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I now have rich textbook content and images to put together a comprehensive ECG overview. Here it is:

Electrocardiogram (ECG) - Complete Overview

What is an ECG?

An ECG (electrocardiogram) is a non-invasive recording of the electrical activity of the heart at the body surface. It provides direct measurement of the rate, rhythm, and time-dependent electrical vector of the heart, and gives fundamental information about the origin and conduction of the cardiac action potential. - Medical Physiology

The Cardiac Conduction System

Cardiac conduction system and ECG deflections

The electrical impulse travels through the heart in this sequence:

Tissue Activated	ECG Deflection
SA node discharge	None (silent)
Atrial muscle	P wave
AV node + His bundle	None (isoelectric PR segment)
Ventricular myocardium	QRS complex
Completion of ventricle	None (ST segment)
Ventricular repolarization	T wave

The SA node fires spontaneously, spreading depolarization through both atria via Bachmann's bundle and the internodal tracts
At the AV node, conduction slows deliberately - creating the PR delay between atrial and ventricular contraction
From the His bundle, the impulse travels through the left and right bundle branches into the Purkinje fibers, rapidly depolarizing the ventricles

ECG Paper & Calibration

Vertical axis: 0.1 mV per 1 mm square
Horizontal axis: 0.04 sec per small box (1 mm) | 0.2 sec per large box (5 mm)
Five large boxes = 1.0 second

The Normal Waveforms, Segments & Intervals

P Wave

Represents atrial depolarization
Normally upright in inferior leads (II, III, aVF)
Duration: 0.06-0.12 sec (1.5-3 small boxes)
Amplitude: 2-3 mm

PR Interval

From start of P wave to start of QRS
Represents time for conduction through the AV node
Normal: 0.12-0.20 sec (3-5 small boxes)
Prolonged in AV block; shortened in pre-excitation (WPW)

QRS Complex

Represents ventricular depolarization
Normally starts with a small Q (septal), then tall R (ventricular mass), then S wave
Normal duration: up to 0.10 sec (2.5 small boxes)
Wide QRS (>0.12 sec) = bundle branch block or ventricular origin

ST Segment

From end of QRS (J point) to start of T wave
Represents the plateau phase of the ventricular action potential
Normally isoelectric (flat at baseline)
Elevation = ischemia/injury/infarction or pericarditis
Depression = subendocardial ischemia or reciprocal change

T Wave

Represents ventricular repolarization
Normally upright in most leads; inverted in aVR
Peaked/tall T waves = hyperkalemia or hyperacute MI
Inverted T waves = ischemia, ventricular hypertrophy

QT Interval

From start of QRS to end of T wave
Represents the entire ventricular action potential duration
Normal: up to 0.43 sec (rate-corrected QTc)
Shortens with faster heart rates (inverse relationship)
Prolonged QT = risk of torsades de pointes

Key Intervals - Normal Values

(Ganong's Review of Medical Physiology)

Interval	Average	Normal Range	What it Measures
PR interval	0.18 sec	0.12-0.20 sec	AV conduction time
QRS duration	0.08 sec	up to 0.10 sec	Ventricular depolarization
QT interval	0.40 sec	up to 0.43 sec	Ventricular action potential
ST interval (QT - QRS)	0.32 sec	-	Plateau phase

Heart Rate Calculation

Method 1 (regular rhythm): Rate = 60 / R-R interval in seconds

Quick method: Count large boxes in the R-R interval and use the sequence:

300 - 150 - 100 - 75 - 60 - 50

Each number corresponds to 1, 2, 3, 4, 5, or 6 large boxes between R waves. For example, 4 large boxes between R waves = 75 bpm.

The 12 Leads

Bipolar limb leads (measure potential difference between two limbs):

Lead I: Left arm (+) vs Right arm (-)
Lead II: Left leg (+) vs Right arm (-)
Lead III: Left leg (+) vs Left arm (-)

Augmented unipolar limb leads:

aVR (right arm), aVL (left arm), aVF (left foot)

Precordial (chest) leads: V1-V6, placed across the chest wall

Each lead "looks" at the heart from a different angle. This allows localization of pathology to specific regions of the heart.

Systematic ECG Interpretation (Step-by-Step)

Rate - Regular or irregular? Calculate bpm
Rhythm - Sinus (P before every QRS)? Regular?
P wave - Present, upright, normal morphology?
PR interval - Normal (0.12-0.20 sec)?
QRS complex - Narrow or wide? Normal morphology?
Axis - Normal (-30° to +90°)?
ST segment - Elevation or depression?
T wave - Upright and normal?
QT interval - Prolonged?

Key ECG Abnormalities

Finding	Likely Cause
ST elevation	STEMI, pericarditis, Brugada, LBBB
ST depression	NSTEMI, subendocardial ischemia, digoxin
Wide QRS (>0.12 sec)	Bundle branch block, ventricular tachycardia, hyperkalemia
Prolonged PR (>0.20 sec)	First-degree AV block
Absent P waves + irregular rhythm	Atrial fibrillation
Regular wide-complex tachycardia	Ventricular tachycardia
Delta wave + short PR	Wolff-Parkinson-White (pre-excitation)
Peaked T waves	Hyperkalemia
Q waves (pathological)	Old myocardial infarction

ECG in Myocardial Infarction

The ECG changes in acute MI arise from three mechanisms (Ganong's Review of Medical Physiology):

Defect in Infarcted Cells	Current Flow	ECG Change
Rapid repolarization	Out of infarct	ST elevation
Decreased resting membrane potential	Into infarct	ST elevation (via TQ depression)
Delayed depolarization	Out of infarct	ST elevation

Over time, the ECG evolves: ST elevation (minutes-hours) → T wave inversion → pathological Q waves (permanent scar)

Sources: Ganong's Review of Medical Physiology (26th ed.) | Medical Physiology | Miller's Anesthesia (10th ed.) | Textbook of Family Medicine (9th ed.)

Want me to go deeper on any specific aspect - arrhythmia recognition, axis determination, bundle branch blocks, or a particular condition?

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