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Basic ECG Interpretation: Waves, Intervals, and Axis
The ECG Paper (Grid Standards)
The ECG is recorded on standard grid paper at a speed of 25 mm/sec:
| Grid | Time | Voltage |
|---|---|---|
| Small box (1 mm) | 0.04 sec (40 ms) | 0.1 mV |
| Large box (5 mm) | 0.20 sec (200 ms) | 0.5 mV |
The Waveforms
P Wave
- Represents atrial depolarization
- Originates from the SA node in the high lateral right atrium
- Normal: upright in leads I and II, negative in aVR
- Duration correlates with atrial conduction time; broadening indicates slowed conduction
- Atrial repolarization is buried under the QRS and not visible on a normal ECG
PR Interval
- Measured from the start of the P wave to the start of the QRS
- Includes the P wave + isoelectric PR segment (AV node conduction delay)
- Normal: 120-200 ms (3-5 small boxes)
- A short PR (<120 ms) suggests pre-excitation (e.g. WPW); a long PR (>200 ms) = 1st degree AV block
- Sympathetic stimulation shortens it; parasympathetic stimulation lengthens it
QRS Complex
- Represents ventricular depolarization
- Q wave = first negative deflection before R
- R wave = first positive deflection
- S wave = negative deflection after R
- Despite the ventricles being much larger than the atria, QRS duration is similar to the P wave duration because the His-Purkinje system conducts much faster than atrial tissue
- Normal duration: 80-120 ms (2-3 small boxes); >120 ms = bundle branch block
- The J point marks the end of QRS and beginning of the ST segment
ST Segment
- Isoelectric (flat) segment between end of QRS (J point) and start of T wave
- Corresponds to the plateau phase of the ventricular action potential
- Elevation (>1 mm) = ischemia/infarction or pericarditis; Depression = ischemia or strain
T Wave
- Represents ventricular repolarization
- Normally upright in most leads (except aVR, sometimes V1)
- Peaked T waves = hyperkalemia; flattened/inverted = ischemia, hypokalemia, strain
QT Interval
- Measured from start of QRS to end of T wave
- Represents total ventricular depolarization + repolarization time
- Varies inversely with heart rate - must correct using QTc
- QTc formula (Bazett): QTc = QT ÷ √RR (in seconds)
- Upper normal limits: <460 ms in women, <450 ms in men
- Prolonged QT = risk of torsades de pointes
U Wave
- Small deflection after the T wave (not always seen)
- Most prominent in V2-V3; prominent U waves suggest hypokalemia or bradycardia
Heart Rate Calculation
From the ECG, heart rate can be quickly calculated:
| Method | How |
|---|---|
| 300 rule (regular rhythm) | Count large boxes between two R waves → divide 300 by that number |
| 1500 rule (regular rhythm) | Count small boxes between two R waves → divide 1500 |
| 6-second method (irregular) | Count QRS complexes in a 6-second strip × 10 |
Example: R-R interval = 4 large boxes → HR = 300/4 = 75 bpm
The 12 Leads
The 12-lead ECG views the heart from different angles:
Limb leads (frontal plane):
- Bipolar: I, II, III
- Augmented unipolar: aVR, aVL, aVF
Precordial leads (horizontal plane):
- V1 through V6, placed across the chest
Each lead is like a different camera angle on the same cardiac events. Lead II and aVF are best for P wave analysis; V1 is useful for rhythm assessment.
Cardiac Axis
The QRS axis reflects the overall direction of ventricular depolarization in the frontal plane.
Normal Axis: 0° to +90°
- Lead I positive (QRS points left) + Lead aVF positive (QRS points down) = normal axis
Quick axis method using Leads I and aVF:
| Lead I | aVF | Axis |
|---|---|---|
| Positive | Positive | Normal (0° to +90°) |
| Positive | Negative | Left axis deviation (-30° to -90°) |
| Negative | Positive | Right axis deviation (+90° to +180°) |
| Negative | Negative | Extreme axis deviation (-90° to ±180°) |
Clinical causes of axis deviation:
Left Axis Deviation (more negative than -30°):
- Left anterior fascicular block
- Left ventricular hypertrophy
- Inferior MI
- WPW syndrome (right-sided accessory pathway)
Right Axis Deviation (more positive than +90°):
- Right ventricular hypertrophy
- Left posterior fascicular block
- Pulmonary embolism / cor pulmonale
- Dextrocardia
- Normal in children and thin young adults
Normal ECG Values at a Glance
| Parameter | Normal Range |
|---|---|
| Heart rate | 60-100 bpm |
| P wave duration | <120 ms (<3 small boxes) |
| PR interval | 120-200 ms (3-5 small boxes) |
| QRS duration | <120 ms (<3 small boxes) |
| QT interval (QTc) | <450 ms (men), <460 ms (women) |
| P axis (frontal) | 0° to +75° (upright in I and II) |
| QRS axis | -30° to +90° |
Systematic Approach to Reading an ECG
A reliable order to avoid missing findings:
- Rate - count QRS complexes or use large-box method
- Rhythm - regular or irregular? Is there a P before every QRS?
- Axis - check leads I and aVF
- P waves - morphology, axis, one per QRS?
- PR interval - short, normal, or prolonged?
- QRS duration - narrow or wide (bundle branch block)?
- ST segment - elevation or depression?
- T waves - upright, inverted, peaked?
- QT interval - calculate QTc
- U waves - present or absent?
Sources: Goldman-Cecil Medicine, 26e | Costanzo Physiology, 7e | Harrison's Principles of Internal Medicine, 22e | Braunwald's Heart Disease, 12e
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