Cardiac Conduction System & ECG (complete)

Question 1

Describe the relationship between ventricular APs of individual cardiac myocytes and the surface electrocardiogram

Answer 1

• R wave => phase 0
• ST segment => phase 2
• T wave => phase 3
• Segment after T wave => phase 4

Question 2

What is the sequence of the activation of the cardiac electrical conduction system?

Answer 2

• Upper portion of septum is depolarized from left to right
• then depolarization downward in the septum to the apex
• Depolarization from endocardium to epicardium
• Depolarization moves upward from apex in the free walls of both ventricles
• Depolarization of the base of the ventricles

OVERALL GOAL: to help push the blood in the correct direction!

Question 3

Describe the P wave

Answer 3

• Represents atrial depolarization

Question 4

Describe the QRS complex

Answer 4

• Represents depolarization of ventricles

Question 5

Describe the T wave

Answer 5

• Represents repolarization of ventricles

- When K+ channel takes over

Question 6

Describe the PR interval

Answer 6

• Index of conduction time across the AV node

- Depolarization pauses at the bundle of His after depolarization of atria and before that of ventricles

Question 7

Describe the QT interval

Answer 7

• Represents total duration of depolarization and repolarization

Question 8

Describe the effects of left or right bundle branch block on cardiac conduction

Answer 8

Right: QRS widening w/ delayed conduction to RV

Left: QRS widening w/ delayed conduction to LV

Question 9

What are the three major mechanisms by which disturbances in cardiac conduction cause tachyarrhythmias?

Answer 9

1) Abnormal reentry pathways
2) Ectopic foci
3) Triggered activity

Question 10

Describe abnormal reentry pathways as a mechanism leading to arrhythmia

Answer 10

• Most common cause of severe tachycardias

- Reentry occurs w/ unidirectional block and slowed conduction

Question 11

Describe ectopic foci as a mechanism leading to arrhythmia

Answer 11

• when cells outside conduction system acquire automaticity
• and if depolarization rate exceeds that of SA node => abnormal rhythm
• Can be isolated ectopic beats or sustained arrhythmias

Question 12

Describe the concept of triggered activity as a mechanism leading to arrhythmia

Answer 12

• abnormal afterdepolarizations may be triggered by the preceding AP
• Early afterdepolarization before AP => tachyarrhythmia
• Delayed afterdepolarizations appear after an AP => also triggers arrhythmias

Question 13

What are the three types of atrioventricular block?

Answer 13

1) First Degree
2) Second Degree
3) Third Degree

Question 14

Describe first degree AV block

Answer 14

• Conduction is delayed

- BUT all P waves conduct to the ventricles

Question 15

Describe second degree AV block

Answer 15

Some P waves conduct — not all

Question 16

Describe third degree AV block

Answer 16

• No P waves conduct

- Ventricular pacemaker now needed