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Electrical Conduction
System & ECG Basics

Explore how the heart generates and conducts electrical impulses, and learn to read the fundamental waves of an electrocardiogram (ECG).

Anatomy & Physiology

What Is the Conduction System?

The heart has its own built-in electrical wiring. Specialized cardiac muscle cells generate and propagate electrical impulses that coordinate each heartbeat: a precise sequence of atrial contraction followed by ventricular contraction.

Key Structures of the Conduction Pathway

SA Node
The "pacemaker" in the right atrium. Fires at \( 60-100~\text{bpm} \).
AV Node
Located at the interatrial septum. Delays the signal \( \sim 0.1~\text{s} \).
Bundle of His
Carries the impulse from atria to ventricles via the interventricular septum.
Bundle Branches
Carry the impulse down the left and right sides of the septum.
Purkinje Fibers
Spread the signal rapidly through ventricular walls, causing contraction.

The Conduction Pathway

SA
SA Node
AV
AV Node
Bundle of His
Right BB
Left BB
Purkinje Fibers
Right Atrium
Left Atrium
Right Ventricle
Left Ventricle

What Is an ECG?

An electrocardiogram (ECG or EKG) is a recording of the heart's electrical activity over time. Electrodes on the skin detect tiny voltage changes as the cardiac impulse spreads. The result is a characteristic waveform with labeled segments.

The Normal ECG Waveform

P
QRS
T
P
QRS
T
Baseline
P Wave
Atrial depolarization (atria contract)
QRS Complex
Ventricular depolarization (ventricles contract)
T Wave
Ventricular repolarization (ventricles recover)

Depolarization vs Repolarization

Depolarization Repolarization
Definition Electrical charge reversal that triggers contraction Restoration of resting charge after contraction
Ion Movement \( \text{Na}^+ \) rushes in (cell becomes positive) \( \text{K}^+ \) flows out (cell returns to negative)
ECG Deflection P wave, QRS complex T wave
Mechanical Result Muscle contraction Muscle relaxation / reset

ECG Intervals and Segments

PR Interval
Start of P wave to start of QRS. Represents atrial depolarization + AV node delay. Normal: \( 0.12 - 0.20~\text{s} \)
QRS Duration
Width of the QRS complex. Represents ventricular depolarization speed. Normal: \( < 0.12~\text{s} \)
QT Interval
Start of QRS to end of T wave. Total ventricular activity. Normal: \( 0.36 - 0.44~\text{s} \)

Calculating Heart Rate from an ECG

ECG paper moves at 25 mm/s, so each small box (1 mm) equals 0.04 s and each large box (5 mm) equals 0.20 s. Two common methods:

1
300 Method: Count the number of large boxes between two consecutive R waves. Heart rate = \( \frac{300}{\text{number of large boxes}} \)
2
1500 Method: Count the number of small boxes between two R waves. Heart rate = \( \frac{1500}{\text{number of small boxes}} \)

The SA Node: The Heart's Natural Pacemaker

Each part of the conduction system can generate impulses on its own, but at different intrinsic rates. The SA node fires the fastest, so it dominates.

1
SA Node: \( 60 - 100~\text{bpm} \) (normal sinus rhythm)
2
AV Node: \( 40 - 60~\text{bpm} \) (junctional escape rhythm)
3
Purkinje Fibers: \( 20 - 40~\text{bpm} \) (ventricular escape rhythm)

Why the AV Delay Matters

The AV node introduces a critical \( \sim 0.1~\text{s} \) delay before passing the impulse to the ventricles. This brief pause ensures the atria finish contracting and completely empty their blood into the ventricles before the ventricles contract.

SA fires
0.0 s
Atria contract
AV Delay
\( \sim 0.1 \) s
Ventricles contract
Reset

Example 1: Reading a Normal ECG

P
R
Q
S
T
PR Interval
QT Interval

Example 2: Calculating Heart Rate

R1
R2
1
2
3
4
4 large boxes

Example 3: Recognizing Abnormal Patterns

Prolonged PR
\( \text{PR} > 0.20~\text{s} \) suggests a first-degree AV block: the AV node delays the signal too long.
Wide QRS
\( \text{QRS} > 0.12~\text{s} \) suggests a bundle branch block: the impulse takes a detour through ventricular muscle instead of the fast conduction fibers.
ST Elevation
The ST segment rises above baseline, suggesting acute myocardial infarction (heart attack): injured tissue changes repolarization.

How to Read an ECG Strip

1
Check the rhythm: Are the R-R intervals regular or irregular?
2
Calculate the rate: Use the 300 method: \( \text{HR} = \frac{300}{\text{large boxes between R-R}} \)
3
Identify P waves: Is there a P wave before every QRS? Are they upright and uniform?
4
Measure the PR interval: Is it between 0.12 and 0.20 s (3-5 small boxes)?
5
Measure the QRS duration: Is it narrow (\( < 0.12~\text{s} \))? Check for bundle branch blocks.
6
Examine the ST segment and T wave: Is the ST segment at baseline? Is the T wave upright?

Common Mistakes

✕ Confusing P with T
The P wave comes before the QRS complex, and the T wave comes after. If you lose your place, find the tall R peak first, then look left for P and right for T.
✕ Forgetting the AV Delay
Students often say the AV node "conducts quickly." In fact, it deliberately slows the signal to allow atrial emptying. The PR interval reflects this delay.
✕ ECG = Heart Pumping
The ECG records electrical activity only. A flat line (asystole) means no electrical signal, but a normal-looking ECG does not guarantee normal pumping (pulseless electrical activity).

Summary: ECG Components at a Glance

ECG Feature Electrical Event Mechanical Result Normal Value
P Wave Atrial depolarization Atrial contraction \( < 0.12~\text{s} \), \( < 2.5~\text{mm} \)
PR Interval AV node conduction delay Atrial emptying \( 0.12 - 0.20~\text{s} \)
QRS Complex Ventricular depolarization Ventricular contraction \( < 0.12~\text{s} \)
T Wave Ventricular repolarization Ventricular relaxation Upright in most leads
QT Interval Total ventricular activity Systole + early diastole \( 0.36 - 0.44~\text{s} \)

Question 1 of 5

Which structure is known as the heart's natural pacemaker because it initiates each heartbeat?

Question 2 of 5

The QRS complex on an ECG represents which electrical event?

Question 3 of 5

If there are 5 large boxes between two consecutive R waves on an ECG strip, what is the approximate heart rate?

Question 4 of 5

What is the primary purpose of the delay at the AV node?

Question 5 of 5

A patient's ECG shows a PR interval of 0.28 seconds. Which condition does this most likely indicate?