Question 1

What does an ECG record?

Accepted Answer

An ECG records the electrical changes associated with the heart beat. It does not directly show the mechanical movement of each chamber. The P wave, QRS complex and T wave reflect different phases of cardiac electrical activity.

Question 2

Why is the SA node called the pacemaker?

Accepted Answer

The SA node, or sinuatrial node, is located in the upper part of the right atrial wall near the opening of the superior vena cava. It normally initiates each impulse and sets the heart rate at about 70–75 beats per minute.

Question 3

What is the function of the AV node?

Accepted Answer

The AV node lies in the lower part of the interatrial septum and delays the impulse for about 0.1 second. This allows atrial contraction to be completed before the impulse reaches the ventricles through the bundle of His.

Question 4

How do the bundle of His and Purkinje fibres conduct the impulse?

Accepted Answer

The bundle of His continues from the AV node and divides into right and left bundle branches in the interventricular septum. Purkinje fibres then spread the impulse through the ventricular walls, leading to ventricular depolarisation and contraction.

Question 5

What do the P, QRS and T waves mean?

Accepted Answer

The P wave represents atrial depolarisation. The QRS complex represents ventricular depolarisation, with Q, R and S as components of one complex. The T wave represents ventricular repolarisation, while atrial repolarisation is normally hidden within the QRS complex.

ECG & the Conducting System — One Beat Traced in 3D

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