ECG - Cardiac Conduction

Question 1

What is AV block?

Answer 1

Interruption of AV conducting system

Question 2

What are characteristics of 1st degree AV block?

Answer 2

Transmission of all p-waves with prolonged PR interval

Question 3

What is the definition of 2nd degree heart block?

Answer 3

Failure of conduction of some of the p-waves into the ventricles

Question 4

What are the characteristics of Mobitz type I heart block?

Answer 4

Progressive PR prolongation, subsequently dropping a QRS complex

Question 5

What is the physiological cause of Mobitz type I AV block?

Answer 5

Progressive prolongation of conduction time in the AV node

Question 6

Is the rate of progression to complete AV block low or high for Mobitz type I AV block?

Answer 6

Low

Question 7

What are characteristics of mobitz type II AV block?

Answer 7

Normal PR interval, but lack of conduction of some P-waves. Conduction ratios are calculated for number of P-waves:number of QRS complexs

Question 8

Is the rate of progression to complete heart block low or high in Mobitz type II AV block?

Answer 8

High

Question 9

What is the key to making the distinction between mobitz type I and type II AV block?

Answer 9

The timing of the non-conducted p-wave

Question 10

What is the following?

Answer 10

1st degree HB with non-conducted premature atrial ectopic beat (occuring before expected p-wave with different morphology)

Question 11

What conduction ratio of mobtiz type II is classed as untypable?

Answer 11

Conduction ratio of 2:1 - can’t distinguish if mobtiz type I or type II

Question 12

What are the characteristics of third degree AV block?

Answer 12

AV dissociation - no P-waves transmiited

Question 13

What area of the heart maintains rhythm in 3rd degree AV block?

Answer 13

Ventricular myocytes = ventricularly paced rhythm

Question 14

Why does 3rd degree AV block occur?

Answer 14

Due to diffuse damage of AV conducting system

Question 15

What happens to the QRS complex in 3rd degree HB?

Answer 15

Becomes Broad complex due to ventricular rhythms

Question 16

What would the rate be in complete HB with ventriclar rhythm?

Answer 16

Bradycardic

Question 17

When might you get complete HB with narrow complex QRS?

Answer 17

AV node dysfunction with discharging focus at bundle of his

Question 18

Why does RBBB produce the characeristic “rabbit’s ears” shape in lead V1?

Answer 18

Intial depolarisation from LBBB is towards V1, producing positive r wave. This is followed by rapid left ventricular depolarisation (s-wave) followed by slow RV depolarisation (2nd wide r-wave). This is known as an RSR complex, which is broad complex

Question 19

Why do you get the characteristic slurred S-wave in lead V6 in RBBB?

Answer 19

Due to delayed depolarisation of the RV due to RBBB

Question 20

Why can you get ST and T-wave changes in RBBB in right sided chest leads?

Answer 20

Due to altered pattern of repolarisation in the RV

Question 21

What are the varients of QRS complex seen in RBBB in lead V1?

Answer 21

• RSR
• Notched QRS
• One large R-wave

Question 22

What criteria are used to identify complete RBBB?

Answer 22

• QRS > 0.12s
• Slurred S wave in V6 and/or RSR pattern in V1
• Overall positive QRS complex in V1

Question 23

Will the QRS complex be overall positive, negative or isoelectric in lead V1?

Answer 23

Overall positive - A MUST FOR DIAGNOSIS

Question 24

Why is the RBBB liable to compressive damage from pressure in the RV?

Answer 24

Due to close proximity to the subendocardium

Question 25

What can cause RBBB?

Answer 25

Can be normal, but can also be caused by * **Chronic increase in RV pressure** - Cor pulmonale, PHTN * **Acute increase in RV pressure** - PE * **Septal MI** - LAD * **Congenital Defects** * **Myocarditis** * **Cardiomyopathy** * **Iatrogenic causes**

Question 26

Why do you get the characteristic M shaped QRS complex in V6 in LBBB?

Answer 26

Due to depolarisation originating from right side first and spreading to the left. This causes intial r-wave, followed by a negative deflection representing RV depolarisation away from V6, and a subsequent second R wave in V6 due to slowed LV depolarisation

Question 27

Why does LBBB produce deep wide S-waves in lead V1?

Answer 27

Due to delayed depolarisation of the LV

Question 28

Where else would you get a M-shaped QRS complex in LBBB other than V6?

Answer 28

Lead I

Question 29

What are the main criteria for complete LBBB?

Answer 29

* **QRS prolongation \> 0.12 s** * **Broad R waves in Lead I and V6 with no q-waves** * **Broad S waves in septal leads**

Question 30

What can happen to T-waves in LBBB?

Answer 30

Due to abnormal depolarisation/repolarisation of LV * **Complete lack of concordance** * **ST elevation an depression can occur**

Question 31

What can cause LBBB?

Answer 31

* **LAD MI** - Anteroseptal infarct * **Hypertrension** * **Myocarditis** * **Cardiomyopathy** * **Aortic Valve disease** - endocarditis, AVR, AS

Question 32

What are the 3 main divisions of the LBBB?

Answer 32

* **Anterior Fascicle** * **Septal fascicle** * **Posterior Fascicle**

Question 33

Why does the QRS complex remain within the normal range if one of the fascicles of the LBB becomes damaged?

Answer 33

Due to extensive anastamoses with the other fascicles - Depolarisation occurs at same rate, just in different direction through the fascicle

Question 34

What is the blood supply of the AV node?

Answer 34

Branch of posterior descending artery, supplied by RCA

Question 35

What location of MI can cause AV node damage with 1st, 2nd or 3rd degree HB as a result?

Answer 35

Inferior MI

Question 36

What do the septal arteries of the LAD supply?

Answer 36

* **RBB** * **Main Body of LBB** * **Anterior fascicle**

Question 37

WHat conduction pathologies on ECG can an anterior MI give rise to?

Answer 37

* **RBBB** * **LBBB** * **LAFB**

Question 38

What is acceleration dependent aberrancy?

Answer 38

Development of R/LBBB above a certain heart rate, due to abnormal refractory period of the affected branch. This means that above certain heart rates the BB does not repolarise fast enough to conduct a beat, and ends up resulting in a physiological BBB

Question 39

What are features of bifascicular block?

Answer 39

Typical features of RBBB plus either left or right axis deviation.

Question 40

What is bifascicular block?

Answer 40

the combination of RBBB with either LAFB or LPFB.

Question 41

What is the most common type of bifascicular block?

Answer 41

RBBB + LAFB

Question 42

What are causes of bifascicular block?

Answer 42

* **Ischaemic heart disease** (40-60% cases) * **Hypertension** (20-25%) * **Aortic stenosis** * **Anterior M**I (occurs in 5-7% of acute AMI) * **Primary degenerative disease of the conducting system** (Lenegre’s / Lev’s disease) * **Congenital heart disease** * **Hyperkalaemia** (resolves with treatment)