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

Reflects transmission through AV node (0.12-0.20 sec)

Question 5

What is the QRS interval?

Answer 5

Time for the whole ventricle to depolarise

(normally about 0.08 sec)

Question 6

What is the QT interval?

Answer 6

Time for ventricles to depolarise and repolarise

(varies with heart rate, normally 0.42 sec at 60 bpm)

Question 7

Why can’t you see atrial repolarisation?

Answer 7

Coincides with ventricular depolaristion

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 the ventricle depolarise at different times and directions

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

Question 9

Why is the T - wave positive?

Answer 9

AP longer in endocardial cells than epicardial cells, wave of repolarisation runs in opposite direction to wave of depolarisation

i.e. 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 III?

Answer 10

Main vector of depolarisation is in line with the axis of recording from left leg with respect to the 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

Gives you 3 other perspective on events in the heart

ie 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

Main vector of depolarisation is shown by the arrow, will produce negative going blip when recorded from V1, positive going blip from V6, and flip over around V3/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)

can determine heart rate:

Measure R-R interval and work out how many occur in 60 sec

Count 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 does STEMI or NSTEMI stand for?

Answer 20

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

ST should be flat because cells are in refractory state – st elevation is indication of severity of heart attack

More dead tissue means more elevation