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ECG (Electrocardiogram) - Complete Notes for 3rd Year Medical Students

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1. INTRODUCTION

• Heart rate
• Heart rhythm
• Conduction pathways
• Ischemia / infarction patterns
• Hypertrophy and metabolic disturbances

"Electrocardiography allows simultaneous recording of myocardial activation from multiple vantage points on the body's surface, thereby permitting analysis of electrical activation in different myocardial regions." - Goldman-Cecil Medicine

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2. THE CARDIAC CONDUCTION SYSTEM

1. SA node - located in the high lateral epicardial right atrium near the superior vena cava. Spontaneously depolarizes at the highest rate - it is the dominant pacemaker (60-100 bpm intrinsic rate).
2. Atria - the wave front spreads through both atria. The Bachmann bundle speeds conduction to the left atrium. This produces the P wave.
3. AV node - the only electrical connection between atria and ventricles (the AV rings are electrically insulated). Physiologic delay occurs here. This produces the PR interval.
4. Bundle of His - exits the AV node; capable of rapid conduction.
5. Bundle branches - the His bundle bifurcates into the right bundle branch and the left bundle branch. The left further divides into the left anterior fascicle and left posterior fascicle.
6. Purkinje system - distal ramifications that rapidly deliver current to ventricular myocytes.
7. Ventricular myocardium - depolarization produces the QRS complex; repolarization produces the T wave.

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3. ECG PAPER AND TECHNICAL STANDARDS

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4. THE ECG LEADS

Limb Leads (Frontal Plane)

Precordial Leads (Horizontal Plane)

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5. ECG WAVEFORMS - WHAT EACH REPRESENTS

P Wave

• Represents: Atrial depolarization (both right and left atria)
• Normal duration: < 0.12 sec (< 3 small boxes)
• Normal amplitude: < 2.5 mm in limb leads; < 1.5 mm in precordial leads
• Morphology: Upright in leads I, II, aVF, V4-V6; inverted in aVR
• Abnormalities:
  • Broad/bifid P (P mitrale) → Left atrial enlargement
  • Tall/peaked P (P pulmonale, >2.5 mm) → Right atrial enlargement
  • Absent P waves → AF, junctional rhythm, SA block

PR Interval

• Represents: Time from start of atrial depolarization to start of ventricular depolarization (includes AV nodal delay)
• Normal: 0.09 - 0.20 sec (90-200 msec)
• Prolonged (>0.20 sec): 1st degree AV block (delay usually in AV node)
• Short (<0.09 sec): Ventricular pre-excitation (WPW syndrome), junctional rhythm, or enhanced AV nodal conduction

QRS Complex

• Represents: Ventricular depolarization
• Normal duration: 0.075 - 0.11 sec (75-110 msec)
• Nomenclature:
  • Capital letters (Q, R, S) = amplitude ≥ 5 mm (0.5 mV)
  • Lowercase letters (q, r, s) = amplitude < 5 mm
  • Q/q = initial negative deflection
  • R/r = positive deflection
  • S/s = negative deflection after a positive deflection
  • QS = entirely negative deflection
  • R' = second positive deflection (after S wave)
• Prolonged QRS (>0.11 sec): Bundle branch block, ventricular rhythm, pre-excitation

ST Segment

• Represents: Early ventricular repolarization (plateau phase of action potential)
• Normal: Isoelectric (at baseline)
• J point: Junction between the end of QRS and beginning of ST segment
• Abnormalities:
  • ST elevation → Acute STEMI, pericarditis, Prinzmetal angina, LBBB, LVH
  • ST depression → NSTEMI/UA, subendocardial ischemia, digoxin effect, hypokalemia

T Wave

• Represents: Ventricular repolarization
• Normal: Upright in I, II, V3-V6; inverted in aVR (and often V1)
• Amplitude: < 10 mm precordial; < 5 mm limb leads
• Abnormalities:
  • Tall, peaked T waves → Hyperkalemia (early), STEMI (hyperacute T waves)
  • T wave inversion → Ischemia, ventricular hypertrophy strain pattern, RBBB, LBBB, CNS events

QT Interval

• Represents: Total ventricular depolarization + repolarization
• Measured: QRS onset to end of T wave; in leads II, V5, V6
• Must be rate-corrected: Use Bazett's formula: QTc = QT / √RR (RR in seconds)
• Normal QTc:
  • Males: 390-450 msec
  • Females: 390-460 msec
• Prolonged QTc: Risk of Torsades de Pointes (TdP) and sudden cardiac death
  • Causes: Drugs (antiarrhythmics, antipsychotics, antibiotics), hypokalemia, hypomagnesemia, hypocalcemia, congenital long QT syndrome
• Short QTc (<350 msec): Hypercalcemia, digoxin, congenital short QT syndrome

U Wave

• Small deflection after the T wave; best seen in V2-V3
• Represents: Repolarization of Purkinje fibers / interventricular septum (debated)
• Prominent U waves: Hypokalemia, bradycardia, hypothermia
• Note: Prominent U waves can complicate QTc measurement

J Wave (Osborn Wave)

• Positive deflection at the J point (QRS downstroke)
• Seen in: Hypothermia (most common), hypercalcemia, brain injury
• May indicate risk of idiopathic ventricular fibrillation

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6. HEART RATE CALCULATION

HR = 60 / R-R interval (in seconds) = 60,000 / R-R interval (in msec)

300 - 150 - 100 - 75 - 60 - 50 (for 1, 2, 3, 4, 5, 6 large boxes) Formula: HR = 300 / (number of large boxes)

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7. NORMAL INTERVAL SUMMARY TABLE

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8. QRS AXIS

Quick method using Leads I and aVF:

Causes of Axis Deviation:

• Left anterior fascicular block (most common)
• Left ventricular hypertrophy
• Inferior STEMI
• WPW (right-sided accessory pathway)

• Right ventricular hypertrophy
• Left posterior fascicular block
• Pulmonary embolism / cor pulmonale
• WPW (left-sided accessory pathway)
• Normal in children and tall thin adults

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9. SYSTEMATIC APPROACH TO ECG INTERPRETATION

Step-by-Step:

1. Rate - calculate using large-box method
2. Rhythm - regular or irregular? Is there a P for every QRS? Is there a QRS for every P?
3. P wave - present, morphology, axis (upright in I and II = sinus origin)
4. PR interval - normal, prolonged, or short?
5. QRS complex - duration, morphology, pathological Q waves?
6. QRS axis - using leads I and aVF
7. ST segment - elevation or depression?
8. T waves - normal, inverted, tall/peaked?
9. QT interval - calculate QTc
10. Overall interpretation - put it all together

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10. NORMAL SINUS RHYTHM (NSR) - Criteria

• Regular rhythm
• Rate 60-100 bpm
• P wave upright in I, II, aVF; inverted in aVR
• Every P followed by QRS; every QRS preceded by P
• PR interval 120-200 msec
• QRS < 120 msec
• Normal axis

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11. COMMON ARRHYTHMIAS

Sinus Arrhythmia

• Subtle variation in heart rate with respiration
• Inspiration → faster rate; expiration → slower rate
• Normal variant - no treatment required

Sinus Tachycardia

• Rate > 100 bpm; otherwise normal ECG
• Causes: exercise, anxiety, fever, pain, anemia, hypovolemia, hyperthyroidism
• Treat the underlying cause

Sinus Bradycardia

• Rate < 60 bpm; otherwise normal ECG
• Causes: athletes, sleep, hypothyroidism, sick sinus syndrome, medications (beta-blockers)

Atrial Fibrillation (AF)

• Irregularly irregular rhythm
• No distinct P waves - replaced by chaotic fibrillatory baseline
• Narrow QRS (unless aberrant conduction)
• Rate: variable (ventricular rate depends on AV node conduction)

Atrial Flutter

• Regular sawtooth flutter waves at ~300 bpm (P waves)
• Usually 2:1 AV block → ventricular rate ~150 bpm
• Sawtooth most visible in leads II, III, aVF

Supraventricular Tachycardia (SVT)

• Regular narrow-complex tachycardia, rate 150-250 bpm
• P waves may be hidden in QRS or just after QRS
• Includes AVNRT, AVRT

Ventricular Tachycardia (VT)

• ≥3 consecutive wide QRS complexes at rate > 100 bpm
• AV dissociation (independent P waves), fusion beats, capture beats
• Emergency - risk of degeneration to VF

Ventricular Fibrillation (VF)

• Chaotic, irregular wide complexes - no discernible QRS/T
• No cardiac output - cardiac arrest
• Treat with immediate defibrillation

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12. AV BLOCKS

1st Degree AV Block

• PR interval > 200 msec (one large box)
• All P waves conduct to ventricles
• Usually benign; no treatment needed

2nd Degree AV Block - Mobitz Type I (Wenckebach)

• Progressive PR prolongation until a P wave is not conducted (dropped QRS)
• Then cycle repeats
• Site of block: usually AV node
• Usually benign

2nd Degree AV Block - Mobitz Type II

• Fixed PR interval with sudden dropped QRS (no progressive lengthening)
• Site of block: His-Purkinje system (below the AV node)
• More serious - can progress to complete heart block
• May need pacemaker

3rd Degree (Complete) AV Block

• Complete dissociation between P waves and QRS complexes
• P waves and QRS have independent regular rates
• Ventricular escape rhythm: rate 20-40 bpm, wide QRS (if ventricular) or narrow QRS (if junctional)
• Emergency - needs pacemaker

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13. BUNDLE BRANCH BLOCKS

Right Bundle Branch Block (RBBB)

• QRS ≥ 0.12 sec (≥ 120 msec)
• RSR' pattern ("rabbit ears") in V1
• Wide, slurred S wave in leads I, V5, V6
• Secondary T wave changes (T wave inversion in V1-V3)

Left Bundle Branch Block (LBBB)

• QRS ≥ 0.12 sec
• Broad, notched R wave in I, aVL, V5, V6 (no Q wave in V5-V6)
• QS or rS in V1
• ST and T wave changes opposite to QRS direction (discordant)

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14. VENTRICULAR HYPERTROPHY

Left Ventricular Hypertrophy (LVH)

• S in V1 + R in V5 or V6 > 35 mm
• Or: R in aVL > 11 mm

• Strain pattern: ST depression + T wave inversion in lateral leads (I, aVL, V5, V6)
• Left axis deviation
• Wide QRS (though < 120 msec)

Right Ventricular Hypertrophy (RVH)

• R > S in V1 (tall R wave in V1, R ≥ 7 mm)
• Right axis deviation (> +90°)
• Strain pattern: ST depression/T inversion in V1-V3, II, III, aVF

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15. ISCHEMIA AND INFARCTION

Sequence of ECG Changes in STEMI:

Pathological Q Waves

• Width ≥ 0.04 sec (1 small box) or duration ≥ 40 msec
• Depth ≥ 25% of R wave amplitude in the same complex
• Represent dead/electrically silent myocardium (transmural infarction)
• Note: Septal Q waves in I, aVL, V5, V6 are normal (small, narrow)

Localizing the Infarct by Territory:

STEMI vs. Pericarditis ST Elevation:

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16. ELECTROLYTE DISTURBANCES ON ECG

Hyperkalemia (progressive changes with rising K+):

1. Tall, peaked, narrow ("tented") T waves (earliest sign)
2. PR prolongation
3. P wave flattening/disappearance
4. Wide QRS
5. Sine wave pattern
6. VF/asystole (at K+ > 7-8 mEq/L)

Hypokalemia:

• Flattening of T waves
• Prominent U waves (most characteristic)
• ST depression
• Prolonged QU interval (mistaken for long QT)
• Risk of TdP at severe levels

Hypercalcemia:

• Short QT interval (shortened ST segment)
• Broad T waves
• J waves (Osborn waves) at very high levels

Hypocalcemia:

• Prolonged QT interval (prolonged ST segment)
• Risk of TdP

Hypomagnesemia:

• PR prolongation
• Prolonged QT
• T wave changes
• Risk of TdP (often co-exists with hypokalemia)

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17. DRUG EFFECTS ON ECG

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18. OTHER IMPORTANT ECG PATTERNS

Wolff-Parkinson-White (WPW)

• Short PR interval (< 120 msec)
• Delta wave (slurred upstroke of QRS)
• Widened QRS
• ST/T changes
• Caused by accessory pathway (Bundle of Kent) bypassing AV node
• Risk: pre-excited AF → VF (never use AV-nodal blocking drugs in AF with WPW)

Brugada Pattern

• RBBB-like pattern + ST elevation in V1-V3 (downsloping/coved type)
• Associated with sudden cardiac death in structurally normal heart
• Accentuated by fever, sodium channel blockers, vagal tone

Pulmonary Embolism (PE)

• Most common finding: Sinus tachycardia
• Classic (but uncommon): S1Q3T3 - deep S in I, Q wave + T inversion in III
• Right heart strain: RBBB, RAD, P pulmonale
• T wave inversions in V1-V4

Pericarditis

• Stage 1: Diffuse ST elevation (concave/saddle-shaped) + PR depression in all leads except aVR/V1 (PR elevated in aVR)
• Stage 2: ST and PR return to baseline
• Stage 3: Diffuse T wave inversion
• Stage 4: ECG normalizes

Hypothermia

• Sinus bradycardia
• Osborn (J) waves - positive deflection at J point; most prominent in leads V4-V6
• QT prolongation
• Prolonged PR and QRS
• Shivering artifact

Long QT Syndrome

• Congenital (LQT1, LQT2, LQT3 - different triggers and T wave morphologies)
• Acquired (drugs, electrolytes)
• Risk: Torsades de Pointes (polymorphic VT) → syncope / sudden death

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19. QUICK CLINICAL CLUES TABLE

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20. KEY FORMULAS SUMMARY

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• Goldman-Cecil Medicine, International Edition (Chapter 42 - Electrocardiography)
• Medical Physiology (Boron & Boulpaep), Chapter 21 - Box 21-2, Table 21-5
• Miller's Anesthesia, 10th Edition - Chapter 13 (Electrical Events and the ECG)
• Roberts & Hedges' Clinical Procedures in Emergency Medicine - Electrocardiography chapter