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I now have comprehensive content from authoritative sources (Guyton & Hall physiology, Harrison's Internal Medicine, Costanzo Physiology, Braunwald's Heart Disease, Rosen's Emergency Medicine). Here is a complete ECG guide:

Complete Guide to ECG (Electrocardiogram)

What Is an ECG?

An ECG is a graphical recording of the electrical activity generated by the heart. Metallic electrodes attached to the limbs and chest wall detect tiny voltage signals, which are amplified and printed on graph paper. It is non-invasive, inexpensive, and one of the most versatile tools in medicine - detecting arrhythmias, conduction defects, ischemia/infarction, electrolyte disturbances, and drug toxicities.

Harrison's Principles of Internal Medicine, 22E

1. The ECG Paper

Standard ECG paper runs at 25 mm/second:

Each small box (1 mm) = 0.04 seconds (40 ms)
Each large box (5 small boxes) = 0.20 seconds (200 ms)
Vertically: 1 mV = 10 mm (2 large boxes)

2. Electrical Basis - Why the Waves Exist

The heart generates electrical impulses in a specific sequence:

SA node fires spontaneously (automaticity) → depolarization spreads through right and left atria
Impulse reaches the AV node → delayed (allowing atria to finish contracting)
Impulse travels down the Bundle of His → splits into right and left bundle branches → through Purkinje fibers → ventricular muscle depolarizes
Ventricles then repolarize

Each of these steps creates a measurable wave on the ECG.

Guyton and Hall Textbook of Medical Physiology

3. The Waveforms

ECG waveform diagram showing P wave, QRS complex, T wave, and key intervals

Figure from Harrison's Principles of Internal Medicine, 22E - The classic ECG waveform showing P, QRS, T, U, J point, and all key intervals.

Figure from Costanzo Physiology 7th Edition - ECG measured from lead II showing labeled segments.

Wave/Segment	What It Represents	Normal Features
P wave	Atrial depolarization	Upright in II, inverted in aVR; duration < 120 ms
PR segment	AV node conduction delay	Isoelectric (flat)
PR interval	Start of P wave to start of QRS	120-200 ms (3-5 small boxes)
QRS complex	Ventricular depolarization	< 100-110 ms (< 2.5 small boxes)
ST segment	Ventricular plateau (phase 2 of action potential)	Isoelectric; corresponds to plateau of action potential
T wave	Ventricular repolarization	Upright in I, II, V3-V6; inverted in aVR
QT interval	Start of QRS to end of T wave; full ventricular depolarization + repolarization	Corrected QTc: < 460 ms (women), < 450 ms (men)
U wave	Possibly Purkinje repolarization	Small, same direction as T wave
J point	Junction between end of QRS and start of ST segment	At baseline

Key physiology notes:

Atrial repolarization is NOT visible on a normal ECG - it is buried within the QRS complex
The QT interval varies inversely with heart rate; QTc corrects for this (Framingham formula: QT + 0.154 × [1000 - RR])
Costanzo Physiology 7th Edition; Harrison's 22E

4. The 12 Leads

The 12-lead ECG gives 12 different "views" of the same electrical activity. Think of placing cameras around the heart at different angles.

Limb Leads (Frontal Plane)

Bipolar limb leads (Einthoven's triangle):

Lead I = Left arm (+) vs Right arm (-) → 0°
Lead II = Left leg (+) vs Right arm (-) → +60°
Lead III = Left leg (+) vs Left arm (-) → +120°

Einthoven's Law: Lead I + Lead III = Lead II (voltages sum at any instant)

Augmented unipolar limb leads:

aVR = Right arm → -150° (looks at heart from right shoulder)
aVL = Left arm → -30° (looks from left shoulder)
aVF = Left foot → +90° (looks from below)

Chest (Precordial) Leads (Horizontal Plane)

Electrodes placed on the anterior chest wall, all referenced to Wilson's central terminal:

Lead	Position	What it "sees"
V1	4th ICS, right sternal border	Right ventricle / septum
V2	4th ICS, left sternal border	Septum
V3	Between V2 and V4	Anterior wall
V4	5th ICS, midclavicular line	Anterior wall
V5	Anterior axillary line	Lateral wall
V6	Midaxillary line	Lateral wall

Normal R-wave progression: R waves grow from V1 (small) to V5 (tallest), then slightly decrease at V6. The "transition zone" where R = S is normally at V3/V4.

Guyton and Hall; Harrison's 22E

Normal ECGs recorded from the three standard bipolar limb leads (I, II, III) - Guyton and Hall Textbook of Medical Physiology

5. Systematic Approach to Reading Any ECG

Use this 8-step method every single time:

Step 1 - Rate

Count R waves:

300 ÷ number of large boxes between R waves = rate (quick method)
Or: 1500 ÷ number of small boxes between R waves
Or: Count R waves in a 10-second strip × 6
Normal: 60-100 bpm

Step 2 - Rhythm

Is it regular or irregular?
Is there a P wave before every QRS? Is every P followed by a QRS?
Normal sinus rhythm: regular rate 60-100 bpm, P wave upright in lead II, P-wave axis normal

Step 3 - P Wave

Morphology, duration, axis
Right atrial enlargement: tall peaked P waves (P pulmonale, > 2.5 mm in II)
Left atrial enlargement: broad notched P waves (P mitrale, > 120 ms) or deep biphasic P in V1

Step 4 - PR Interval

Normal: 120-200 ms
Prolonged (> 200 ms) = first-degree AV block
Variable PR with dropped beats = second-degree AV block
PR < 120 ms = pre-excitation (WPW syndrome)

Step 5 - QRS Complex

Normal: < 110 ms
Widened QRS (> 120 ms) = bundle branch block, ventricular rhythm, hyperkalemia, drugs
Right BBB pattern: rSR' in V1, wide S in I and V6
Left BBB pattern: broad, notched R in I and V6, no septal Q, QS in V1

Step 6 - Axis

Using leads I and aVF:

Lead I positive + aVF positive = Normal axis (0° to +90°)
Lead I positive + aVF negative = Left axis deviation (more negative than -30°)
Lead I negative + aVF positive = Right axis deviation (more positive than +90°)

Causes of LAD: LBBB, left anterior fascicular block, inferior MI, LVH Causes of RAD: RVH, RBBB, left posterior fascicular block, lateral MI

Step 7 - ST Segment and T Wave

ST elevation (> 1 mm in limb leads, > 2 mm in precordial leads): STEMI, pericarditis, Brugada, early repolarization, LV aneurysm
ST depression (horizontal or downsloping): subendocardial ischemia/NSTEMI, digoxin effect, LVH strain, hypokalemia, RBBB/LBBB
T wave changes: inversions (ischemia, PE, RVH), tall peaked T waves (hyperacute MI, hyperkalemia)

Step 8 - QT Interval

Calculate QTc using Bazett or Framingham formula
QTc prolonged (> 450 ms men, > 460 ms women): Torsades de Pointes risk - caused by drugs (quinolones, macrolides, antipsychotics, antiarrhythmics), hypokalemia, hypomagnesemia, hypokalemia, long QT syndrome
QTc shortened: Hypercalcemia, digoxin, short QT syndrome

6. Key Pathological Patterns

Ischemia and MI

Ischemia creates "currents of injury" - voltage gradients between normal and ischemic tissue that cause ST displacement.

Harrison's 22E, Chapter 247

Pattern	Meaning	Leads Affected
Hyperacute tall T waves	Earliest STEMI (minutes)	Over infarct zone
ST elevation	Transmural injury / STEMI	Over infarct zone
Reciprocal ST depression	Mirror of ST elevation	Opposite leads
ST depression	Subendocardial ischemia / NSTEMI	Often diffuse
T wave inversion (symmetric, narrow)	Ischemia (chronic or Wellens)	Over ischemic zone
Q waves (> 40 ms, > 1/4 R height)	Completed infarct / necrosis	Over infarct zone

Localization of MI by leads:

Territory	Culprit Artery	Leads with Changes
Anterior	LAD	V1-V4
Lateral	LCx	I, aVL, V5-V6
Inferior	RCA (or LCx)	II, III, aVF
Posterior	RCA/LCx	ST depression V1-V3 (reciprocal); tall R in V1
Right ventricle	RCA proximal	V1; right-sided leads V4R

Rosen's Emergency Medicine; Harrison's 22E

Arrhythmias (Core Patterns)

Arrhythmia	Key ECG Finding
Sinus tachycardia	Rate > 100, normal P waves, normal QRS
Sinus bradycardia	Rate < 60, normal morphology
Atrial fibrillation	No discrete P waves, irregularly irregular R-R, fibrillatory baseline
Atrial flutter	"Sawtooth" flutter waves at ~300/min, regular or variable block (2:1, 3:1, 4:1)
SVT / AVRT	Narrow complex tachycardia ~150-220/min; P waves often buried in QRS or ST segment
Ventricular tachycardia (VT)	Wide complex tachycardia, AV dissociation, fusion beats, capture beats; rate > 100
Ventricular fibrillation	Chaotic, no organized complexes
1st degree AV block	PR > 200 ms, all P waves conduct
2nd degree AV block (Mobitz I / Wenckebach)	Progressive PR lengthening until dropped QRS
2nd degree AV block (Mobitz II)	Fixed PR, suddenly dropped QRS - more dangerous
3rd degree (complete) AV block	P and QRS completely dissociated, escape rhythm
WPW	Short PR (< 120 ms), delta wave (slurred QRS upstroke), wide QRS

Hypertrophy

Left Ventricular Hypertrophy (LVH):

Sokolow-Lyon: S in V1 + R in V5 or V6 > 35 mm
Cornell: R in aVL > 11 mm, or R in aVL + S in V3 > 28 mm (men), > 20 mm (women)
May also show ST depression and T wave inversion in lateral leads ("strain pattern")

Right Ventricular Hypertrophy (RVH):

Tall R in V1 (R > S in V1), right axis deviation, T wave inversion in right precordial leads

Electrolyte Effects

Electrolyte	ECG Changes
Hyperkalemia	Peaked narrow T waves → PR prolongation → wide QRS → sine wave → VF/asystole
Hypokalemia	Flattened T waves, prominent U waves, ST depression, QT prolongation
Hypercalcemia	Short QT interval
Hypocalcemia	Long QT interval (prolonged ST segment)

Drug Effects

Drug	ECG Change
Digoxin	"Scooped" ST depression (reverse tick), T wave changes, short QT
Class IA antiarrhythmics (quinidine)	Prolonged QT, wide QRS
Class IC (flecainide)	Wide QRS, PR prolongation
Beta blockers / CCBs	Bradycardia, AV block
Tricyclic antidepressants	Wide QRS, right axis deviation, prolonged QT

7. Memory Aids

"Rate, Rhythm, P, PR, QRS, Axis, ST/T, QT" - 8 steps, never skip any.

Lead groupings to remember:

Inferior = II, III, aVF
Lateral = I, aVL, V5, V6
Anterior/septal = V1-V4
aVR = "looks in" from right shoulder - usually negative (inverted P, QRS, T)

Normal intervals (in order from short to long):

QRS < 110 ms
PR 120-200 ms
QTc < 450-460 ms

Bundle branch block rule: Wide QRS + look at V1:

rSR' in V1 (rabbit ears) = RBBB
QS or wide R in V1 = LBBB

8. Clinical Significance of Each Lead

Each lead "looks" at a specific wall of the heart. When you see ST changes, look for reciprocal changes in anatomically opposite leads - this dramatically increases the specificity for MI. For example:

Inferior STEMI (II, III, aVF elevation) → reciprocal ST depression in aVL (150° away)
Anterior STEMI (V1-V4) → may show reciprocal depression in inferior leads

Summary Table - Quick Reference

Finding	Think of
Short PR	WPW, LGL, AV junctional rhythm
Long PR	1st degree AV block, drugs, hypervagal
Wide QRS	BBB, VT, WPW, hyperkalemia, toxins
Long QT	Drugs, electrolytes, congenital LQTS
ST elevation	STEMI, pericarditis, Brugada, early repol
ST depression	Ischemia, NSTEMI, LVH, drugs, posterior MI
Peaked T	Hyperacute MI, hyperkalemia
Inverted T	Ischemia, PE (right heart strain), CNS events
U wave	Hypokalemia, bradycardia
Irregular rhythm	AF, MAT, frequent ectopics
No P waves + irregular	Atrial fibrillation
Sawtooth waves	Atrial flutter
AV dissociation	VT, complete heart block

Sources: Guyton and Hall Textbook of Medical Physiology | Costanzo Physiology 7th Edition | Harrison's Principles of Internal Medicine 22E | Rosen's Emergency Medicine | Braunwald's Heart Disease

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