EKG - Conduction Defects

Question 1

Conduction Defects

Answer 1

A Conduction Block or Defect is defined as any obstruction or delay of the normal
pathways of electrical conduction within the heart. This can occur anywhere within
the conduction system.

Question 2

There are three main types of conduction defects:

Answer 2

○ Sinus Node Block - Sinus node exit block
○ AV Block - This refers to a block at the AV
node or Penetrating Fibers
○ Bundle Branch Block - Conduction defects in
one or both of the bundle branches.
Sometimes, the only part of the bundle is
blocked, resulting in Fascicular Blocks.

Question 3

The term AV Block suggests any conduction block that occurs
between_____

Answer 3

the SA Node and the Bundle Branches.

Question 4

There are three varieties of AV Blocks:

Answer 4

○ First-Degree
○ Second-Degree (two different types)
○ Third-Degree (AKA Complete AV Block)

Question 5

First-Degree AV Block

Answer 5

● A First-Degree AV Block is characterized
by a prolonged delay in the conduction at
the AV Node or Bundle of His.
● Atrial depolarization happens normally,
and despite the delay at the AV Node or
Bundle of His, every atrial impulse does
eventually make it through to the
ventricles (1:1 ratio of P waves to QRS).
● So, technically, a First-Degree AV Block
isn’t really a “block”- it’s a “delay.”

Question 6

What is this showing?

Answer 6

First degree AV block

Question 7

Second-Degree AV Blocks

Answer 7

Unlike First-Degree AV Block, not every atrial impulse is eventually
able to make it to the ventricles in Second-Degree AV Blocks.
○ For this reason, the ratio of P waves to QRS complexes is greater
than 1:1 (fewer QRS complexes)

Question 8

There are two types of Second-Degree AV Blocks:

Answer 8

○ Mobitz Type I - Commonly called “Wenckebach”
○ Mobitz Type II

Question 9

Second-Degree Type I

Answer 9

● When referring to Wenckebach, the block
is almost always within the AV node.
● The diagnosis of Wenckebach requires the progressive lengthening of
each successive PR Interval until one P wave fails to conduct through
to the ventricles and a QRS complexes is missed.
● Remember, “Type wone is Wenckebach, weird, and iwwegular.”

Question 10

What is this showing?

Answer 10

Second-Degree AV Block Type I
Remember, “Type wone is Wenckebach, weird, and iwwegular.”

Question 11

Second-Degree AV Block Type II

Answer 11

● Mobitz Type II is similar to Wenckebach in
that the ratio of P waves to QRS
complexes is not 1:1, but there is one key
difference.
○ Prolongation of the PR Interval does
not occur, but instead conduction is
an all-or-nothing phenomenon.
● The diagnosis of Mobitz Type II requires the presence of a dropped
QRS complex without progressive lengthening of the PR Interval.

Question 12

What is this showing?

Answer 12

Second-Degree AV Block - Type II

Question 13

Third-Degree AV Block

Answer 13

○ The location of the block can be at
the AV Node or below.
● No atrial impulse makes it through to
depolarize the ventricles, so it is often
also called a “Complete Heart Block.”
● The atria and ventricles continue to fire,
but at their own intrinsic rates

Question 14

Third-Degree AV Block EKG characteristics

Answer 14

● P waves will march out regularly, at a rate of 60-100 bpm.
● Wide QRS complexes also march out, but at a rate of 15-40 bpm.
○ These QRS complexes represent a ventricular escape rhythm

Question 15

How is a Third degree AV block differentiated?

Answer 15

Total AV dissociation

Question 16

Bundle Branch Blocks

Answer 16

● The term Bundle Branch Block refers to a
block in either the right or the left bundle
branch.
○ Causes possible widening of the QRS
and changes in the QRS morphology

Question 17

Right Bundle Branch Block (RBBB) EKG features

Answer 17

○ Wide QRS complex (may be only slight).
○ The QRS complex in the leads above the RV
(V1 and V2) assume a unique, virtually
diagnostic morphology- RSR’ (“rabbit ears”)
○ Repolarization is also dysfunctional and MAY
appear as ST depression and T Wave inversion
in V1 and V2 (not always).

Question 18

What is this showing?

Answer 18

Right Bundle Branch Block (RBBB)
“rabbit ears”

Question 19

Left Bundle Branch Block (LBBB)

Answer 19

● Blockage of the LBB causes a delay in left
ventricular depolarization.
● Rather than the LV being totally
depolarized by the LBB conduction
system, a lot of the depolarization comes
from the RV depolarization, just delayed.

Question 20

Left Bundle Branch Block (LBBB) EKG Features

Answer 20

○ Wide QRS complex (may be slight)
○ The QRS complex in the leads over the lateral
LV (I, AVL, V5, and V6) assume a characteristic
morphology- Broad or notched R Wave with
prolongation of the R Wave upstroke.
○ May also see ST depression and T Wave
inversion in the left lateral leads

Question 21

What is this showing?

Answer 21

Left Bundle Branch Block (LBBB) showing a Broad or notched R Wave with
prolongation of the R Wave upstroke

Question 22

The Left Bundle Branch is actually composed of three separate fascicles:

Answer 22

○ Left Anterior Fascicle
○ Left Posterior Fascicle
○ Septal Fascicle

Question 23

One of the main effects that hemiblocks have is _____

Answer 23

Axis Deviation

Question 24

Left Anterior Fascicular Hemiblock

Answer 24

Left Anterior Hemiblock causes current to stop flowing in the anterior fascicle, so the current circling around from the posterior fascicle is dominant

Question 25

Left Posterior Fascicular Hemiblock

Answer 25

Left Posterior Hemiblock causes current to stop flowing in the posterior fascicle, so the current circling around from the anterior fascicle is dominant

Question 26

Bifascicular Block EKG Findings

Answer 26

EKG findings in Bifascicular Block include a combination of features of both
Hemiblock and a Right Bundle Branch Block

Question 27

Bifascicular Block comprised of RBBB and Left Anterior Hemiblock:

Answer 27

○ Wide QRS with RSR’ in V1 and V2, AND
○ Left Axis Deviation

Question 28

Bifascicular Block comprise of RBBB and Left Posterior Hemiblock:

Answer 28

○ Wide QRS with RSR’ in V1 and V2, AND
○ Right Axis Deviation