Electrical System of the Heart and ECGs

ECG

Electrocardiogram; a recording of the heart's electrical activity.

Node

A specialized area of cardiac muscle that generates electrical impulses.

SA node

The heart's natural pacemaker; located in the right atrium.

SA node function

Starts the electrical impulse that begins each normal heartbeat.

SA node normal rate

About 60–100 beats per minute.

AV node

Receives the impulse from the SA node and briefly delays it before sending it to the ventricles.

AV node location

Right atrium.

AV node function

Delays the electrical impulse so the atria can finish contracting before the ventricles contract.

AV bundle

Also called the Bundle of His; carries the impulse from the AV node toward the ventricles.

Bundle branches

Carry electrical impulses toward the right and left ventricles.

Purkinje fibers

Spread electrical impulses through the ventricles and cause them to contract.

Cardiac conduction pathway

SA node → AV node → Bundle of His → bundle branches → Purkinje fibers.

P wave

Atrial depolarization.

Atrial depolarization

Leads to atrial contraction.

QRS complex

Ventricular depolarization.

Ventricular depolarization

Leads to ventricular contraction.

T wave

Ventricular repolarization.

Ventricular repolarization

Occurs as the ventricles recover/reset electrically.

Atrial repolarization

Occurs during the QRS complex but is hidden by the stronger ventricular signal.

PR interval

From the beginning of the P wave to the beginning of the QRS complex.

QT interval

Represents the electrical activity associated with ventricular contraction and recovery.

Prolonged PR interval

May indicate a problem with electrical conduction.

Prolonged QT interval

May indicate heart damage or an electrolyte problem.

SA node damage

If the SA node fails, the AV node can act as a backup pacemaker.

AV node backup rate

About 40–60 beats per minute.