CVS Session 7 (Lecture 7.1)

Question 1

What property of the heart causes the electrodes of the ECG to detect its activity?

Answer 1

1) Myocardium is a large mass of muscle undergoing electrical changes at the same time.
2) This generates a large changing electrical field which can be detected by electrodes on the body surface.

Question 2

Describe the rules of the ECG detection for depolarisation.

Answer 2

1) Depolarisation coming towards an electrode = upward deflection
2) Depolarisation going away from an electrode = downward defelction

Question 3

Describe the rules of the ECG detection for repolarisation.

Answer 3

1) Repolarisation coming towards an electrode = downward deflection
2) Repolarisation going away from an electrode = upward defelction

Question 4

Describe the rules of the ECG detection for amplitude.

Answer 4

Amplitude depends on

1) How much muscle is depolarising
2) How directly towards the electrode the excitation is moving

Question 5

Describe the P wave of the ECG (view from Apex)

Answer 5

1) Atrial depolarisation

Will produce a small upward deflection (small because little muscle but moving towards electrode)

Question 6

Describe the Q wave of the ECG (view from Apex)

Answer 6

2) Spread from the septum

Excitation spreads about halfway down the septum, then out across the axis of the heart :

producing a small downward deflection (downward because moving away from electrode and small because not directly away

Question 7

Describe the R wave of the ECG (view from Apex)

Answer 7

3) Spread through ventricular myocardium

Depolarisation spreads through the ventricular myocardium along an axis slightly to the left of the septum

Produces a large upward deflection (upward because moving towards electrode, large because : directly towards and lots of muscle

Question 8

Describe the S wave of the ECG (view from Apex)

Answer 8

4) End of depolarisation

Depolarisation finally spreads upwards to the base of the ventricles

Produces a downward deflection (downward because moving away, small because not moving directly away)

Question 9

Describe the T wave of the ECG (view from Apex)

Answer 9

5) Ventricular repolarisation

After 280ms repol begins on epicardial surface
Spreads through myocardium in opposite way to depol

Produces a medium upward deflection (upward because moving away, medium because timing in different cells dispersed)

Question 10

What happens to atrial repolarisation?

Answer 10

Lost in QRS complex

Question 11

What happens to the directions and amplitude as the electrode is moved? Describe for the R wave

Answer 11

Change in accordance to position of electrode thus the ‘view’ of that electrode for the heart

For the R wave:

An electrode viewing the R wave head on will see a large upward deflection
Viewing sideways on sees no signal
Viewing end on sees a large downward signal

Question 12

What do amplifiers do in the ECG?

Answer 12

Take signal coming in on the negative input
Invert it
Add it to the signal from the positive input before amplifying the total

Question 13

What does a negative electrode observe?

Answer 13

Looks from the opposite direction to an equivalent positive electrode.

Therefore convert negative view to an equivalent positive view, then add to actual positive view, taking account of direction (VECTORS)

Question 14

Describe the lead II set up.

Answer 14

Positive electrode bottom left
Negative electrode top right
Equivalent positive of negative bottom left
Two views from bottom left

Question 15

Describe lead I set up.

Answer 15

Positive electrode top left
Negative electrode top right
Equivalent positive of negative bottom left
Looks from left side

Question 16

Describe lead III set up.

Answer 16

Positive bottom left
Negative top left
Equivalent positive - bottom right
Straight up view from bottom