Conduction Blocks

Question 1

First Degree AV Block

Answer 1

A disease of the electrical conduction system of the heart in which the PR interval is lengthened beyond 0.20 seconds.

Question 2

In first-degree AV block, the impulse conducting from atria to ventricles through the atrioventricular node (AV node) is _________ (delayed/accelerated).

Answer 2

Delayed

Question 3

What could be a physiological cause of first-degree AV block?

Answer 3

• There could be an issue with the L-type calcium channels that kicks in the slow AP.
  
  • They become slower.
  • One reason they could become slower is permanent deactivation of these L-type calcium channels could lead to a slower rise of phase 0, which dictates conduction.

Question 4

Second Degree AV blocks

Answer 4

Diagnosed when one or more (but not all) of the atrial impulses fail to conduct to the ventricles due to impaired conduction.

Question 5

What are the two types of second degree AV blocks?

Answer 5

• Mobitz 1/ Wenckebach
• Mobitz 2/ Wenckebach

Question 6

EKG of Mobitz 1

Answer 6

PR interval is progressively lengthening from one bear ro another until a beat is dropped

Question 7

Of the two types of second degree AV blocks. Which is more serious?

Answer 7

mobitz 2

• More serious because it is known predictive. There’s sporadic drop of the QRS. This is due to the AV node suddenly losing all its calcium channels. Then they recovery.
• may progress suddenly to complete heart block
• Impairement usually below the AV node

Question 8

What are the PR intervals getting longer and longer with second degree AV block?

Answer 8

• The first excitation comes through the AV node and it’s experiencing ischemia. This is causing the calcium channels to depolarize slower than usual.
• Recovery drom inactivation is becoming slower and slower

NOTE: The lesser the ratio, the more severe it is. Ex: 3 progressively lengthening PR intervals and 1 complete block is more severe that 2 progressively lengthening PR intervals and 1 block.

Question 9

Third degree AV block

Answer 9

A complete failure of the AV node to conduct any impulses from the atria to the ventricles

*Because the impulse is blocked, an accessory pacemaker in the lower chambers will typically activate the ventricles. This is known as an escape rhythm.

Question 10

What is the appearance of a third degree AV block on an ECG?

Answer 10

1. The P waves with a regular P-to-P interval represent the first rhythm.
2. The QRS complexes with a regular R-to-R interval represent the second rhythm. The PR interval will be variable, as the hallmark of complete heart block is lack of any apparent relationship between P waves and QRS complexes.
3. QRS becomes wide bc the flow is slow from one muscle cell to another; there fore the QRS will spread over the ventricle but in a slow fashion, leading to a wide QRS.
4. The QRS and the T aave have opposite directions

Question 11

What key factor makes third degree AV block so severe?

Answer 11

Venous return is not connected to cardiac output

REMEMBER: What goes in must come out but in this case that’s not going to happen

Question 12

Mechanism of Right and Left Bundle Branch Blocks

Answer 12

• SA node is stimulated, so you have the p wave
• The AV node is then stimulated so you have excitation down the bundle.
• There’s a difference of if the left or right bundle are damaged.
  
  • If both bundles are damaged, you have them knocked out and so you’d have the same issue as the third degree block.
  • If only one of the bundles are damaged, one side of the heart will get normal transmission and normal excitation, the other bundle will get abnormal transmission/excitation that is delayed; so both ventricles won’t be excited/contracted at the same time.