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I now have enough content to give a thorough answer. Here is a complete, structured guide to basic ECG interpretation.
ECG Basic Interpretation
What is an ECG?
An ECG (electrocardiogram) measures tiny potential differences on the body's surface that reflect the electrical activity of the heart. Because different parts of the heart activate in a specific sequence - atria before ventricles, and ventricles in a defined pathway - each phase of that sequence produces a recognizable wave or deflection.
The ECG Paper
- Horizontal axis (time): each small box = 0.04 s; each large box = 0.2 s; 5 large boxes = 1.0 second
- Vertical axis (voltage): 1 mm = 0.1 mV; standard calibration = 10 mm/mV
Waves, Intervals & Segments
| Component | What it represents | Normal value |
|---|---|---|
| P wave | Atrial depolarization | Duration < 120 ms; amplitude < 2.5 mm |
| PR interval | Atrial depolarization + AV node conduction time (start of P to start of QRS) | 120-200 ms (3-5 small boxes) |
| PR segment | AV node conduction only (end of P to start of QRS) - isoelectric | Part of PR interval |
| QRS complex | Ventricular depolarization | 60-100 ms (< 3 small boxes); typically similar duration to P wave |
| ST segment | Plateau of ventricular action potential - isoelectric | Flat, at baseline; elevation or depression is pathological |
| T wave | Ventricular repolarization | Upright in most leads; same general direction as QRS |
| QT interval | Total ventricular electrical activity: first depolarization to last repolarization (includes QRS + ST + T) | Rate-dependent; corrected QTc < 440 ms in men, < 460 ms in women |
Note: The difference between intervals and segments - intervals include the bounding waves; segments do not.
Atrial repolarization is not visible on a standard ECG because it is buried within (and masked by) the QRS complex.
A Systematic Approach to Reading an ECG
Step 1 - Rate
Two methods:
- Precise: Rate = 60 / R-R interval (in seconds)
- Quick (300 rule): Count large boxes between two consecutive R waves. Rate = 300 / number of large boxes
- 1 box = 300 bpm | 2 = 150 | 3 = 100 | 4 = 75 | 5 = 60 | 6 = 50
Normal resting rate: 60-100 bpm
Step 2 - Rhythm
Ask three questions:
- Where is the pacemaker? (Normal = SA node)
- What is the conduction path? (SA node → AV node → His-Purkinje → ventricles)
- Is it regular and at the correct rate?
Normal sinus rhythm: P wave before every QRS; QRS after every P; regular P-P and R-R intervals; rate 60-100 bpm.
Step 3 - Intervals & Durations
- Prolonged PR (> 200 ms) → AV conduction delay (1st-degree AV block)
- Short PR (< 120 ms) → pre-excitation (e.g. WPW) or enhanced AV conduction
- Wide QRS (> 120 ms) → bundle branch block or ventricular origin
- Prolonged QTc → risk of torsades de pointes (electrolyte disturbance, drugs, congenital)
Step 4 - Axis
The mean electrical axis of the ventricles represents the overall direction of ventricular depolarization in the frontal plane.
Normal axis: -30° to +90°
| Axis | Range | Common causes |
|---|---|---|
| Normal | -30° to +90° | - |
| Left axis deviation (LAD) | < -30° | Left ventricular hypertrophy, left anterior fascicular block, inferior MI |
| Right axis deviation (RAD) | > +90° | Right ventricular hypertrophy (e.g. pulmonary stenosis, tetralogy of Fallot), left posterior fascicular block |
| Extreme (NW) axis | -90° to ±180° | Ventricular tachycardia, severe RVH |
Quick axis check using leads I and aVF:
- Both positive → normal axis
- Lead I positive, aVF negative → LAD
- Lead I negative, aVF positive → RAD
Axis and hypertrophy: When a ventricle hypertrophies, the axis shifts toward it because (a) more muscle generates more electrical potential on that side, and (b) depolarization takes longer on that side, so the hypertrophied ventricle remains positive when the other has already depolarized.
Step 5 - P wave morphology
- Peaked P > 2.5 mm in lead II → right atrial enlargement ("P pulmonale")
- Bifid (notched) P in lead II, or biphasic P in V1 → left atrial enlargement ("P mitrale")
Step 6 - QRS morphology
- Tall R waves in left leads (I, aVL, V5-V6) + deep S in right leads → left ventricular hypertrophy
- Tall R in right leads (V1-V2) + deep S in left leads → right ventricular hypertrophy
- Delta wave (slurred QRS upstroke) → pre-excitation
Step 7 - ST segment & T waves
- ST elevation: STEMI, pericarditis, Prinzmetal angina, Brugada
- ST depression: NSTEMI/unstable angina, ischemia, digoxin effect, reciprocal changes
- T wave inversion: ischemia, RVH strain, hypertrophic cardiomyopathy, pulmonary embolism
- Tall peaked T waves: hyperkalemia (early), hyperacute STEMI
Step 8 - Putting it all together
After assessing all the above, form a summary statement (e.g. "Normal sinus rhythm at 72 bpm, normal axis, no ST-T changes").
Key Normal Reference Values (Summary)
| Parameter | Normal range |
|---|---|
| Heart rate | 60-100 bpm |
| PR interval | 120-200 ms |
| QRS duration | < 120 ms |
| QT interval (corrected) | < 440 ms (men), < 460 ms (women) |
| Mean QRS axis | -30° to +90° (mean ~59°) |
Sources: Costanzo Physiology 7th Ed., p. 149; Medical Physiology (Boron & Boulpaep), p. 731; Guyton & Hall Medical Physiology, p. 156-157
Would you like to go deeper on any specific part - arrhythmia recognition, bundle branch blocks, ST changes in MI, or axis determination?
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