📋 The Complete ECG Guide for Clinical Medical Students

PART 1 - ECG Basics: The Paper, Leads & Waves

The ECG Paper

• Standard speed: 25 mm/sec
• Each small square = 1 mm = 0.04 sec (40 ms)
• Each large square = 5 mm = 0.20 sec (200 ms)
• Amplitude standard: 10 mm = 1 mV

The 12 Leads - What They "See"

• aVR is the "mirror" lead - normally negative
• Precordial lead placement: V1 right sternal border 4th ICS; V2 left sternal border 4th ICS; V3-V6 progressing laterally (see diagram below)

The Normal ECG Waveform - A to Z

          R
          |
    P     |          T          U
   /\     |         /\         /\
--/  \----|--------/  \-------/  \--  (baseline)
          |\      /
          | \    /
          |  \  /
          |   \/
          S
   |--PR--|--QRS--|--ST--|----QT----|

Heart Rate Calculation

• Regular rhythm: 300 ÷ number of large squares between R-R peaks
  • 1 large square = 300 bpm, 2 = 150, 3 = 100, 4 = 75, 5 = 60, 6 = 50
• Irregular rhythm: Count QRS complexes in a 10-second strip × 6

QRS Axis (Frontal Plane)

• Normal: -30° to +90°
• Left axis deviation (LAD): more negative than -30° → most common cause is left anterior fascicular block
• Right axis deviation (RAD): more positive than +90° → RVH, PE, left posterior fascicular block
• Quick trick:
  • Lead I positive + aVF positive = normal axis
  • Lead I positive + aVF negative = LAD
  • Lead I negative + aVF positive = RAD
  • Both negative = "Northwest" / extreme axis

R-Wave Progression (Precordial Leads)

• R wave normally grows from V1 to V5 (transition at V3-V4 where R = S)
• Poor R-wave progression (small r waves persisting through V4) = anterior MI or LVH
• Diagram of ventricular depolarization vectors:

PART 2 - The Systematic Approach (14 Steps - Harrison's)

1. Standardization/calibration (10mm = 1mV, 25mm/sec)
2. Rhythm (sinus vs. non-sinus)
3. Heart rate
4. PR interval / AV conduction
5. QRS duration
6. QT/QTc interval
7. Mean QRS electrical axis
8. P wave morphology
9. QRS voltages
10. R-wave progression (precordial)
11. Abnormal Q waves
12. ST segments
13. T waves
14. U waves

PART 3 - Arrhythmias

Bradyarrhythmias

AV Blocks

Tachyarrhythmias

Narrow Complex (QRS < 120 ms) = Supraventricular

Wide Complex (QRS ≥ 120 ms)

PART 4 - Bundle Branch Blocks

Right Bundle Branch Block (RBBB)

• rSR' ("rabbit ears" or M-pattern) in V1
• Wide S wave in I, V5, V6
• T-wave inversion in V1-V3 (secondary change, expected)
• Can be normal variant OR indicate RV strain, PE, congenital heart disease

Left Bundle Branch Block (LBBB)

• Broad, notched R wave ("W"-shaped in V1, "M"-shaped in V5/V6)
• No septal q waves in lateral leads (I, V5, V6)
• T-wave inversion in V5, V6 (secondary, expected)
• LBBB in a chest pain patient = treat as STEMI equivalent (it hides ischemia and is an independent risk marker)
• Isolated LBBB in an otherwise healthy person carries 2× increased cardiovascular risk

• WiLLiaM MaRRoW: W in V1 = LBBB (Left); M in V1 = RBBB (Right)
• Or: LBBB = "Leave the patient" (more dangerous); RBBB = "Reassure" (usually benign)

PART 5 - Myocardial Ischemia and Infarction

Mechanism (Why ECG Changes Occur)

Evolutionary Changes in STEMI

Localization of MI by Lead Groups

NSTEMI / Unstable Angina

• ST depression (subendocardial ischemia) or T-wave inversion
• No ST elevation, but troponin distinguishes NSTEMI (elevated) from UA (normal)

Wellens Syndrome (professor favourite!)

• Deeply inverted or biphasic T waves in V2-V3 in a pain-FREE patient
• Indicates critical LAD stenosis - do NOT stress test!

De Winter T Waves

• ST depression + tall peaked T waves in V1-V6 = proximal LAD occlusion equivalent (STEMI variant - needs cath lab)

PART 6 - Other High-Yield ECG Patterns

LVH (Left Ventricular Hypertrophy)

• Sokolow-Lyon criteria: S in V1 + R in V5 or V6 ≥ 35 mm
• R in aVL ≥ 11 mm (Cornell criterion)
• Often associated with "strain pattern" (ST depression + T inversion in lateral leads)

RVH (Right Ventricular Hypertrophy)

• Tall R in V1 (R > S in V1)
• Right axis deviation
• ST depression + T inversion in V1-V3

Pericarditis

• Diffuse ST elevation (saddle-shaped, concave upward) in multiple lead groups (not fitting one territory)
• PR depression (reciprocal PR elevation in aVR) - pathognomonic early sign
• No reciprocal ST depression (vs. STEMI)

Pulmonary Embolism (PE)

• Sinus tachycardia (most common, ~44%)
• S1Q3T3 pattern: deep S in lead I, Q wave + T inversion in lead III
• New RBBB, right axis deviation
• T inversions in V1-V4 (RV strain)

Hyperkalemia

• Peaked/tented T waves (early)
• Wide QRS (later)
• Flat/absent P waves
• Sine-wave pattern → VF (severe)

Hypokalemia

• Prominent U waves (larger than T in same lead)
• T-wave flattening
• Prolonged QT
• Risk of torsades de pointes

Hypercalcemia / Hypocalcemia

• Hypercalcemia: Short QT (short ST segment)
• Hypocalcemia: Long QT (prolonged ST segment)

Long QT Syndrome

• QTc > 440 ms (men), > 460 ms (women)
• Risk of torsades de pointes (polymorphic VT) → VF → sudden death
• Causes: congenital channelopathies, drugs (amiodarone, sotalol, quinolones, antipsychotics), electrolyte disturbance (↓K, ↓Mg, ↓Ca)

Wolff-Parkinson-White (WPW)

• Short PR interval (< 120 ms)
• Delta wave (slurred upstroke of QRS)
• Wide QRS
• Due to accessory pathway (Bundle of Kent) bypassing AV node

Brugada Syndrome

• Coved-type ST elevation in V1-V2 (right chest leads)
• RBBB pattern
• Risk of sudden cardiac death - important channelopathy

PART 7 - The "Things That Impress a Professor" Checklist

1. "I will use a systematic approach..." (then use the 14-step Harrison's method)
2. Always state rate, rhythm, axis, intervals first
3. Describe P waves: "present/absent, morphology, relation to QRS"
4. Know the difference between primary vs. secondary ST-T changes
5. In any wide-complex tachycardia: "This is VT until proven otherwise"
6. In any new LBBB with chest pain: "This is a STEMI equivalent"
7. Know reciprocal changes (ST depression in leads opposite to ST elevation) - confirms STEMI
8. State the "territory and likely culprit artery" for any ischemic pattern
9. Mention QTc (corrected QT), not just QT
10. With AV blocks: always classify (1st/2nd Mobitz I or II/3rd) and state the clinical risk

Quick-Reference Summary Card

🧠 How to Actually Learn ECGs (Not Just Memorize Them)

The #1 Mindset Shift

• Why does LBBB hide ischemia? (altered depolarization changes ST baseline)
• Why is Mobitz II dangerous? (infranodal block - escape rhythm is unreliable)
• Why does PE cause S1Q3T3? (acute RV dilation rotates the heart electrically)

Study Tips by Stage

Stage 1 - Build the Foundation (Week 1)

• Read ECGs in one fixed order every single time (use the 14-step Harrison's system from the guide above) - never "spot read"
• Draw the normal ECG waveform from memory every day until it's automatic
• Learn the five boxes trick for axis (I + aVF quadrant method) - 30 seconds per ECG
• Memorize normal values as numbers, not ranges: "PR is 3-5 small boxes, QRS is under 3 boxes, QT is under half the RR interval"

Stage 2 - Pattern Recognition (Week 2)

• Learn the "Big 5" patterns your prof will definitely ask:
  1. STEMI (ST elevation, territory, reciprocal changes)
  2. AF (irregularly irregular, no P waves)
  3. Complete heart block (complete P-QRS dissociation)
  4. LBBB vs RBBB (WiLLiaM MaRRoW mnemonic)
  5. VT vs SVT with aberrancy (AV dissociation = VT)
• For each pattern: describe it out loud as if presenting to your professor

Stage 3 - Volume Practice (Week 3 onward)

• Do at least 5 ECGs per day - this is the only thing that works
• Time yourself: aim to read a full 12-lead in under 3 minutes
• Read one "difficult" or "tricky" case weekly (Wellens, De Winter, posterior MI, Brugada)

Memory Tricks That Actually Work

Common Mistakes to Avoid

1. Calling rate without rhythm first - always state rhythm before rate
2. Missing reciprocal changes - for every STEMI, always look at the opposite leads for ST depression (confirms the diagnosis)
3. Ignoring the P wave - most arrhythmia diagnoses live or die on P-wave analysis
4. Not correcting QT - always report QTc not raw QT
5. Diagnosing ischemia in LBBB - LBBB makes ST interpretation unreliable (use Sgarbossa criteria if needed)
6. Forgetting to compare - always ask "is there a prior ECG?" - this is the most clinically useful step

Free Practice Resources

Websites (Free, No Login Needed)

• ECG Wave-Maven (Harvard): ecg.bidmc.harvard.edu - 400+ real clinical cases with answers. The gold standard for self-study.
• Practical Clinical Skills: practicalclinicalskills.com/ecg - short courses, drills, and quizzes
• ECG Made Simple: ecgmadesimple.ca - free videos + interactive materials
• Dr. Smith's ECG Blog: hqmeded-ecg.blogspot.com - real cases daily, great for advanced learning

Apps (Free or Freemium)

• ECG Quiz (iOS/Android) - rapid-fire pattern recognition
• Cardioserv - good for systematic approach drills
• Geeky Medics ECG - medical student focused

YouTube Channels

Your Weekly Practice Schedule

The "Professor Presentation" Formula

"This is a 12-lead ECG recorded at 25 mm/sec and 10 mm/mV. The rhythm is [sinus/AF/etc], rate is [X] bpm, the PR interval is [X] ms, QRS duration is [X] ms, and QTc is [X] ms. The axis is [normal/LAD/RAD]. P waves are [present/absent/abnormal]. There are [no/these] ST changes: [describe with leads]. My conclusion is [diagnosis]. The most important clinical concern is [action]."