Session 6 Lectures - The ECG

Question 1

In what direction to

ventricles depolarise?

Answer 1

From endocardium to epicardium

Question 2

What is the normal rhythm of the heart referred to as?

Answer 2

Sinus rhythm

Question 3

Where do the right and left bundle branches lie?

Answer 3

Subendocardially in intraventicular septum

Question 4

When current is carried from the Bundle of His - what depolarises first and in what direction?

Answer 4

The atrioventricular septum

Depolarises left to right

Question 5

What is last in the ventricles to be depolarised?

Answer 5

The base of the heart (towards the atria)

Question 6

Where does the ECG record changes?

Answer 6

On the extracellular surface of the cardiac myocytes

Question 7

In what direction is a wave travelling to cause a positive deflection? (moving up)

Answer 7

The wave is travelling towards the electrode

Question 8

In what direction should a wave be travelling to cause a negative deflection? (DOWN)

Answer 8

Away from the electrode

Question 9

What will you see on the voltmeter if the heart was at resting state?

Answer 9

A flat line
Positive charge (because the intracellular environment is more negative and we are measuring extracellular environment)

Question 10

Does the depolarisation of the SA node register on the ECG?

Answer 10

No! The depolarisation is too small to register on the ECG

Question 11

What causes the first current on the ECG and what is this called?

Answer 11

Depolarisation of the atria

It causes the P wave which is a positive deflection as the current is moving in the direction of the positive electrode

Question 12

What causes the flat line after the P wave?

Answer 12

The delay at the AV node (which allows for the atria to finish emptying into the ventricles)

Question 13

What causes the Q wave?

Answer 13

Depolarisation of the intraventricular septum
It causes a small negative deflection because the depolarisation is travelling left to right (moving away from the view at apex)

Question 14

What causes the R wave - why is it the largest deflection?

Answer 14

Depolarisation of the apex of the ventricles
It is so large because there is a large muscle mass so greater electrical activity
The R wave is a large positive deflection because the wave is moving towards the electrode

Question 15

What will happen to the R wave in LV hypertrophy?

Answer 15

The R wave will increase - greater deflection because there will be more muscle mass so greater electrical conductivity

Question 16

What causes the S wave?

What direction is the deflection?

Answer 16

The depolarisation of the base of the ventricles

It produces a small downwards deflection because moving slightly away from view

Question 17

What part of the wave is atrial depolarisation and which is ventricular depolarisation?

Answer 17

Atrial = the P wave 
Ventricular = the QRS complex 
Q = iv septum
R = apex 
S = base

Question 18

How many electrodes are placed on the body and how many views does this give?

Answer 18

10 electrodes = 4 on the limbs and 6 on the chest
12 views of the heart
- 6 from the limb electrodes (AVR, AVL, AVF, 1, 2 and 3)
- 6 from the electrodes on the chest

Question 19

Which limb leads are looking at the heart inferiorly?

Answer 19

Leads 2,3 and AVF

Question 20

Which limb leads are looking at the left side of the heart?

Answer 20

Lead 1 and AVL

Question 21

On ECG paper what is the size of the small squares and large squares?

Answer 21

Small = 1mm x 1mm
Large = 5mm x 5mm
5 small squares = width of 1 large square

Question 22

On the ECG paper what are along the axis?

Answer 22

Horizontal = time (ms)
Vertical = voltage (mv)

Question 23

What is the normal ECG speed?

How many squares does this equal?

Answer 23

25mm/second
If small square = 1mm
25 small squares = 25mm
25 small squares = 5 large squares 
So it will travel 5 large squares or 25 small squares a second

Question 24

How long does it take to travel 1 small square and 1 large square?
How many squares can you travel in a second?

Answer 24

1 small square = 0.04 seconds
1 large square = 0.2 seconds
So in 1 second you can travel 5 large squares or 25 small squares

Question 25

How many large squares can you travel in 3 seconds on an ECG?

Answer 25

1 large square = 0.2 seconds
5 large squares in 1 seconds
15 large squares in 3 seconds
(as there are 5 small squares in 1 large square you can also travel 75 small squares in 3 seconds)

Question 26

How many squares in 6 seconds?

Answer 26

1 large square = 0.2 seconds
5 large squares = 1 second
30 large squares = 6 seconds
(there are 5 small squares in 1 large square so 150 small squares in 6 seconds)

Question 27

Which waves on the ECG make up 1 cardiac cycle?

Answer 27

PQRST complex = 1 cardiac cycle or Heartbeat

Question 28

If you count 4 squares between an R-R interval, what is the heart rate?

Answer 28

300 large squares (no for 1 minute) divided by 4 = 75

So the patient has a heart rate of 75 BPM

Question 29

What should you count on an ECG if heart rate is irregular?

Answer 29

Count the number of QRS complexes in 6 seconds and then multiply by 10
30 large squares = 6 seconds
Count for 30 large squares and the number of QRS complexes x 10

Question 30

What is the PR interval?

What is the normal length?

Answer 30

The start of the P wave until the flat line afterwards changes (to either the Q wave or the R wave if there is no Q wave)
The PR complex should be 3-5 small boxes

Question 31

What is the QRS interval?

What is the normal length?

Answer 31

It is the start of Q until the end of S

It should be less than 3 small boxes

Question 32

What is the QT interval?

What is the normal length?

Answer 32

The start of Q until the end of T

It should be 11-12 small boxes

Question 33

What HR = sinus tachycardia and which = sinus bradycardia?

Answer 33

Sinus tachycardia = over 100BPM

Sinus bradycardia = under 60BPM