Session 7 - The ECG

Question 1

What do will you see on an ECG if there is no change in membrane potential

Answer 1

Nothing as the ECG only records changes

Question 2

Describe the conduction over the heart

Answer 2

Starts in the SA node then spreads over the atria to the AV node where there is a delay of c.120ms. Then spreads down the septum from L to R then over the ventricular myocardium from the inside to the outside

Question 3

Describe repolarisation

Answer 3

Happens after about 280ms

Spreads in the opposite direction to ventricle depolarisation - outside to inside

Question 4

What would you see if depolarisation was spreading towards an electrode

Answer 4

An upward signal

Question 5

What would you see if depolarisation was spreading away from an electrode

Answer 5

A downward signal

Question 6

What would you see if repolarisation was spreading towards an electrode

Answer 6

A downward signal

Question 7

What does the amplitude of a signal show

Answer 7

How much muscle is depolarising and how directly towards the electrode the excitation is moving

Question 8

What does the P wave show

Answer 8

Atrial depolarisation

Question 9

What does the QRS complex show

Answer 9

Ventricular depolarisation

Question 10

Why is atrial repolarisation not seen

Answer 10

As is lost in the QRS complex

Question 11

What does the T wave show

Answer 11

Ventricular repolarisation

Question 12

What is the cardiac axis

Answer 12

The average direction of spread of depolarisation as seen from the front and estimated from leads 1,2 and 3

Question 13

What would you see if there was left axis deviation and why might this occur

Answer 13

Biggest signal displacement would be seen in lead 1 not in lead 2
Left ventricular hypertrophy

Question 14

What would you see in right axis deviation and why might this occur

Answer 14

The greatest deflection is seen in lead 3

Right ventricular hypertrophy

Question 15

What do you see in atrial fibrillation

Answer 15

No P waves and irregular contraction of the ventricles

Question 16

What does the P-R interval measure and what are its normal values

Answer 16

The delay between atrial depolarisation and ventricular depolarisation
Normally 120-220ms (3-5 small squares)

Question 17

What do you see in first degree heart block

Answer 17

Elongated PR interval indicating a conduction problem

Question 18

What do you see in second degree heart block

Answer 18

Some P waves that are not followed by a QRS complex. Ratio is not 1:1

Question 19

What do you see in third degree heart block

Answer 19

No relationship between P waves and QRS complex

Question 20

In third degree heart block why are the QRS complexes a different shape

Answer 20

They are longer as there is an abnormal focus of depolarisation within the ventricle

Question 21

What might you see in an MI

Answer 21

Pathological Q waves
ST elevation
T wave inversion

Question 22

What is the normal paper speed

Answer 22

25mm/second