ECG Basics - A Comprehensive Guide

1. What is an ECG?

2. The Cardiac Conduction System (The ECG's Electrical Source)

3. The ECG Paper and Technical Standards

• Standard paper speed: 25 mm/sec
• Standard calibration: 10 mm = 1 mV
• A standard ECG records 10 seconds of activity, with 12 leads arranged in groups: I, II, III → aVR, aVL, aVF → V1, V2, V3 → V4, V5, V6

4. The ECG Waveforms

P Wave

• Represents: Atrial depolarization
• Normal duration: < 120 ms (< 3 small boxes)
• Atrial repolarization is NOT seen on the normal ECG - it is buried under the QRS complex (atrial muscle mass is too small)
• Morphology reflects atrial structure: if conduction slows through the atria, the P wave widens

PR Interval

• Measured from: Onset of P wave to onset of QRS complex
• Represents: Conduction time through atrial muscle + AV node + His-Purkinje system (the AV nodal delay is the isoelectric PR segment within the interval)
• Normal: 90-200 ms (0.09-0.20 sec)
• Sympathetic stimulation shortens it; parasympathetic stimulation (or AV node disease) prolongs it

QRS Complex

• Represents: Ventricular muscle depolarization
• Normal duration: 75-110 ms (< 3 small boxes)
• Much larger amplitude than the P wave because of the large ventricular muscle mass
• Contains tremendous information about ventricular structure and function

• Q wave = first negative deflection before any positive deflection
• R wave = first positive deflection
• S wave = negative deflection after the R wave
• Additional deflections are labeled R', S', etc.

ST Segment

• Measured from: J point (junction of QRS end and ST start) to onset of T wave
• Represents: Plateau phase of the ventricular action potential (isoelectric - no net current flow)
• Normally isoelectric (flat, at baseline)
• Elevation or depression is a major clinical sign

T Wave

• Represents: Ventricular repolarization
• Normally upright in most leads
• T waves are concordant with the QRS (i.e., upright where QRS is predominantly positive) in normal ECG

U Wave

• Small deflection following the T wave (occasionally seen)
• May represent repolarization of the Purkinje fibers or mid-myocardial "M cells"
• Prominent U waves: hypokalemia, bradycardia, certain drugs

QT Interval

• Measured from: QRS onset to T wave offset
• Represents: Total ventricular depolarization and repolarization (ventricular systole)
• Must be corrected for heart rate using Bazett's formula: QTc = QT / √RR (in seconds)
• Normal QTc: Males ≤450 ms; Females ≤460 ms

J Point (Osborn Wave)

• The J point marks the end of QRS and start of ST segment
• A positive deflection at the J point = J wave (Osborn wave) - classically seen in hypothermia, also in hypercalcemia and brain injury

5. Normal ECG Intervals - Reference Table

6. Calculating Heart Rate

Heart rate = 60,000 ÷ RR interval (in ms)

• 1 box = 300 bpm
• 2 boxes = 150 bpm
• 3 boxes = 100 bpm
• 4 boxes = 75 bpm
• 5 boxes = 60 bpm
• 6 boxes = 50 bpm

7. The 12 Leads - What Each One "Sees"

Limb Leads (Frontal Plane)

Precordial (Chest) Leads (Horizontal Plane)

8. The Electrical Axis

• Both positive → Normal axis
• I positive, aVF negative → LAD (check if truly left: leads II)
• I negative, aVF positive → RAD
• Both negative → Extreme axis deviation

9. Bundle Branch Blocks and Fascicular Blocks

10. Key Abnormalities to Recognize

ST Changes

• ST elevation (≥1 mm in limb leads, ≥2 mm in precordial): STEMI, pericarditis (diffuse, with PR depression), Brugada, early repolarization
• ST depression: Ischemia (subendocardial), reciprocal changes, digoxin effect (scooped)

Pathological Q Waves

• Width ≥40 ms OR depth ≥25% of R wave height = significant
• Indicate prior transmural infarction (electrically dead tissue)

P-Wave Abnormalities

• Broad, notched P (P mitrale): Left atrial enlargement - LA conduction is delayed
• Tall, peaked P (P pulmonale, >2.5 mm in II): Right atrial enlargement

PR Interval Changes

QTc Prolongation

• QTc >450 ms (males) or >460 ms (females)
• Risk of torsades de pointes (polymorphic VT)
• Causes: drugs (amiodarone, sotalol, macrolides, antipsychotics), hypokalemia, hypomagnesemia, congenital long QT syndrome

11. Systematic Approach to Reading an ECG

1. Rate - Calculate from RR interval
2. Rhythm - Regular or irregular? P before every QRS? QRS after every P?
3. Axis - Look at leads I and aVF
4. Intervals - PR, QRS duration, QTc
5. P wave morphology - Size, shape, axis
6. QRS morphology - Bundle branch blocks, R-wave progression, Q waves
7. ST segments - Elevation or depression vs. J point
8. T waves - Upright, inverted, peaked?
9. U waves - Present?
10. Compare with prior ECG if available

12. Summary Cheat Sheet

ECG Calculations Using Seconds - Made Easy

The Foundation: The Grid

1 small box  =  0.04 sec  (40 ms)
1 large box  =  0.20 sec  (200 ms)   [= 5 small boxes]

Step 1 - Converting Boxes to Seconds

• 3 boxes → 0.3... → 0.03 × 4 → actually just say: 3 boxes = 0.12 sec
• Or memorize: boxes × 4 = milliseconds (e.g., 3 × 4 = 12 → 0.12 sec)

Step 2 - Measuring Each Interval

P Wave Duration

• Count small boxes from start to end of P wave
• Normal: < 3 small boxes (< 0.12 sec)

Example: P wave spans 2 boxes
2 × 0.04 = 0.08 sec ✓ Normal

PR Interval

• Count small boxes from start of P to start of QRS
• Normal: between 0.09 sec and 0.20 sec (roughly 2-5 small boxes)

Normal range in boxes:
  0.09 sec ÷ 0.04 = ~2.25 boxes (minimum)
  0.20 sec ÷ 0.04 =  5 boxes (maximum = exactly 1 large box)

Example: PR spans 4 small boxes
4 × 0.04 = 0.16 sec ✓ Normal

Example: PR spans 6 small boxes
6 × 0.04 = 0.24 sec ✗ Prolonged → 1st degree AV block

QRS Duration

• Count small boxes from start to end of QRS
• Normal: ≤ 2.5-3 small boxes (≤ 0.11 sec)

Normal upper limit = 0.11 sec ÷ 0.04 = ~2.75 boxes

Example: QRS spans 2 boxes
2 × 0.04 = 0.08 sec ✓ Normal

Example: QRS spans 4 boxes
4 × 0.04 = 0.16 sec ✗ Wide → Bundle branch block

QT Interval

• Count small boxes from start of QRS to end of T wave
• Normal depends on heart rate, so we correct it

QTc = QT (in seconds) ÷ √RR interval (in seconds)

1. Measure QT: spans 10 small boxes → 10 × 0.04 = 0.40 sec
2. Measure RR: spans 20 small boxes → 20 × 0.04 = 0.80 sec
3. √0.80 = 0.894
4. QTc = 0.40 ÷ 0.894 = 0.447 sec = 447 ms
5. Check: Males ≤ 450 ms ✓ (just within normal)

• Males: ≤ 0.45 sec (450 ms)
• Females: ≤ 0.46 sec (460 ms)

Step 3 - Heart Rate Calculation

Formula Method

HR = 60 ÷ RR interval (in seconds)
  OR
HR = 1500 ÷ RR interval (in small boxes)
  OR
HR = 300 ÷ RR interval (in large boxes)

• RR interval = 15 small boxes
• 15 × 0.04 = 0.60 sec
• HR = 60 ÷ 0.60 = 100 bpm
• Same result: 1500 ÷ 15 = 100 bpm ✓

The "300 Rule" - Fastest Clinical Method

The "10-Second Strip" Method (for irregular rhythms like AF)

Count all QRS complexes in the 10-second strip × 6 = HR

Step 4 - All Normal Values in "Box Language"

Quick Clinical Decisions in Seconds

The One-Glance Summary

SMALL BOX = 0.04 sec  |  LARGE BOX = 0.20 sec

INTERVALS (count small boxes × 0.04):
  P wave     < 3 boxes  = < 0.12 sec
  PR         2-5 boxes  = 0.08-0.20 sec
  QRS        < 3 boxes  = < 0.12 sec

HEART RATE:
  300 ÷ (large boxes between R waves)
  OR  1500 ÷ (small boxes between R waves)
  OR  beats on 10-sec strip × 6

QTc = QT(sec) ÷ √RR(sec)    →   ≤ 0.45 sec (M), ≤ 0.46 sec (F)

ECG Calculations - The Best Ones, Made Easy

The Only Rule You Need to Memorize First

1 small box = 0.04 sec
1 large box = 0.20 sec

1. Heart Rate - 3 Methods, Pick One

The 300 Rule (fastest, for regular rhythms)

1 box = 300    2 = 150    3 = 100
4 = 75         5 = 60     6 = 50

Tip: Just memorize the sequence: 300, 150, 100, 75, 60, 50

The 1500 Rule (most accurate, regular rhythms)

HR = 1500 ÷ small boxes between R waves

Example: 20 small boxes → 1500 ÷ 20 = 75 bpm

The ×6 Rule (best for irregular rhythms like AF)

Count QRS complexes in 10-sec strip × 6 = HR

Example: 9 complexes → 9 × 6 = 54 bpm

2. Intervals - Count Boxes, Multiply by 0.04

The rule of 3: P wave, PR max, and QRS are all judged against 3 small boxes (0.12 sec)

3. QTc - Corrected QT (Bazett's Formula)

QTc = QT (sec) ÷ √RR (sec)

If QT is less than half the RR interval → QT is probably normal

4. What the Numbers Mean Clinically

One Card Summary

┌─────────────────────────────────────────┐
│  1 small box = 0.04 sec                 │
│  1 large box = 0.20 sec                 │
├─────────────────────────────────────────┤
│  RATE  →  300 ÷ large boxes (R to R)    │
│           300/150/100/75/60/50          │
├─────────────────────────────────────────┤
│  PR    →  3-5 small boxes (0.12-0.20s)  │
│  QRS   →  < 3 small boxes (< 0.12s)    │
│  QTc   →  QT ÷ √RR  ≤ 0.44-0.46s      │
│  Shortcut: QT < ½ RR = likely normal   │
└─────────────────────────────────────────┘