Irregular rhythm: Count QRS complexes in 6 seconds × 10
• Normal: 60–100 bpm; <60 = bradycardia; >100 = tachycardia

2. Rhythm

• Is it regular or irregular?
• Is every P wave followed by a QRS? (AV relationship)
• Does every QRS have a preceding P wave?
• Normal sinus rhythm (NSR): regular P before every QRS; upright P in I, II; inverted in aVR; rate 60–100

3. Axis

• LAD: left anterior hemiblock, LVH, inferior MI
• RAD: RVH, RBBB, left posterior hemiblock, pulmonary disease

4. P Wave Morphology

• Broad, notched P in II = left atrial enlargement (P mitrale)
• Tall, peaked P in II (>2.5 mm) = right atrial enlargement (P pulmonale)

5. PR Interval

• Short (<0.12 s): pre-excitation (WPW), junctional rhythm, ectopic atrial pacemaker
• Long (>0.20 s): 1st degree AV block
• Progressive lengthening then dropped QRS = Mobitz I (Wenckebach)
• Consistent PR with sudden dropped QRS = Mobitz II
• No relationship between P and QRS = 3rd degree (complete) AV block

6. QRS Complex

• Width: narrow (<0.12 s) = normal conduction; wide (≥0.12 s) = bundle branch block or ventricular origin
• RBBB: rSR' pattern in V1 ("M-shaped"), wide S in I, V5, V6
• LBBB: broad notched R in I, V5, V6; QS in V1
• LVH: Sokolow–Lyon: S in V1 + R in V5/V6 ≥ 35 mm
• RVH: tall R in V1, right axis deviation
• Pathological Q waves: ≥ 0.04 s wide, ≥ 25% of R height → prior MI

7. ST Segment

• Elevation (≥1 mm in ≥2 contiguous leads):
  • Convex (dome) elevation = STEMI (myocardial infarction)
  • Concave (saddle-shaped) = pericarditis
  • Diffuse elevation = early repolarization variant
• Depression: ischemia (subendocardial), digitalis effect, hypokalemia

8. T Wave

• Inversion: ischemia, LVH strain, RVH strain, PE (V1–V4), BBB
• Peaked/tall T: hyperkalemia, early MI ("hyperacute T")

9. QT Interval (corrected = QTc using Bazett: QT ÷ √RR)

• Prolonged QTc: drugs (antiarrhythmics, antibiotics, antipsychotics), hypokalemia, hypomagnesemia, congenital long QT → risk of Torsades de Pointes
• Short QTc (<0.35 s): hypercalcemia, digoxin, short QT syndrome

Localization of Ischemia / MI by Leads

Analysis of Your ECG

• QRS duration: 0.09 s (normal)
• Heart rate (ЧСС): 62 beats/min (normal sinus rate)
• QRS axis: 73° (normal)
• QT actual: 0.36 s (normal)
• QT corrected: 0.34 s (within normal limits)
• P duration: not defined (не определен)
• PR interval: not measured

• Rhythm: Regular, P waves visible before each QRS in lead II — normal sinus rhythm, rate ~62 bpm
• Axis: 73° — normal
• P waves: Visible and upright in inferior leads, inverted in aVR — normal morphology
• QRS: Narrow (0.09 s), no bundle branch block; normal R-wave progression in V1–V6
• ST segments: Appear isoelectric across all leads — no ST elevation or depression evident
• T waves: Upright in I, II, V4–V6; appears inverted in aVR (normal)
• QT/QTc: 0.34 s — normal (no prolongation)

⚠️ This analysis is for educational purposes. Clinical decisions require evaluation by a licensed physician in context with the patient's symptoms, history, and other investigations.

Patient & Recording Details

Machine-Reported Measurements

Systematic Lead-by-Lead Analysis

Rhythm & Rate

• Rate: 63 bpm — normal
• Rhythm: Regular R-R intervals
• P waves: Present and upright in leads I, II, aVF; inverted in aVR
• Conclusion: Normal sinus rhythm (NSR)

Axis

• Lead I: positive QRS
• aVF: positive QRS
• Axis = 73° → Normal axis (0° to +90°)

P Waves

• Upright in II, III, aVF — normal sinus origin
• Not particularly tall (no right atrial enlargement) or broad/notched (no left atrial enlargement)
• Normal P wave morphology

PR Interval

• Visually appears 0.14–0.16 s (within normal 0.12–0.20 s)
• No AV block

QRS Complex

• Duration 0.08 s — narrow (no bundle branch block)
• No pathological Q waves visible in inferior (II, III, aVF) or anterior (V1–V4) leads
• R-wave progression: R wave increases from V1 → V5, normal transition at V3–V4
• No signs of LVH (no voltage criteria met visually)
• No signs of RVH

ST Segments

• Isoelectric across all 12 leads
• No ST elevation (rules out acute STEMI)
• No ST depression (no subendocardial ischemia)

T Waves

• Upright in I, II, V4–V6
• Inverted in aVR — normal
• No T-wave inversion in chest leads suggesting ischemia or strain

QT / QTc

• QTc = 0.34 s — normal, no prolongation risk

Overall Interpretation

Normal 12-lead ECG

⚠️ This interpretation is for educational purposes only. All clinical decisions must be made by a licensed physician in conjunction with the patient's symptoms, clinical history, and other investigations.