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ECG (Electrocardiogram) - Full MBBS Level Guide

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1. What is an ECG?

• Standard recording speed: 25 mm/sec
• Each small box (1 mm) = 0.04 sec horizontally, 0.1 mV vertically
• Each large box (5 mm) = 0.2 sec horizontally, 0.5 mV vertically
• Standard paper runs for 10 seconds total

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2. ECG Waveforms and What They Represent

The Waveforms

Why is atrial repolarization not seen?

QRS complex nomenclature

• Capital letters (Q, R, S) = large deflections ≥ 5 mm (0.5 mV)
• Lowercase letters (q, r, s) = small deflections < 5 mm
• Q/q waves = initial negative deflection
• R/r = positive deflection
• S/s = negative deflection following an R wave
• QS complex = entirely negative complex (no R wave at all)

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3. Normal ECG Intervals (Table)

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4. Heart Rate Calculation

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

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5. The 12 ECG Leads

Bipolar Limb Leads (Standard Leads)

• Lead I: Right arm (-) vs Left arm (+). Records lateral heart wall.
• Lead II: Right arm (-) vs Left leg (+). Parallel to the long axis of the heart; best for rhythm assessment.
• Lead III: Left arm (-) vs Left leg (+).

Augmented Unipolar Limb Leads

• aVR (augmented Voltage Right arm): P and QRS are normally negative (inverted) - looks at the heart from the right shoulder
• aVL (augmented Voltage Left arm): P wave upright; QRS may be upright or inverted
• aVF (augmented Voltage Left Foot): P wave and QRS normally positive (upright)

Precordial (Chest) Leads V1-V6

• Poor R wave progression (transition at V4 or later) = may suggest anterior MI or clockwise rotation.

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6. Electrical Axis

• If QRS is positive in both Lead I and Lead II = normal axis
• If QRS is positive in I, negative in aVF = LAD
• If QRS is negative in I, positive in aVF = RAD
• Find the lead where QRS is most isoelectric; axis is perpendicular to that lead

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7. Stepwise Approach to ECG Interpretation

1. Rate - count the heart rate (regular or irregular)
2. Rhythm - is it sinus rhythm? (P before every QRS, P upright in I and II)
3. Axis - calculate frontal plane axis
4. Intervals - measure PR, QRS, QT
5. P wave - morphology, duration, axis
6. QRS complex - amplitude, duration, Q waves, bundle branch blocks
7. ST segment - elevation or depression
8. T wave - morphology (upright/inverted/peaked)
9. U wave - present or absent
10. Overall interpretation - put it all together

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8. Common ECG Abnormalities (MBBS Core Topics)

A. Sinus Rhythms and Rate Abnormalities

B. P Wave Abnormalities

C. PR Interval Abnormalities - AV Blocks

D. QRS Abnormalities - Bundle Branch Blocks

• Broad QRS ≥ 120 ms
• rSR' pattern ("M" pattern) in V1 (rabbit ears)
• Wide, slurred S wave in leads I and V6
• Secondary ST-T changes (ST depression, T wave inversion in V1-V3)
• Mnemonic: MaRRoW (M-shaped in V1, W-shaped in V6 for RBBB)

• Broad QRS ≥ 120 ms
• Broad, notched R wave ("M"-shaped) in V5, V6, I, aVL
• No septal q waves in I, V5, V6 (loss of normal septal q)
• WiLLiaM pattern: W in V1, M in V6 for LBBB
• ST elevation in V1-V3 is normal in LBBB (do not diagnose STEMI without using Sgarbossa criteria)

E. QRS Voltage Abnormalities - Hypertrophy

• Sokolow-Lyon criteria: S in V1 + R in V5 or V6 > 35 mm
• R in aVL > 11 mm
• Often with "strain pattern" = ST depression + T wave inversion in I, aVL, V5, V6

• R > S in V1 (dominant R in V1)
• Right axis deviation
• "Strain" ST depression in V1-V3
• Causes: pulmonary hypertension, mitral stenosis, cor pulmonale

• QRS amplitude < 5 mm in all limb leads AND < 10 mm in all precordial leads
• Causes: pericardial effusion, obesity, emphysema, hypothyroidism, anasarca

F. Q Waves - Ischemia and Infarction

• Width ≥ 0.04 sec (1 small box)
• Depth ≥ 1/4 of the R wave amplitude

G. ST Segment Changes

H. T Wave Abnormalities

I. QT Interval Abnormalities

• Prolonged QTc (> 450 ms men, > 460 ms women):
  • Causes: drugs (antiarrhythmics, antipsychotics, antibiotics - azithromycin, fluoroquinolones), hypokalemia, hypomagnesemia, hypocalcemia, congenital long QT syndrome, hypothermia
  • Risk: Torsades de pointes (polymorphic VT that can degenerate to VF)
• Short QTc (< 340 ms): hypercalcemia, digoxin, short QT syndrome

J. Arrhythmias

• AV dissociation = VT
• Fusion beats or capture beats = VT
• Extreme right axis deviation = VT
• Concordance (all QRS deflections same direction in V1-V6) = VT
• QRS > 160 ms = more likely VT
• "If in doubt, treat as VT" - it is safer

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9. Electrolyte Effects on ECG

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10. Special ECG Patterns (High Yield for MBBS)

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11. ECG in Specific Clinical Conditions

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12. ECG Progression in Acute MI

1. Hyperacute phase (minutes-hours): Tall, peaked (hyperacute) T waves - first change
2. Acute phase (hours): ST elevation; Q waves may begin
3. Evolving phase (hours-days): Q waves deepen, ST elevation begins to resolve, T wave inversion appears
4. Chronic/old phase (days-weeks): Q waves persist (permanent scar), ST normalizes, T waves may normalize

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13. Ambulatory ECG Monitoring

• Holter monitor: Continuous 24-48 hour recording; for palpitations, syncope, presyncope
• Event recorder: Weeks to months; patient-activated; for infrequent episodes
• Implantable loop recorder: Up to 3 years; subcutaneous implant; auto-triggered; for very rare unexplained syncope
• Mobile cardiac telemetry: Real-time transmission to a monitoring center

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Quick Reference Summary Table

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Method 1: The Large Box Method (Quickest - use this in exams)

Example A

HR = 300 ÷ 4 = 75 bpm ✓ Normal

Example B

HR = 300 ÷ 2 = 150 bpm ✓ Tachycardia (e.g., atrial flutter with 2:1 block)

Example C

HR = 300 ÷ 6 = 50 bpm ✓ Bradycardia

Memory Sequence (memorize this row):

Mnemonic: "300, 150, 100, 75, 60, 50" - just count the boxes and pick the number.

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Method 2: RR Interval Formula (Most Accurate)

Example A - Normal rhythm

HR = 60,000 ÷ 800 = 75 bpm

Example B - Tachycardia

HR = 60,000 ÷ 400 = 150 bpm

Example C - Bradycardia

HR = 60,000 ÷ 1200 = 50 bpm

How to measure RR in ms from paper:

• Count small boxes between two R peaks
• Multiply by 40 ms (since each small box = 0.04 sec = 40 ms)
• Example: 20 small boxes × 40 ms = 800 ms → HR = 75 bpm

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Method 3: 10-Second Strip Count (Best for Irregular Rhythms - e.g., AF)

Example A - Atrial Fibrillation

HR = 11 × 6 = 66 bpm (controlled AF)

Example B - Irregular but fast

HR = 18 × 6 = 108 bpm (uncontrolled AF / tachycardia)

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Side-by-Side Comparison

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Exam Tip

• For regular rhythms - use the large box method (fastest)
• For irregular rhythms (atrial fibrillation) - always use the 10-second count method
• For precise calculation (e.g., checking QTc) - use the ms formula
• The RR interval in seconds is also what you plug into Bazett's formula for corrected QT: QTc = QT ÷ √(RR in seconds)

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Heart Rate Counting + Frontal Plane Axis Calculation

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PART 1 - How to Count the Heart Rate

Step 1: Is the rhythm regular or irregular?

• Look at the R-R intervals across the strip - are they evenly spaced?
• Regular → use large box or RR formula
• Irregular (like AF) → use the 10-second count method

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Method A - Large Box Method (Fastest, for regular rhythms)

Memorize: "300, 150, 100, 75, 60, 50" - one number per finger on your hand, starting from 1 large box.

• Between 3 boxes (100 bpm) and 4 boxes (75 bpm)
• Estimate: ~85 bpm ← you can interpolate for fractional boxes

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Method B - RR Interval Formula (Most precise)

1. Count small boxes between two R peaks
2. × 40 ms (each small box = 0.04 sec = 40 ms)

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Method C - 10-Second Strip Count (For irregular rhythms)

• You count 14 QRS complexes across the whole 10-second ECG strip
• HR = 14 × 6 = 84 bpm

Always use this method for atrial fibrillation because the RR intervals vary beat to beat - no single RR is representative.

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PART 2 - How to Calculate the Frontal Plane Axis

What is the frontal plane axis?

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The 6 Frontal Plane Leads and Their Positions

         aVR (-150°)    -90°    aVL (-30°)
              \           |           /
               \          |          /
                \         |         /
    ±180° ------+-------- + --------+------ 0° (Lead I)
                /         |         \
               /          |          \
              /           |           \
           III (+120°)   +90°       II (+60°)
                          |
                         aVF (+90°)

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Step-by-Step Axis Calculation

Quick Method (Leads I and aVF only - use in exams)

• Lead I: tall upright R wave (positive QRS)
• Lead aVF: upright R wave (positive QRS)
• → Normal axis ✓

• Lead I: positive QRS
• Lead aVF: negative QRS (deep S wave dominates)
• → Left axis deviation (LAD)

• Lead I: negative (S > R)
• Lead aVF: positive (R > S)
• → Right axis deviation (RAD)

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Precise Method (using the isoelectric lead)

• Lead aVF is isoelectric (R ≈ S, net = 0)
• Lead I is strongly positive

• Lead II is isoelectric
• Lead I is deeply negative (S dominant)

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Causes of Axis Deviation (High Yield)

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Summary Cheat Sheet

STEP 1 - Count HR:
  Regular rhythm   → 300 ÷ large boxes (or 60,000 ÷ RR ms)
  Irregular rhythm → Count QRS in 10 sec × 6

STEP 2 - Axis (quick):
  I+ aVF+ = Normal
  I+ aVF- = LAD
  I- aVF+ = RAD
  I- aVF- = Extreme RAD (no man's land)

STEP 3 - Axis (precise):
  Find isoelectric limb lead
  Axis = perpendicular to it
  Confirm direction using most positive lead