Electrical Conduction System and ECG

What is the function of the cardiac conduction system?

To generate and spread electrical impulses so the heart contracts in a coordinated way.

Conduction pathway in order

SA node → AV node → AV bundle/Bundle of His → right and left bundle branches → Purkinje fibers.

What is the pacemaker of the heart?

SA node.

Where is the SA node located?

In the right atrial wall near the superior vena cava.

What does the SA node do?

Starts the electrical impulse that sets heart rate.

What happens after the SA node fires?

The impulse spreads through the atria, causing atrial depolarization.

What does atrial depolarization lead to?

Atrial contraction.

Where is the AV node located?

In the inferior interatrial septum near the tricuspid valve.

What does the AV node do?

Delays the impulse before it enters the ventricles.

Why is the AV node delay important?

It gives the atria time to finish filling the ventricles before ventricular contraction.

What is the AV bundle also called?

Bundle of His.

What does the AV bundle do?

Carries the impulse from the AV node into the interventricular septum.

Where do bundle branches travel?

Down the interventricular septum toward the apex.

What do Purkinje fibers do?

Spread the impulse through the ventricular myocardium.

Why do ventricles contract from apex upward?

Purkinje fibers spread the impulse to the apex first, pushing blood upward toward the arteries.

What does an ECG measure?

Electrical activity of the heart at the body surface.

What does an ECG not directly measure?

Mechanical contraction, blood pressure, valve movement, or blood flow.

Why can an ECG be used to infer contraction?

Depolarization normally triggers contraction, so electrical patterns suggest mechanical activity.

What does the P wave represent?

Atrial depolarization.

What happens mechanically after the P wave?

Atrial contraction.

What does the QRS complex represent?

Ventricular depolarization.

What happens mechanically after the QRS complex?

Ventricular contraction.

Where is atrial repolarization on the ECG?

Hidden by the QRS complex.

What does the T wave represent?

Ventricular repolarization.

What happens mechanically around/after the T wave?

Ventricles relax.

What does the PR interval represent?

Time from atrial depolarization to ventricular depolarization, including AV node delay.

What does the ST segment represent?

Time when ventricles are depolarized, corresponding to the plateau phase.

What does the QT interval represent?

Time from ventricular depolarization through ventricular repolarization.

Draw-card: Normal ECG

Draw and label P wave, QRS complex, T wave, PR interval, ST segment, and QT interval.

P wave = which chambers depolarize?

Atria.

QRS complex = which chambers depolarize?

Ventricles.

T wave = which chambers repolarize?

Ventricles.

If the SA node fails, what may happen?

Other autorhythmic cells may take over at a slower rate.

What does abnormal ECG rhythm suggest?

Possible abnormal conduction, depolarization, repolarization, or heart rhythm.