ECG Made Easy

What Is an ECG?

The ECG Paper

• Horizontal axis = Time - each small box = 0.04 seconds; each large box (5 small boxes) = 0.2 seconds
• Vertical axis = Voltage (amplitude) - each small box = 0.1 mV

The Cardiac Conduction System (What Generates the Waves)

SA Node → Atria → AV Node → Bundle of His → 
Left & Right Bundle Branches → Purkinje Fibers → Ventricular Muscle

The PQRST Waves - One at a Time

P Wave

• What it is: Atrial depolarization (the atria contracting)
• Origin: SA node fires, signal spreads through both atria
• Normal: Smooth, rounded, upright in leads I, II; duration < 0.12 sec (3 small boxes); amplitude 2-3 mm
• Simple memory: "P = atria Pumping"

PR Interval

• What it is: Time from start of atrial depolarization to start of ventricular depolarization
• Includes: The P wave + the flat PR segment (= AV node delay)
• Normal: 0.12 - 0.20 sec (3-5 large boxes)
• Clinical use: Prolonged PR = heart block; short PR = pre-excitation (e.g., WPW)

QRS Complex

• What it is: Ventricular depolarization (both ventricles contracting)
• Q wave: First downward deflection (small, normal septal Q)
• R wave: First upward deflection (the tall peak)
• S wave: First downward deflection after the R wave
• Normal duration: < 0.12 sec (< 3 small boxes)
• Clinical use: Wide QRS (> 0.12 sec) = bundle branch block or ventricular rhythm
• Simple memory: "QRS = ventricles Squeezing"

Why is QRS as short as the P wave even though ventricles are larger? Because the His-Purkinje system conducts much faster than atrial conduction tissue - so ventricles depolarize just as quickly. - Costanzo Physiology, 7th ed.

ST Segment

• What it is: The flat line between end of QRS and start of T wave - corresponds to the plateau phase of the ventricular action potential
• Normal: Flat (isoelectric), hugging the baseline
• Clinical use: ST elevation = STEMI (heart attack); ST depression = ischemia or NSTEMI

T Wave

• What it is: Ventricular repolarization (the ventricles "recharging")
• Normal: Upright in leads I, II, V2-V6; always inverted in aVR
• Clinical use: Tall peaked T waves = hyperkalemia; inverted T waves = ischemia

QT Interval

• What it is: From start of QRS to end of T wave - represents the full ventricular action potential (depolarization + repolarization)
• Normal: Rate-dependent; roughly < 440 ms in men, < 460 ms in women (corrected QTc)
• Clinical use: Prolonged QT = risk of Torsades de Pointes (dangerous arrhythmia)

The 12 Leads - What They Look At

How to Read an ECG - A Systematic Approach

1. Rate

• Regular rhythm: Count the large boxes between two R waves, divide 300 by that number
  • 1 box = 300 bpm, 2 boxes = 150, 3 = 100, 4 = 75, 5 = 60
• Irregular rhythm: Count R waves in the 10-second strip and multiply by 6
• Normal resting rate: 60-100 bpm

2. Rhythm

• Is it regular (equal spacing between R waves) or irregular?
• Is there a P wave before every QRS? Is there a QRS after every P?
• If irregular and no visible P waves: think atrial fibrillation

3. Axis

• Look at leads I and aVF
• Both positive (upright QRS) = normal axis (-30° to +90°)
• Lead I positive, aVF negative = left axis deviation (LAHB, LBBB)
• Lead I negative, aVF positive = right axis deviation (RVH, PE)

4. P Wave Analysis

• Present and upright in lead II? (= sinus rhythm)
• Absent or chaotic? (= AF, flutter)
• Widened (> 0.12 sec)? = Left atrial enlargement ("P mitrale")
• Peaked (> 2.5 mm in II)? = Right atrial enlargement ("P pulmonale")

5. PR Interval

• Normal: 0.12-0.20 sec
• Short (< 0.12 sec): WPW or LGL syndrome
• Prolonged (> 0.20 sec): 1st degree heart block
• Progressively lengthening until a QRS is dropped: 2nd degree Mobitz I (Wenckebach)
• Constant PR with random dropped QRS: 2nd degree Mobitz II
• No relationship between P and QRS: 3rd degree (complete) heart block

6. QRS Complex

• Wide or narrow?
• Right bundle branch block (RBBB): wide QRS, "RSR' pattern" (rabbit ears) in V1
• Left bundle branch block (LBBB): wide QRS, broad notched R in V5/V6
• Q waves: pathological Q wave = old MI (width > 0.04 sec or depth > 25% of R wave height)

7. ST Segment & T Waves

• ST elevation: check which leads to localize the infarct territory (see table below)
• ST depression: ischemia, NSTEMI, digoxin effect
• T wave inversion: ischemia, ventricular strain, PE (V1-V4)

8. QT Interval

• Correct for rate (QTc = QT ÷ √RR interval in seconds)
• Prolonged QTc: drugs (antiarrhythmics, antipsychotics, antibiotics), electrolyte disturbances, congenital long QT syndrome

Cardiac Cycle Correlation

• P wave - atrial systole (the atrial "kick" - the last 25% of ventricular filling)
• QRS - mitral valve closes, ventricular systole begins
• ST segment - ventricular ejection (aortic valve is open)
• T wave - ventricular repolarization, aortic valve closes
• TP segment - diastole (ventricular filling)

Quick Normal Values Reference

Common ECG Abnormalities at a Glance