Chapter 4 conduction blocks

Question 1

Define conduction block?

Answer 1

any obstruction or delay of the flow of electricity along the normal pathway of electrical conduction is called a conduction block.

Question 2

what are the 3 types of conduction blocks?

Answer 2

1. sinus node block: SA node fires but signal is not transmitted to surrounding tissue.
2. AV block: between SA node and purkinje fibers includes the AV node and his bundle.
3. Bundle branch block: only a part of the branch may be blocked (fascicular block or a hemiblock)

Question 3

describe AV blocks?

Answer 3

3 degrees: 1st, 2nd, and 3rd degree block

they are diagnosed by carefully examining the relationship of the P-waves to the QRS complexes

Question 4

describe 1st degree AV block?

Answer 4

there is a delay in conduction at the AV node or His bundle (just below the AV node).
EKG cannot tell the difference between a block in the AV node and one in the His bundle.
Elongated PR interval. do to delay in AV node
PR interval has to be longer than the 0.2 seconds

BUT EVERY atrial depolarization makes it to the ventricles, so every QRS is preceded by the P-wave.
can be seen in normal or diseased hearts.

Question 5

Describe 2nd degree AV block?

Answer 5

NOT every atrial pulse is able to pass through the AV node into the ventricles. 2 types:

• Mobitz type I second-degree AV block (wenckeback block): BLOCK WITHIN the AV NODE
• Mobitz type II second-degree AV block

Question 6

describe a Mobitz type I second-degree AV block (wenckeback block)?

Answer 6

Block within the AV node.
The block/delay is variable, increasing with each ensuing impulse. Each successive atrial impulse encounter a longer and longer delay in the AV node until one impulse (usually every third or fourth fails to make it through)
EKG shows progressive PR lengthening then a P-wave followed by NO QRS. (dropped beat). After the dropped beat the sequence repeats itself.
they can be 4:3 3:2 etc. ratios of P to QRS’s.

Question 7

Describe Mobitz type II second-degree AV block?

Answer 7

block below the AV node in the His bundle.
Progressive lengthening of the PR interval does not occur. Conduction is an ALL-OR-NOTHING phenomenon.
EKG appearance: 2 normal beats with normal PR distance and then a P-wave with no QRS (dropped beat)
2:1 and 3:1 ratios are seen.

Question 8

out of the 2 types of AV blocks which one is more dangerous?

Answer 8

Mobitz type II block since it can signify a more serious heart disease and capable of progressing suddenly to 3rd degree heart block.
mobitz type II mandates insertion of a pacemaker.

Question 9

describe the 3rd degree AV block?

Answer 9

NO atrial impulses at all make it through to activate the ventricles. Also called COMPLETE HEART BLOCK.
The result is AV dissociation as the atria and ventricles beat at different rates.
ATRIA 60-100bpm
VENTRICLES 30-45bpm

Question 10

what does a 3rd degree AV block look like on an ECG?

Answer 10

P-wave are seen at 60-100bpm and have no relationship with the QRS complexes that are much slower.
QRS complexes are bizarre and wide, just like premature ventricular contraction (PVCs)

Question 11

compare PVC (premature ventricular contractions) to 3rd degree AV block.

Answer 11

PVC are premature occurring before the next expected beat and even the slowest ventricular tachycardia will be faster than the patients normal rhythm.
Ventricular escape beat occurs after a long pause and always slower than normal beats.

Question 12

how to diagnose the different types of AV blocks?

Answer 12

1. first degree: PR interval is greater than 0.2 seconds, all beats conducted to ventricles
2. second degree: some beats are conducted through to ventricles
   -Mobitz type I (wenckeback): progressively longer PR interval
   -Mobitz type II: periodic QRS drop, elongated PR interval
   3rd degree: NO beats conducted (AV dissociation)

Question 13

Describe Bundle branch block?

Answer 13

block/slowing of current flow in one of the branches.

It is DIAGNOSED by: looking at the width and configuration of the QRS complexes.

Question 14

Describe Right bundle branch block?

Answer 14

Right ventricular depolarization is delayed, it does not begin until the left ventricle is almost fully depolarized.
1. QRS may be wider than 0.12 seconds
2. V1 and V2 show a second R’ wave and the whole complex is the rabbit ears R-S-R’ prime.
I, aVl V5 and V6 show a deep S-wave.
RABBIT EARS WILL NOT ALWAYS BE SEEN, just a wide QRS.

Question 15

Describe Left bundle branch block?

Answer 15

Left ventricular depolarization is delayed:
1. QRS may be wider than the 0.12 seconds
2. QRS comples in leads overlying the left ventricle (I, aVL, V5 and V6) will have tall R waves. that will be either broad on top or notched (true rabbit ears are less common)
Leads overlying the right ventricle show reciprocal broad deep S waves,

Question 16

Describe how bundle branch blocks affect repolarization?

Answer 16

There is an ST depression in the respective leads overlying the right and left ventricles depending on which side experiences the block.
ST-depression and T-wave inversion is seen in right precordial leads during right bundle branch block.
similar abnormalities can occur with ventricular hypertrophy.

Question 17

what is “critical rate”

Answer 17

right or left bundle branch blocks may only appear when a particular heart rate is achieved. If the heart rate is so rapid that the bundle cannot repolarize in time there will be a block.

Question 18

Describe the hemiblocks of the ventricles?

Answer 18

septal fascicle and left and anterior and left posterior fascicle. Hemiblock refers to one of these fascicles, hemiblocks appear only in the left ventricle.
Hemiblock cause axis deviaiton
- Left anterior hemiblock = left axis deviation (-30 to -90 degrees)
- Left POSTERIOR hemiblock = right axis deviation (90 to 180 degrees).
QRS complex will be negative in lead I and positive in lead aVF.
UNLIKE COMPLETE BLOCK - HEMIBLOCKS DO NOT MAKE THE QRS WIDE. Also NO ST segment and T-wave repolarization changes.

Question 19

when can hemiblock be seen?

Answer 19

• left anterior hemiblock can be seen in both normal and diseased hearts
• Left posterior hemiblocks is exclusive to province of sick hearts.

Question 20

what is bifascicular block?

Answer 20

when a right bundle branch block and hemiblock occur together. Thus only one fascicle of the left bundle branch is supplying electrical current to the bulk of both ventricles.

Question 21

what is the criteria for bifascicular block? (right bundle branch block + Left anterior hemiblock)

Answer 21

Right bundle branch block combined with left anterior hemiblock: RIGHT bundle branch block
- QRS wider than 0.12 seconds
- RSR' in V1 and V2
LEFT anterior hemiblock:
-left axis deviation

Question 22

what is the criteria for bifascicular block? (right bundle branch block + Left posterior hemiblock)

Answer 22

RIGHT bundle branch block
- QRS wider than 0.12 seconds
- RSR' in V1 and V2
LEFT posterior hemiblock:
-Right axis deviation

Question 23

name the 2 types of block that underachieve?

Answer 23

• nonspecific intraventricular conduction delay: QRS is wider then 0.10 seconds with no other criteria for either bundle branch block or bifascicular block.
• incomplete bundle branch block: rabbit ears but the QRS is below 0.12 seconds