ECG Interpretation - A Complete, Easy Guide

1. The ECG Paper - Understanding the Grid

• Horizontal axis = Time. Each tiny small box = 0.04 seconds. Each large box (5 small boxes) = 0.20 seconds.
• Vertical axis = Voltage (amplitude). Each small box = 0.1 millivolt (mV). Each large box = 0.5 mV.

2. The Waves - What Each One Means

P Wave - "The Atria Wake Up"

• Represents depolarization (electrical activation) of the atria (the upper chambers).
• Looks like a small, smooth bump.
• Normal: less than 0.12 sec wide (3 small boxes), less than 2.5 mm tall.
• If the P wave is wide or notched: the atria may be enlarged.
• If there are no P waves: could be atrial fibrillation (A-fib).

PR Interval - "The Signal Waits at the Gate"

• Measured from the start of P to the start of QRS.
• This is the time it takes for the electrical signal to travel from the atria down through the AV node (the gatekeeper) into the ventricles.
• Normal: 0.12 - 0.20 seconds (3 to 5 small boxes).
• Too short (<0.12 sec): signal bypassing the AV node (e.g., Wolff-Parkinson-White syndrome).
• Too long (>0.20 sec): AV block (1st degree heart block - signal delayed).
• Getting progressively longer until a beat is dropped: 2nd degree Mobitz I (Wenckebach).
• Consistently long + dropped beats: 2nd degree Mobitz II.
• No relationship between P waves and QRS: 3rd degree (complete) heart block - very serious.

QRS Complex - "The Ventricles Fire"

• Represents depolarization (contraction activation) of the ventricles (the large pumping chambers).
• It looks like a sharp spike - the biggest wave on the ECG.
• The Q is the first downward deflection, R is the upward peak, S is the downward deflection after R.
• Normal: less than 0.12 sec (3 small boxes wide).
• Ventricles are big, but depolarize quickly because the His-Purkinje system (a super-fast highway) carries the signal at high speed.
• Wide QRS (>0.12 sec): signal traveling slowly through the ventricles - called a bundle branch block, or could be a ventricular origin beat.
• Pathological Q waves: if a Q wave is wider than 1 small box or deeper than 1/3 the height of the QRS - suggests old myocardial infarction (heart attack).

ST Segment - "The Heart Holds Its Breath"

• The flat line between the end of S and the start of T.
• Represents the period when the ventricles are all depolarized but not yet repolarizing.
• Normally sits on the isoelectric (flat) baseline - called the J point.
• ST Elevation: the segment rises above baseline - sign of acute heart attack (STEMI - ST elevation MI) or pericarditis.
• ST Depression: the segment dips below baseline - sign of myocardial ischemia (insufficient blood flow) or non-STEMI.

T Wave - "The Heart Recovers"

• Represents repolarization (recovery) of the ventricles.
• Normally a smooth, rounded, upright wave (in most leads).
• Inverted T waves: ischemia, ventricular strain, bundle branch block.
• Tall/peaked T waves: early ischemia, or hyperkalemia (high potassium - can be life-threatening).
• Flat T waves: hypokalemia (low potassium), ischemia.

QT Interval - "Total Recovery Time"

• Measured from the start of Q to the end of T.
• Represents the total time for ventricles to depolarize and repolarize.
• Normal: roughly 0.35-0.45 seconds (varies with heart rate - use the corrected QTc).
• Prolonged QT: risk of dangerous arrhythmia called Torsades de Pointes (can cause sudden death). Causes: drugs (many antibiotics, antipsychotics), low magnesium/potassium, congenital.

U Wave - "The Quiet Afterthought"

• A small bump after the T wave, not always visible.
• Prominent U waves suggest hypokalemia (low potassium).

3. The 12 Leads - Different Camera Angles

• ST elevation in V1-V4 = anterior wall MI (LAD artery blocked)
• ST elevation in II, III, aVF = inferior wall MI (RCA artery blocked)
• ST elevation in I, aVL, V5-V6 = lateral wall MI (LCx artery blocked)

4. Heart Rate - How to Calculate It

Example: 3 big boxes between R waves = 300 ÷ 3 = 100 bpm

• Normal rate: 60-100 bpm
• Bradycardia: <60 bpm
• Tachycardia: >100 bpm

5. Rhythm - Is It Regular?

• Regular: same R-R intervals throughout - normal sinus rhythm, or a regular arrhythmia.
• Regularly irregular: a pattern to the irregularity (e.g., dropped beats in 2nd degree block).
• Irregularly irregular: no pattern at all - classic for atrial fibrillation (A-fib).

6. The Axis - Is the Heart Pointing the Right Direction?

7. A Systematic Checklist for Every ECG

1. Rate - 60-100 bpm?
2. Rhythm - Regular? Any P waves?
3. P wave - Present and normal shape?
4. PR interval - 0.12-0.20 sec?
5. QRS duration - Less than 0.12 sec?
6. QRS axis - Normal?
7. QRS voltage - Low voltage? High voltage (LVH)?
8. R-wave progression - Does R grow V1→V6?
9. Q waves - Any pathological Q waves?
10. ST segments - Elevated or depressed?
11. T waves - Upright and normal?
12. QT interval - Prolonged?
13. U waves - Prominent?
14. Compare with old ECG - Any new changes?

8. Common Abnormalities at a Glance

Key Normal Values - Quick Reference

ECG Axis - Diagrams and Easy Explanation

What Is the "Axis"?

The Einthoven Triangle + Reference System

• Left side: The Einthoven triangle - right arm (RA), left arm (LA), left leg (LL) form the 3 corners
• Middle: How you plot the mean QRS vector using leads I, II, and III together
• Right side: The degree reference system (0° on the right, +60° bottom right, +120° bottom left, 180° on the left, -120° top left, -60° top right)

How to Calculate the Axis (Guyton & Hall Method)

• Lead I net deflection (R - S) = +5 mm → plotted on the Lead I axis (horizontal)
• Lead III net deflection = +10 mm → plotted on the Lead III axis (diagonal)
• Draw perpendiculars from each measurement, where they intersect = the mean electrical axis arrow
• Result here: the arrow points to ~59° - normal!

Normal vs. Abnormal Axis - The Quick Rule

Left Axis Deviation (LAD) - From Guyton & Hall

• Lead I: tall upright R wave
• Lead II: smaller upright complex
• Lead III: predominantly downward (negative) QRS

• Left ventricular hypertrophy (hypertension, aortic stenosis)
• Left anterior fascicular block (LAFB)
• Left bundle branch block (LBBB)
• Inferior MI (loss of inferior forces)
• Wolff-Parkinson-White syndrome

Right Axis Deviation (RAD) - From Guyton & Hall

• Lead I: predominantly downward (negative) deep S wave
• Lead II: small complex
• Lead III: tall upright dominant R wave

• Right ventricular hypertrophy (pulmonary hypertension, pulmonary stenosis, tetralogy of Fallot)
• Left posterior fascicular block (LPFB)
• Right bundle branch block (RBBB)
• Pulmonary embolism (acute RV strain)
• Dextrocardia
• Normal variant in tall thin people

Summary Diagram in Words - The Axis "Wheel"

           -90° (straight up)
              |
-180° --------+-------- 0° / 360°  (Lead I positive pole)
              |
           +90° (straight down, aVF positive pole)

  NORMAL ZONE: -30° to +90°  (down-left quadrant)
  LAD:  more negative than -30°  (upper-left)
  RAD:  more positive than +90°  (lower-right)
  EXTREME: -90° to ±180°  (upper-right, "northwest")

Why Does the Axis Matter Clinically?