A first look at the ECG

Question 1

What are your standard limb leads?

Answer 1

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 and how long does it usually last?

Answer 4

Time from atrial depolarisation to ventricular depolarisation

Mainly due to transmission through the AV node (normally about 0.1-0.2 sec)

Should really be called P - Q interval

Question 5

What is the QRS interval?

Answer 5

Time for the whole of the ventricle to depolarise

(normally about 0.08 sec)

Question 6

What is the QT interval?

Answer 6

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

Question 7

Why can’t you see atrial repolarisation?

Answer 7

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

Different parts of ventricle depolarise at different times & different directions

1. Interventricular septum depolarises from left to right
2. Bulk of ventricle depolarises from endocardial to epicardial surface – travels towards the electrode on the left leg – hence the upwards spike
3. Upper part of interventricular septum depolarises

Question 9

Why is the T - wave positive?

Answer 9

AP is longer in endocardial cells than in epicardial cells

Wave of repolarisation runs in opposite direction to wave of depolarisation.

i.e. a wave of repolarisation moving away from recording electrode produces another positive-going blip

Question 10

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

Answer 10

Main vector of depolarisation is in line with axis of recording from left leg with respect to right arm.

Question 11

What are the augmented limb leads?

Answer 11

Question 12

What extra information do the augmented limb leads give you?

Answer 12

A. Gives you 3 other perspective on events in the heart

i.e. recordings from SLLs I, II, III and aVR, aVL, aVF give you 6 different views of events occurring in the frontal (or vertical) plane

Question 13

Can you correctly label the vectors to the correct limb leads?

Answer 13

Question 14

Should AVR be positive or negative?

Answer 14

Negative, travels away from the heart

Question 15

Which wave is:

1. aVL
2. aVF
3. aVR

Answer 15

Question 16

What extra information do the precordial (chest) leads give you?

Answer 16

Arranged in front of heart

Look at the same events, but in horizontal plan

Main vector of depolarisation is shown by arrow,

Produces negative going blip when recorded from V1, a positive going blip from V6,

Flips over somewhere around V3 or V4. “Progression”

Question 17

Look

Answer 17

Question 18

On which plane is the spread of depolarisation measured when using:

1. Limb leads
2. Precordial leads

Answer 18

1. Frontal
2. Transverse

Question 19

What does the rhythm strip tell you?

Answer 19

Paper should run at 25mm/sec

Calibrating pulse is 0.2 sec = 1 large square (5mm) (5 arge squares per second)

Determine the heart rate:

Measure the R-R interval and work out how many occur in 60 sec, or better ..

Count the R waves in 30 large squares (= 6 sec) and multiply by 10

60-100 beats per min = normal

Below 60 beats per minute = bradycardia

Above 100 beats per minute = tachycardia

Question 20

What is the difference between STEMI & NSTEMI?

Answer 20

ST elevated myocardial infarction or non-ST elevated myocardial infarction.