ECG interpretation

Question 1

What is an ECG?

Answer 1

It is an electrocardiogram which is a picture of the electrical conduction of the heart printed onto graphed paper that looks at rate and rhythm.

Question 2

How many leads are there?

Answer 2

12 leads. 10 physical leads placed onto patient.

Question 3

What are the leads?

Answer 3

• RA
• RR
• LL
• LA
• V1, V2, V3, V4, V5, V6

Question 4

Why is it considered 12 lead ECG if only 10 leads?

Answer 4

The six limbs leads are called Lead I, II, III and aVL, aVR, aVF. ‘a’ means augmented.

Whilst there are six precordial leads which are V1-V6 across the precordium (across torso).

Question 5

What is a wave? Name the types of waves.

Answer 5

A positive or negative deflection from baseline that indicates a specific electrical event.

The waves on an ECG include:
• P wave
• Q wave
• R wave
• S wave
• T wave 
• U wave

Question 6

What is an interval? Name the types of intervals.

Answer 6

The time between two specific ECG events.

The intervals commonly measured on an ECG include:

• PR interval
• QRS interval (also called QRS duration)
• QT interval
• RR interval

Question 7

What is a segment? Name the types of segments.

Answer 7

The length between two specific points on an ECG that are supposed to be at the baseline amplitude (not negative or positive).

The segments on an ECG include:
• PR segment
• ST segment
• TP segment

Question 8

What is a complex?

Answer 8

The combination of multiple waves grouped together. The only main complex on an ECG is the QRS complex.

Question 9

What is a point?

Answer 9

There is only one point on an ECG termed the J point, which is where the QRS complex ends and the ST segment begins.

Question 10

What does the P wave represent?

Answer 10

Atrial depolarization.

Question 11

What is atrial depolarization?

Answer 11

Contraction of the atria.

Beginning of the cardiac cycle.

This occurs because the SA node, located in the right atria, generates an action potential and sends it through the Bachmann’s bundle to depolarise (contract) the atrial muscle cells.

Question 12

What does the QRS complex represent?

Answer 12

Ventricular depolarization.

Question 13

What is ventricular depolarization?

Answer 13

Contraction of the ventricles.

After the P wave.

It represents the action potential moving from the AV node, through the bundle of His, left and right branches and Purkinje fibres into the ventricular muscle tissues causing them to contract.

Question 14

What does the T wave represent?

Answer 14

Ventricular repolarization.

Question 15

What is ventricular repolarization?

Answer 15

Relaxation of the ventricles.

Occurs after the QRS complex.