A First Look at an ECG

Question 1

What do the standard limb leads look at in an ECG?

Answer 1

Fast events such as depolarisation and repolarisation in the vertical or frontal plane

Question 2

State the 3 standard limb lead

Answer 2

SLL I = left arm wrt right arm
SLL II = left leg wrt right arm
SLL III = left leg wrt left arm

Question 3

Describe how limb leads can record depolarisation

Answer 3

SLL II records from the left leg wrt the right arm - ie it is the difference in potential between the left leg and right arm

The main wave of depolarisation passes down the ventricles and through the body fluids towards the electrode on the left leg

As the wave of depolarisation passes the electrode on the left leg it creates a positive potential relative to the electrode on the right arm

Question 4

What events do the P, QRS and T wave correspond to?

Answer 4

P wave is caused by atrial depolarisation

QRS complex is caused by ventricular depolarisation

T wave is caused by ventricular repolarisation

Question 5

What event does the PR interval correspond

Answer 5

The PR interval = time from atrial depolarisation to ventricular depolarisation. This is mainly due to transmission through the AV node (normally about 0.12-0.2 sec).

Question 6

How long does the QRS complex last for?

Answer 6

0.08 sec

Question 7

What event does the QT interval correspond to?

Answer 7

QT interval = time spent while ventricles are depolarised (varies with heart rate, but normally about 0.42 sec at 60 bpm)

Question 8

Why is atrial depolarisation not seen on an ECG?

Answer 8

atrial repolarisation coincides with ventricular depolarisation. Ventricular depolarisation involves much more tissue depolarising much faster so it swamps any signal from atrial repolarisation.

Question 9

Why is the QRS complex so complex?

Answer 9

Different parts of the ventricle depolarise at different times in different directions:

1. The interventricular septum depolarises from left to right.
2. The bulk of the ventricle depolarises from the endocardial to the epicardial surface.
3. The upper part of the interventricular septum depolarises from bottom to top.

Question 10

State the normal sinus rhythm, sinus tachycardia and sinus bradycardia

Answer 10

Normal- 60-100 beats per minute.
Bradycardia- below 60 beats per minute.
Tachycardia- above 100 beats per minute.

Question 11

What extra information do the augmented limb leads give you?

Answer 11

Gives another perspective of the the recordings from SLLs

Question 12

What do the precordial (chest leads) tell?

Answer 12

These are arranged in front of the heart and therefore look at the same events, but in the horizontal (or transverse) plane

Question 13

What does STEMI and UNSTEMI mean?

Answer 13

• STEMI = ST elevated myocardial infarction
• NSTEMI = non-ST elevated myocardial infarction
• Elevation of the ST segment seems to be an indication that something has gone seriously wrong.