Provode notes on ecg interpretation with examples of diseases

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I now have rich, comprehensive content from multiple authoritative textbooks. Here are detailed ECG interpretation notes:

ECG Interpretation - Comprehensive Notes

Sources: Harrison's Principles of Internal Medicine 22E (2025), Tintinalli's Emergency Medicine, Fuster and Hurst's The Heart 15th Ed., Pfenninger & Fowler's Procedures for Primary Care, Costanzo Physiology 7th Ed.

1. The Systematic Approach (14-Parameter Checklist)

Harrison's mandates this systematic review for every ECG to avoid errors of omission:

Standardization / calibration - check 1 mV = 10 mm, paper speed 25 mm/s, lead placement artifacts
Rhythm - sinus vs. non-sinus
Heart rate - normal 60-100 bpm; Rate = 1/R-R interval (in seconds)
PR interval / AV conduction - normal 120-200 ms (3-5 small squares)
QRS interval - normal <120 ms
QT/QTc interval - rate-corrected; prolonged if >440 ms (men) / >460 ms (women)
Mean QRS electrical axis - normal -30° to +90°
P waves - morphology, axis, duration
QRS voltages - high or low
Precordial R-wave progression - R grows from V1 to V5
Abnormal Q waves - >40 ms wide or >25% of R wave height
ST segments - elevation or depression relative to baseline
T waves - inversion, peaking, flatness
U waves - seen in V2-V3; prominent in hypokalemia

Always compare with previous ECGs. A single normal ECG does not exclude ischemia.

2. ECG Waveforms - What Each Represents

Wave/Interval	Electrical Event	Normal Duration
P wave	Atrial depolarization	<120 ms, <2.5 mm tall
PR interval	AV nodal conduction time	120-200 ms
QRS complex	Ventricular depolarization	<120 ms
ST segment	Isoelectric (ventricular plateau)	At baseline
T wave	Ventricular repolarization	Upright in I, II, V3-V6
QT interval	Total ventricular activity (depolarization + repolarization)	<440 ms (rate-adjusted)
U wave	Possibly His-Purkinje repolarization	Low amplitude, follows T

Key concept: The QRS complex is similar in duration to the P wave despite the larger ventricular mass, because conduction velocity in the His-Purkinje system is much faster than in the atrial conducting system.

3. Atrial Abnormalities (P-Wave Changes)

P wave morphology: right atrial overload (tall peaked P in lead II and V1), left atrial abnormality (broad notched P in limb leads, biphasic P in V1)

Right atrial (RA) overload = tall, peaked P waves ("P pulmonale"). Left atrial (LA) abnormality = broad, notched P waves in limb leads ("P mitrale"); biphasic P in V1 with prominent negative component.

Right Atrial Overload ("P pulmonale")

Tall, peaked P waves >2.5 mm in leads II, III, aVF
Seen in: pulmonary hypertension, cor pulmonale, tricuspid stenosis, COPD

Left Atrial Abnormality ("P mitrale")

Broad (>120 ms), notched P waves in lead II
Deep, wide negative terminal deflection in V1
Seen in: mitral stenosis, LV failure, hypertension

4. Ventricular Hypertrophy

QRS changes in LVH and RVH across precordial leads, with corresponding heart vector diagrams

Left Ventricular Hypertrophy (LVH)

Voltage criteria: SV1 + RV5 (or RV6) >35 mm (Sokolow-Lyon)
Repolarization abnormality: ST depression + T-wave inversion in lateral leads (I, aVL, V5-V6) - the "strain pattern"
QRS axis may shift leftward
Causes: hypertension, aortic stenosis, hypertrophic cardiomyopathy
Note: Prominent voltages can be a normal variant in young/athletic individuals

Right Ventricular Hypertrophy (RVH)

Tall R wave in V1 (R > S in V1), deep S in V5-V6
Right axis deviation (>+110°)
T-wave inversions in V1-V4 (right "strain" pattern)
Causes: pulmonary hypertension, pulmonary embolism, COPD, mitral stenosis, congenital heart disease

5. Myocardial Ischemia and Infarction

Diagram showing ST vector direction in subendocardial vs. transmural ischemia; A=ST depression in subendocardial, B=ST elevation in transmural

A: Subendocardial ischemia - ST vector points inward, causing ST depression in overlying leads. B: Transmural/epicardial ischemia - ST vector points outward, causing ST elevation.

ST-Elevation MI (STEMI)

Severe acute ischemia causes "currents of injury." The ECG territory localizes the culprit artery:

Territory	Leads with ST Elevation	Culprit Artery
Anterior	V1-V6, I, aVL	Left anterior descending (LAD)
Inferior	II, III, aVF	Right coronary artery (RCA)
Lateral	I, aVL, V5-V6	Left circumflex (LCx)
Posterior	ST depression V1-V3 (reciprocal)	RCA or LCx
Right ventricle	Right-sided leads (V3R-V4R)	Proximal RCA

Evolution of Q-wave infarction:

Hyperacute T waves (early, minutes)
ST elevation (hours)
Q wave formation (hours to days)
T-wave inversion (days)
Q waves may persist or normalize (weeks-months)

Non-STEMI / Subendocardial Ischemia

ST depression in the ischemic zone
ST elevation in aVR (reciprocal to diffuse subendocardial ischemia)
Deep T-wave inversions in V1-V4 with high-grade LAD stenosis = Wellens syndrome (a "warning sign" ECG)

Wellens T-wave Sign

Severe anterior ischemia showing deep T-wave inversions V1-V4 (Wellens pattern), indicating critical LAD stenosis

Deep precordial T-wave inversions V1-V4 = Wellens sign, indicating critical LAD stenosis requiring urgent intervention.

6. Conduction Abnormalities

AV Block

Type	PR Interval	QRS dropped?	Mechanism	ECG Finding
1st degree	>200 ms	No	Slow AV nodal conduction	Constant prolonged PR
2nd degree Mobitz I (Wenckebach)	Progressively lengthens	Yes (periodically)	Incremental AV delay	PR lengthens until P not conducted; then resets
2nd degree Mobitz II	Constant (normal or prolonged)	Yes (suddenly)	Infra-Hisian block	Constant PR, sudden dropped QRS; wide QRS common
3rd degree (complete)	No relationship	All blocked	Complete AV dissociation	P and QRS independent; escape rhythm

Clinical note: Mobitz II is more dangerous than Wenckebach and often requires pacemaker implantation. Complete AV block with a wide QRS escape rhythm indicates infra-Hisian disease and is a pacemaker indication.

Bundle Branch Blocks

Right Bundle Branch Block (RBBB):

QRS >120 ms
RSR' ("rabbit ears") in V1
Wide S in I, V6
Causes: normal variant, pulmonary embolism, RV pressure overload, atrial septal defect, ischemia

Left Bundle Branch Block (LBBB):

QRS >120 ms
Broad, notched R in I, aVL, V5-V6
Deep S (or QS) in V1-V3
No septal Q waves
Causes: ischemia, cardiomyopathy, severe hypertension, fibrosis of conduction system
LBBB + new chest pain = STEMI until proven otherwise (Sgarbossa criteria apply)
LBBB masks ischemic changes and makes standard interpretation unreliable

Fascicular Blocks:

Left anterior fascicular block: QRS axis more negative than -45° (marked left axis deviation)
Left posterior fascicular block: right axis deviation >+110° after excluding RVH (very rare in isolation)

7. Arrhythmias

Atrial Fibrillation (AF)

Atrial flutter ECG: A=regular narrow complex tachycardia 155 bpm, B=flutter waves in II, III, aVF, C=carotid massage unmasking flutter waves

Atrial flutter with flutter waves best seen in II, III, aVF (C shows flutter waves unmasked by carotid sinus massage).

ECG features of AF:

Absent P waves; flat or chaotic (fibrillatory) baseline
Irregularly irregular ventricular rhythm
Narrow QRS (unless bundle branch block or pre-excitation/WPW present)

Causes: ischemic heart disease, valvular disease, cardiomyopathy, alcohol ("holiday heart"), thyrotoxicosis, hypertension

Clinical consequences:

Loss of atrial kick → heart failure in susceptible patients
Rapid ventricular response → reduced filling time
Thromboembolic risk (cardioembolic stroke) - risk increases dramatically if duration >48 hours before cardioversion

Atrial Flutter

Regular saw-tooth flutter waves at ~300 bpm (negative in II, III, aVF)
Usually 2:1 or 4:1 AV block → ventricular rate ~150 or ~75 bpm
Carotid sinus massage slows ventricular rate, unmasking flutter waves

Paroxysmal Supraventricular Tachycardia (PSVT)

Absent normal sinus P waves
Retrograde P wave (inverted, just before or after QRS)
Narrow QRS, regular rate (150-250 bpm)

Ventricular Tachycardia (VT)

Wide QRS (>120 ms), rate >100 bpm
AV dissociation (P waves independent of QRS)
Fusion beats and capture beats (pathognomonic)
May degenerate to ventricular fibrillation (VF)

8. Cor Pulmonale / Pulmonary Embolism

Acute PE ECG findings (often subtle or just sinus tachycardia):

S1Q3T3 pattern: S wave in I, Q wave in III, T-wave inversion in III
Right axis deviation
Right bundle branch block (new)
Sinus tachycardia (most common arrhythmia)
ST-T changes in V1-V4 (can mimic anterior infarction)

Chronic cor pulmonale (COPD):

Low-voltage complexes (due to hyperinflation)
Slow R-wave progression (diminished r waves V1-V3)
P pulmonale in inferior leads

9. Other Disease-Specific ECG Patterns

Disease	Key ECG Finding
Hyperkalemia	Peaked T waves (early) → wide QRS → sine wave → asystole
Hypokalemia	Flattened T waves, prominent U waves, QT prolongation
Hypercalcemia	Short QT interval
Hypocalcemia	Prolonged QT interval
Digoxin toxicity	Scooped ST depression ("Salvador Dali" pattern), PR prolongation, bradyarrhythmias
Wolff-Parkinson-White (WPW)	Short PR (<120 ms), delta wave, wide QRS (preexcitation); risk of AF with rapid ventricular response
Brugada syndrome	Coved ST elevation in V1-V2, RBBB-like pattern; risk of sudden cardiac death
Long QT syndrome	QTc >440-460 ms; risk of torsades de pointes
Pericarditis	Diffuse concave ST elevation, PR depression in most leads; no reciprocal changes (except aVR)
Hypothermia	Osborn (J) waves; bradycardia
Tricyclic antidepressant toxicity	Widened QRS, right axis deviation, prolonged QT

10. Key Limitations of ECG

A single normal ECG does not exclude ischemia - serial ECGs are essential
LBBB, electronic pacemakers, and WPW can mask ischemic changes entirely
LVH with strain can mimic STEMI (pseudo-infarction pattern)
Computerized ECG analyses should not be accepted without careful clinician review
ECG changes in acute PE and anterior STEMI can mimic each other

References:

Harrison's Principles of Internal Medicine 22E (2025), Chapter 247
Tintinalli's Emergency Medicine (Comprehensive Study), Chapters 18, 49
Fuster and Hurst's The Heart, 15th Edition, Chapters 38
Pfenninger & Fowler's Procedures for Primary Care, 3rd Edition, Chapter 5
Costanzo Physiology 7th Edition, Chapter 4

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