A First Look at the ECG

Question 1

What do standard limb leads look at?

Answer 1

Events in the vertical or frontal plane

Question 2

What causes an upward going blip?

Answer 2

A wave of approaching depolarisation

Question 3

Which type of events are transmitted better, fast events or slow events?

Answer 3

Fast events

Question 4

What are the charges found at the right arm?

Answer 4

2 negatives

Question 5

What are the charges found at the left arm?

Answer 5

1 positive

1 negative

Question 6

What are the charges found at the left leg?

Answer 6

2 positives

Question 7

What causes the P wave?

Answer 7

Atrial depolarisation

Question 8

What causes the QRS complex?

Answer 8

Ventricular depolarisation

Question 9

What causes the T wave?

Answer 9

Ventricular repolarisation

Question 10

What does the timing of the PR interval tell you?

Answer 10

Time from atrial depolarisation to ventricular depolarisation is mainly due to transmission through the AV node

Question 11

What does the timing of QRS tell you?

Answer 11

Time for the whole of the ventricle to depolarise

Question 12

What does the timing of QT interval tell you?

Answer 12

Time spent while ventricles are depolarised (varies with HR)

Question 13

Why is atrial repolarisation not seen?

Answer 13

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

Question 14

Why is the QRS complex so complex?

Answer 14

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

Question 15

What occurs at Q?

Answer 15

The interventricular septum depolarises from left to right

Question 16

What occurs at R?

Answer 16

The bulk of the ventricle depolarises from the endocardial to the epicardial surface

Question 17

What occurs at S?

Answer 17

The upper part of the interventricular septum depolarises

Question 18

Why is the T wave positive going?

Answer 18

The action potential is longer in endocardial cells than in epicardial cells, so the wave of repolarisation runs in the opposite direction to the wave of depolarisation i.e a wave of repolarisation moving away from the recording electrode produces another positive going blip

Question 19

Why is the R wave bigger in SLL II than in SLL I or SLL III

Answer 19

The main vector of depolarisation is in line with the axis of recording from the left leg with respect to the right arm

Question 20

What would happen if the heart was rotated to the left or developed hypertrophy on the left, or atrophy on the right?

Answer 20

Causes axis deviation

Question 21

What extra information do the augmented limb leads give you?

Answer 21

-By recording from 1 limb lead with respect to the other 2 combined it gives you 3 other perspective on events in the heart.

Question 22

What extra information do the precordial leads give you?

Answer 22

• These are arranged in front of the heart and therefore look at the same events, but in horizontal (or transverse) plane
• Because the main vector of depolarisation is as shown by the arrow, it will produce a negative going blip when recorded from V1, a positive going blip from V6 and flip over somewhere around V3 or V4

Question 23

What do limb leads look at?

Answer 23

Spread of depolarisation and repolarisation from 6 angles in the frontal plane

Question 24

What do precordial leads look at?

Answer 24

The same events as the limb leads from 6 angles in the transverse plane

Question 25

What can the rhythm strip tell you?

Answer 25

Heart rate

Question 26

What is important about the timing of the rhythm strip?

Answer 26

• Paper should run at 25mm/sec

- Calibrating pulse is 0.2 sec= 1 large square/5mm

Question 27

How is HR measured using a rhythm strip?

Answer 27

Measure the R-R interval and work out how many occur in 60 sec or better
-Count the R waves in 30 large squares which is equal to 6 sec and multiply by 10

Question 28

What are the parameters of different HR?

Answer 28

• Normal= 60-100 bpm
• Bradycardia= <60 bpm
• Tachycardia= >100bpm

Question 29

Other than HR, what can the rhythm strip tell you?

Answer 29

• Is each QRS complex preceded by a P wave?
• Is the PR interval too short or too long?
• Is the QRS complex too wide?
• Is the QT interval too long?

Question 30

What is a STEMI?

Answer 30

ST elevated myocardial infarction

Question 31

What is a NSTEMI?

Answer 31

Non-ST elevated myocardial infarction

Question 32

How does the severity of a STEMI compare to a NSTEMI?

Answer 32

STEMI is worse than NSTEMI