Explain me ECG in simple

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normal ECG waveform labeled P wave QRS T wave

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ECG paper speed segments intervals PR QRS QT explained diagram

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Here's a simple breakdown of ECG (Electrocardiogram).

What is an ECG?

An ECG is a recording of the electrical activity of the heart picked up by electrodes placed on the skin. Every time the heart beats, a tiny electrical signal travels through it — the ECG captures that signal as a wave on paper.

Think of it like this: the heart is a pump, but it needs electrical signals to know when to squeeze. The ECG is a recording of those signals.

The ECG Waveform — What Each Part Means

ECG schematic showing P wave, QRS complex, ST segment, T wave, and key intervals

Harrison's Principles of Internal Medicine — Schematic of the cardiac conduction system

Wave / Segment	What's happening in the heart
P wave	The upper chambers (atria) are told to squeeze. Electrical signal fires from the SA node (the heart's natural pacemaker).
PR interval	The signal pauses at the AV node — a built-in delay so the atria finish squeezing before the ventricles start.
QRS complex	The lower chambers (ventricles) squeeze hard to push blood out to the body and lungs. This is the biggest spike on the ECG.
ST segment	The ventricles are contracting — electrically "flat" in a healthy heart. Any elevation or depression here is a red flag (think: heart attack).
T wave	The ventricles electrically "reset" (repolarize) to prepare for the next beat.
U wave	A small wave sometimes seen after T — not always present; its exact cause is debated.

The Electrical Journey — Step by Step

SA node (fires) → Atria contract (P wave)
        ↓
AV node (delays signal) → PR interval
        ↓
Bundle of His → Left & Right bundle branches → Purkinje fibers
        ↓
Ventricles contract (QRS complex)
        ↓
Ventricles reset (T wave)
        ↓
Ready for the next beat...

How ECG Relates to the Heartbeat (Mechanical Events)

Cardiac cycle diagram showing ECG alongside ventricular pressure, aortic flow, and ventricular volume

Miller's Anesthesia — Electrical and mechanical events during a single cardiac cycle

The diagram above perfectly shows how the electrical events (ECG at bottom) trigger the mechanical events — pressure changes, valve opening/closing, and blood flow.

P wave → atrial kick (final ~25% of ventricular filling)
QRS → mitral/tricuspid valves close → ventricles start squeezing (systole begins)
T wave → aortic/pulmonic valves close → ventricles relax (diastole begins)

Key Intervals to Know

Interval	Normal Value	What it checks
PR interval	120–200 ms	AV node conduction (prolonged = heart block)
QRS duration	< 120 ms	Ventricular conduction (wide = bundle branch block)
QT interval	< 440 ms (men), < 460 ms (women)	Ventricular repolarization (prolonged = risk of dangerous arrhythmia)

What Can ECG Diagnose?

Heart attack (MI) — ST elevation or depression, Q waves
Arrhythmias — irregular R-R intervals, absent P waves (atrial fibrillation), etc.
Heart block — prolonged PR, dropped beats
Bundle branch blocks — wide QRS
Electrolyte abnormalities — e.g., high potassium widens QRS; low potassium prolongs QT

The ECG Paper

Each small square = 0.04 sec (horizontal) and 0.1 mV (vertical)
Each large square = 0.2 sec
Standard speed: 25 mm/second
Heart rate = 300 ÷ number of large squares between two R waves

Bottom line: The ECG is just a picture of the heart's electrical activity. Once you know which wave represents which event — P (atria), QRS (ventricles contracting), T (ventricles resetting) — everything else builds on that foundation.

Sources: Harrison's Principles of Internal Medicine, 21e; Miller's Anesthesia, 10e

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