A first look at ECG

Question 1

What are standard limb leads ?

Answer 1

These look at events in the frontal plane
Recordings can be made from any pair of electrodes
- SLL I = left arm with respect to right arm
- SLL II = left leg with right arm
- SLL III = left leg with left arm

Question 2

What does an approaching wave of depolarisation cause ?

Answer 2

Upward going blip

Question 3

Which events are better transmitted, fast or slow ?

Answer 3

Fast

Question 4

What is the PR interval ?

Answer 4

Time from atrial depol. To ventricular depol. - mainly due to transmission through the AV node (0.12-0.2 sec)

Question 5

What is the QRS interval ?

Answer 5

Time for whole of ventricle to depolarise (~0.08 sec)

Question 6

What is the QT interval ?

Answer 6

Time spent while ventricles are depolarised (varies with HR, ~ 0.42 sec at 60bpm)

Question 7

Why can’t you see atrial repolarisation ?

Answer 7

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

Question 8

How do you explain the three stages of the QRS complex ?

Answer 8

The interventricular septum depolarises from left to right

The bulk of the ventricle depolarises from the endocardial to the epicardial surface – travels towards the electrode on the left leg – hence the upwards spike

The upper part of the interventricular septum depolarises

Question 9

Why is the QRS complex so complex ?

Answer 9

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

Question 10

Why is the T - wave positive ?

Answer 10

The AP 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 11

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

Answer 11

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

Question 12

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

Answer 12

Cause axis deviation

Question 13

What are the augmented limb leads ?

Answer 13

I
II
III
aVR
aVL
aVF

Question 14

What extra information do the augmented limb leads give you ?

Answer 14

By recording from one limb lead with respect to the other 2 combined, it give you 3 other perspectives on events in the heart
i.e. Recordings from SLL I, II, III and aVR, aVL, aVF give you 6 different views of events occuring in the frontal plane

Question 15

What are the precordial limb leads ?

Answer 15

V1-V6

Question 16

What extra information do the precordial leads give you ?

Answer 16

These are arranged in the front of the heart and therefore look at the same events, but in the horizontal 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 a slip over somewhere around V3 or V4 ‘progression’

Question 17

What does the rhythm strip tell you ?

Answer 17

Timing is all important- paper should run at 25mm/sec
Calibrating pulse is 0.2s = 1 large square (5mm)
Heart rate- measure the R-R interval and work out how many occur in 60s or
Count the R waves in 30 large squares (=6s) and multiply by 10

Question 18

What is normal HR ?

Answer 18

60-100 bpm

Question 19

What is considered tachycardia ?

Answer 19

> 100 bpm

Question 20

What is considered bradycardia ?

Answer 20

<60 bpm

Question 21

What is STEMI ?

Answer 21

ST elevated myocardial infacrtion

Question 22

What is NSTEMI ?

Answer 22

NON-ST elevated myocardial infacrtion