CVS S7 - The ECG

Question 1

What does the bundle of His branch into and how does excitation spread to both left and right ventricles?

Answer 1

Right and left purkinje fibre branches

Left splits further into anterior and posterior due to large size of ventricle

Question 2

Outline the course of the spread of excitation through the ventricles

Answer 2

• Travels down septum
• Spreads from endocardial surface to epicardial surface
• Ends at base of the valves

Question 3

What signals would be detected by a positive electrode as wave of depolarization moves towards the electrode and then away?

Answer 3

Upward deflection as depolarization moves towards

Downward deflection as depolarization moves towards

Question 4

How would these signals differ if the wave was repolarization opposed to depolarization?

Answer 4

Downward deflection as repolarization moves towards

Upwards deflection as repolarization moves away

Question 5

What rules govern the amplitude of a signal (ECG)

Answer 5

Amount of muscle depolarizing and the angle at which excitation is moving towards the electrode

Question 6

At which positon around the heart would an electrode obtain no signal for the R wave?

Answer 6

When at 90 degree angle

Question 7

How does the spread of repolarization through the ventricular myocardium differ to depolarization?

Answer 7

Epicardial to endocardial

Question 8

What is the role of amplifiers? (ECG)

Answer 8

• Leads contain one positive and one negative electrode
• Signal from negative electrode is inverted to positive and added to the positive input
• Total is then amplified

Question 9

How do augmented leads differ from Leads I, II and III and how is their signals interpreted?

Answer 9

• Have two negative and one positive electrode
• First the two negative electrodes are combined to one
• This is inverted to positive and combined with the actual positive input
• Gives one combined view

Question 10

What is the axis of the heart and when might this change?

Answer 10

• Relates to the main spread of depolarization through the ventricular wall (R wave)
• Between aVF (+90 degrees) and aVL (-30 degrees)
• Changes in left or right ventricular hypertrophy

Question 11

How would axis deviation be detected on an ECG?

Answer 11

Left shift- R wave bigger on lead I, negative on lead III

Right shift- R wave bigger on lead III, negative on lead I

Question 12

What is ventricular ectopic beats and how would they be detected on an ECG?

Answer 12

• Also known as extrasystoles
• Ventricular cells produce own action potential
• Causes wide QRS complex

Question 13

What is a potential severe consequence of ventricular ectopic beats?

Answer 13

If happens early in T wave of preceding beat can induce ventricular fibrillation

Question 14

How would atrial fibrillation be detected on an ECG?

Answer 14

Absent P waves, irregular QRS complex

Question 15

What is first degree heart block?

Answer 15

Delay in conduction pathway causing prolonged PR interval

Heart block is a communication problem between the atria and the ventricles

Question 16

What is second degree heart block?

Answer 16

Occurs when excitation intermittently completely fails to pass through the AV node or bundle of His

Question 17

Describe Type II second degree heart block (Moritz type II)

Answer 17

• Electrical excitation sometimes fails to pass through the AV node or bundle of His
• Not all atrial contractions are followed by ventricular contractions (no pattern)
• There is high risk of progression to complete heart block

Question 18

Describe Type I second degree heart block (Moritz type I)

Answer 18

• The P-R interval elongates sequentially until a QRS complex is dropped
• The system is then reset

Question 19

Describe complete, third degree block

Answer 19

• There are normal P waves as atrial conduction is unaffected
• Conduction is not transmitted to the ventricles
• The intrinsic ventricular pacemaker potential takes over, leading to a slow ventricular contraction
• The heart rate is too slow to maintain blood pressure and perfusion
• The QRS complex is usually wide

Question 20

What changes would be seen in an ECG of a patient suffering from an MI?

Answer 20

• STEMI
• Inverted T waves
• Pathological Q waves

Question 21

What changes would be seen in the ECG of a patient suffering from angina?

Answer 21

Depression of ST wave

Question 22

How would complete heart block (third degree) be detected on an ECG and why does this change occur?

Answer 22

• P waves independent of QRS complex
• Abnormal shaped QRS complex because of abnormal spread of depolarization through ventricles
• Caused transiently by MI or chronically due to fibrosis around bundle of His

Question 23

What artery would be occluded in hypoxic injury to the AV node?

Answer 23

Right coronary

Question 24

How can bundle branch block be distinguished from ventricular ectopic beats on an ECG?

Answer 24

BBB has widened QRS complex too however normal P waves are present and the PR interval is constant which is not present in rhythms beginning in the ventricles

Question 25

What is the transition point, relating the chest leads and the signal received by them?

Answer 25

It is the point where the R and S waves are equal between V1 where QRS is predominantly downward and V6 where QRS is predominantly upward, indicating the position of the interventricular septum

Question 26

What stage of the cardiac cycle is the QRS complex synonymous to?

Answer 26

Ventricular systole