ECG - Cardiac Conduction Flashcards
What is AV block?
Interruption of AV conducting system
What are characteristics of 1st degree AV block?
Transmission of all p-waves with prolonged PR interval
What is the definition of 2nd degree heart block?
Failure of conduction of some of the p-waves into the ventricles
What are the characteristics of Mobitz type I heart block?
Progressive PR prolongation, subsequently dropping a QRS complex
What is the physiological cause of Mobitz type I AV block?
Progressive prolongation of conduction time in the AV node
Is the rate of progression to complete AV block low or high for Mobitz type I AV block?
Low
What are characteristics of mobitz type II AV block?
Normal PR interval, but lack of conduction of some P-waves. Conduction ratios are calculated for number of P-waves:number of QRS complexs
Is the rate of progression to complete heart block low or high in Mobitz type II AV block?
High
What is the key to making the distinction between mobitz type I and type II AV block?
The timing of the non-conducted p-wave
What is the following?
1st degree HB with non-conducted premature atrial ectopic beat (occuring before expected p-wave with different morphology)
What conduction ratio of mobtiz type II is classed as untypable?
Conduction ratio of 2:1 - can’t distinguish if mobtiz type I or type II
What are the characteristics of third degree AV block?
AV dissociation - no P-waves transmiited
What area of the heart maintains rhythm in 3rd degree AV block?
Ventricular myocytes = ventricularly paced rhythm
Why does 3rd degree AV block occur?
Due to diffuse damage of AV conducting system
What happens to the QRS complex in 3rd degree HB?
Becomes Broad complex due to ventricular rhythms
What would the rate be in complete HB with ventriclar rhythm?
Bradycardic
When might you get complete HB with narrow complex QRS?
AV node dysfunction with discharging focus at bundle of his
Why does RBBB produce the characeristic “rabbit’s ears” shape in lead V1?
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
Why do you get the characteristic slurred S-wave in lead V6 in RBBB?
Due to delayed depolarisation of the RV due to RBBB
Why can you get ST and T-wave changes in RBBB in right sided chest leads?
Due to altered pattern of repolarisation in the RV
What are the varients of QRS complex seen in RBBB in lead V1?
- RSR
- Notched QRS
- One large R-wave
What criteria are used to identify complete RBBB?
- QRS > 0.12s
- Slurred S wave in V6 and/or RSR pattern in V1
- Overall positive QRS complex in V1
Will the QRS complex be overall positive, negative or isoelectric in lead V1?
Overall positive - A MUST FOR DIAGNOSIS
Why is the RBBB liable to compressive damage from pressure in the RV?
Due to close proximity to the subendocardium
What can cause RBBB?
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
Why do you get the characteristic M shaped QRS complex in V6 in LBBB?
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
Why does LBBB produce deep wide S-waves in lead V1?
Due to delayed depolarisation of the LV
Where else would you get a M-shaped QRS complex in LBBB other than V6?
Lead I
What are the main criteria for complete LBBB?
- QRS prolongation > 0.12 s
- Broad R waves in Lead I and V6 with no q-waves
- Broad S waves in septal leads
What can happen to T-waves in LBBB?
Due to abnormal depolarisation/repolarisation of LV
- Complete lack of concordance
- ST elevation an depression can occur
What can cause LBBB?
- LAD MI - Anteroseptal infarct
- Hypertrension
- Myocarditis
- Cardiomyopathy
- Aortic Valve disease - endocarditis, AVR, AS
What are the 3 main divisions of the LBBB?
- Anterior Fascicle
- Septal fascicle
- Posterior Fascicle
Why does the QRS complex remain within the normal range if one of the fascicles of the LBB becomes damaged?
Due to extensive anastamoses with the other fascicles - Depolarisation occurs at same rate, just in different direction through the fascicle
What is the blood supply of the AV node?
Branch of posterior descending artery, supplied by RCA
What location of MI can cause AV node damage with 1st, 2nd or 3rd degree HB as a result?
Inferior MI
What do the septal arteries of the LAD supply?
- RBB
- Main Body of LBB
- Anterior fascicle
WHat conduction pathologies on ECG can an anterior MI give rise to?
- RBBB
- LBBB
- LAFB
What is acceleration dependent aberrancy?
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
What are features of bifascicular block?
Typical features of RBBB plus either left or right axis deviation.
What is bifascicular block?
the combination of RBBB with either LAFB or LPFB.
What is the most common type of bifascicular block?
RBBB + LAFB
What are causes of bifascicular block?
- Ischaemic heart disease (40-60% cases)
- Hypertension (20-25%)
- Aortic stenosis
- Anterior MI (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)
