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Here is a thorough explanation of the ECG, drawing from Guyton & Hall Textbook of Medical Physiology, Miller's Anesthesia, Ganong's Review of Medical Physiology, and Harrison's Principles of Internal Medicine.

Electrocardiogram (ECG)

What Is an ECG?

An ECG records the electrical activity of the heart at the body surface. The heart generates electrical potentials with each beat, and these potentials spread through body fluids to the skin, where electrodes pick them up. The result is a tracing of voltage over time that reflects the sequence of cardiac depolarization and repolarization.

An upward deflection is recorded when the active electrode becomes positive relative to the reference; a downward deflection when it becomes negative.

The Cardiac Conduction System (What Generates the ECG)

The electrical impulse follows a specific path:

SA node (sinoatrial node) - the natural pacemaker, in the right atrium, fires spontaneously
The impulse spreads across both atria - produces the P wave
It reaches the AV node (atrioventricular node) - conduction slows here deliberately, creating a pause that allows atrial contraction to fill the ventricles before they contract
The impulse travels down the Bundle of His, splits into left and right bundle branches
It reaches the Purkinje fibers, spreading to all ventricular muscle cells - produces the QRS complex
Ventricular muscle then repolarizes - produces the T wave

The Normal ECG Waveform

Here is the classic ECG trace from Guyton & Hall:

Normal ECG waveform showing P, Q, R, S, T waves with labeled intervals

Normal ECG - Guyton and Hall Textbook of Medical Physiology

The Waves

Wave	What it represents	Normal appearance
P wave	Atrial depolarization (atria contract)	Small, rounded, upright; 0.1-0.3 mV; duration ~0.08-0.10 s
Q wave	Initial ventricular septal depolarization (small downward deflection)	Small or absent normally; pathological if wide/deep
R wave	Main ventricular depolarization sweeping outward	Tall upward spike; the dominant feature
S wave	Terminal ventricular depolarization at the base	Small downward deflection after R
T wave	Ventricular repolarization	Broader, lower than QRS; same direction as QRS normally
U wave	Possibly late Purkinje repolarization	Small, inconstant; follows T wave

Note: The atrial T wave (atrial repolarization) is not visible - it is buried within the QRS complex.

The Intervals and Segments

Interval	Measured from... to...	Normal duration	What it represents
PR interval	Start of P wave to start of QRS	0.12-0.20 s	AV node conduction delay
QRS duration	Start to end of QRS	< 0.10 s (up to 0.12)	Ventricular depolarization time
ST segment	End of QRS (J point) to start of T wave	Isoelectric (flat)	Ventricular plateau phase - no net current flow
QT interval	Start of QRS to end of T wave	< 0.43 s	Full ventricular action potential duration
RR interval	R wave to next R wave	Depends on heart rate	Duration of one cardiac cycle

(Ganong's Review of Medical Physiology, Table 29-2)

ECG and the Cardiac Cycle

This diagram from Miller's Anesthesia shows how the ECG correlates with mechanical events of the heart:

Cardiac cycle diagram showing ECG in relation to pressures, volumes, and heart sounds

ECG correlated with cardiac mechanical events - Miller's Anesthesia

Key relationships:

P wave coincides with atrial contraction (atrial systole)
QRS coincides with the start of ventricular contraction; the mitral valve closes near the R wave peak
Ventricular systole spans from QRS to end of T wave
T wave ends as the ventricles relax (diastole begins)

The 12-Lead ECG

A standard ECG uses 10 electrodes placed on the body, generating 12 views (leads) of the heart from different angles. Each lead "looks" at the heart from a different direction.

Limb Leads (frontal plane)

These are based on Einthoven's triangle - the two arms and left leg form a triangle around the heart:

Lead I - Right arm (-) to Left arm (+): looks leftward
Lead II - Right arm (-) to Left leg (+): looks inferiorly
Lead III - Left arm (-) to Left leg (+): looks inferiorly-left
aVR - augmented view from right arm
aVL - augmented view from left arm
aVF - augmented view from left foot

Precordial (Chest) Leads (horizontal plane)

Six electrodes across the chest wall - V1 through V6 - look at the heart horizontally.

V1, V2: right side of heart / interventricular septum
V3, V4: anterior wall
V5, V6: lateral wall

How to Read an ECG Systematically

Harrison's Principles of Internal Medicine (2025) recommends analyzing 14 parameters in every ECG:

Standardization/calibration - 1 mV = 10 mm, paper speed = 25 mm/s
Rhythm - regular or irregular?
Heart rate - calculate from RR interval (300 / number of large squares between R waves)
PR interval - AV conduction normal/prolonged/short?
QRS interval - narrow (<120 ms) or wide (>120 ms)?
QT/QTc interval - prolonged QT = arrhythmia risk
Mean QRS electrical axis - normal, left deviation, or right deviation?
P waves - present? one before every QRS?
QRS voltages - high (LVH) or low (effusion, obesity, emphysema)?
R-wave progression - does R wave grow from V1 to V5-V6?
Abnormal Q waves - sign of previous MI
ST segments - elevated (STEMI, pericarditis) or depressed (ischemia)?
T waves - inverted (ischemia, strain) or peaked (hyperkalemia)?
U waves - prominent U wave seen in hypokalemia

Common Clinical Findings

Finding	What it suggests
Prolonged PR > 0.20 s	First-degree AV block
Wide QRS > 0.12 s	Bundle branch block
Peaked tall P waves	Right atrial enlargement
ST elevation	STEMI (heart attack), pericarditis
ST depression	Myocardial ischemia, digoxin effect
Prolonged QTc	Risk of torsades de pointes (dangerous arrhythmia)
No P waves + irregular rhythm	Atrial fibrillation
Absent P + regular wide QRS	Ventricular rhythm
Peaked T waves	Hyperkalemia
Flat/inverted T waves	Ischemia, hypokalemia
Pathological Q waves	Old myocardial infarction

Key Numbers to Remember

Parameter	Normal value
Heart rate	60-100 bpm
PR interval	0.12-0.20 s
QRS duration	< 0.10 s (up to 0.12 s)
QTc	Men < 0.44 s, Women < 0.46 s
P wave amplitude	0.1-0.3 mV
ECG paper speed	25 mm/s (1 small square = 0.04 s)
ECG paper amplitude	1 mV = 10 mm (2 large squares)

Sources: Guyton and Hall Textbook of Medical Physiology; Ganong's Review of Medical Physiology, 26th ed.; Miller's Anesthesia, 10th ed.; Harrison's Principles of Internal Medicine, 22nd ed. (2025)

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