Atrioventricular Block Flashcards
What is an AV block?
involves the partial or complete interruption of impulse transmission from the atria to the ventricles.
What is the most common cause of AV block?
idiopathic fibrosis and sclerosis of the conduction system
What are the 4 types of AV block?
First-degree AV block
Second-degree AV block (type 1)
Second-degree AV block (type 2)
Third-degree (complete) AV block
What is a first degree AV block?
involves the consistent prolongation of the PR interval (defined as >0.20 seconds) due to delayed conduction via the atrioventricular node.
What are the causes of a first-degree AV block?
Enhanced vagal tone: often seen in athletes (non-pathological)
Post myocardial infarction
Lyme disease
Systemic lupus erythematosus
Congenital
Myocarditis
Electrolyte derangements
Drugs: particularly AV blocking drugs such as beta-blockers, rate-limiting calcium-channel blockers, digoxin and magnesium1
Thyroid dysfunction
What are the first degree AV block findings on an ECG?
Rhythm: regular
P wave: every P wave is present and followed by a QRS complex
PR interval: prolonged >0.2 seconds (5 small squares)
QRS complex: normal morphology and duration (<0.12 seconds)
What is the management of a first degree AV block?
Any AV blocking drugs should be stopped. No intervention is usually required if the patient is asymptomatic.
If the patient is symptomatic, a pacemaker may be considered.
What are the complications of a first-degree AV block?
First-degree AV block does not usually progress to higher grade AV blocks. Those with first-degree AV block may be at an increased risk of atrial fibrillation.1
What is the second degree AV block type 1 also know as?
Mobitz type 1 AV block or Wenckebach phenomenon.
What is Mobitz I block?
progressive PR interval prolongation until a P wave fails to conduct
What are the causes of a second-degree AV block type 1?
Increased vagal tone: often seen in athletes (non-pathological)
Drugs: beta-blockers, calcium channel blockers, digoxin, amiodarone
Inferior myocardial infarction
Myocarditis
Cardiac surgery (mitral valve repair, Tetralogy of Fallot repair)
What are the ECG findings of a second degree AV block Type 1?
Rhythm: irregular
P wave: every P wave is present, but not all are followed by a QRS complex
PR interval: progressively lengthens before a QRS complex is dropped
QRS complex: normal morphology and duration (<0.12 seconds), but are occasionally dropped
How do patients with a 2nd degree AV block type 1 typically present?
Patients are usually asymptomatic, but some can develop symptomatic bradycardia and present with symptoms such as pre-syncope and syncope.
What are the 2 clinical findings of type 1 2nd degree AV block?
- Irregular pulse
- Bradycardia
What is the most common cause of second degree heart block?
myocardial infarction (particular anterior MIs)
What is a Second-degree AV block (type 2) also known as?
Mobitz type 2 AV block.
What causes a Mobitz type 2 AV block?
Mobitz type 2 AV block is always pathological, with the block typically occurring at either the bundle of His (20%) or the bundle branches (80%).
Causes of second-degree AV block (type 2) include:3
Myocardial infarction
Idiopathic fibrosis of the conducting system (Lenegre’s or Lev’s disease)
Cardiac surgery (especially surgery occurring close to the septum such as mitral valve repair)
Inflammatory conditions (rheumatic fever, myocarditis, Lyme disease)
Autoimmune (SLE, systemic sclerosis)
Infiltrative myocardial disease (amyloidosis, haemochromatosis, sarcoidosis)
Hyperkalaemia
Drugs (e.g. beta-blockers, calcium channel blockers, digoxin, amiodarone)
Thyroid dysfunction
Is complete heart bock risk higher in Mobitz I or Mobitz II?
Mobitz II (hence a pacemaker is recommended)
What are the ECG finding for a 2nd degree AV block type 2?
Rhythm: irregular (may be regularly irregular in 3:1 or 4:1 block)
P wave: present but there are more P waves than QRS complexes
PR interval: consistent normal PR interval duration with intermittently dropped QRS complexes
QRS complex: normal (<0.12 seconds) or broad (>0.12 seconds)
The QRS complex will be broad if the conduction failure is located distal to the bundle of His
What symptoms of 2nd degree type 2 AV block are typically found?
- Palpitations
- Pre-syncope
- Syncope
What is the management of 2nd degree AV block type 2?
Because of the risk of progression to complete AV block, patients should be placed on a cardiac monitor as soon as possible.
The underlying cause of the AV block should be investigated.
Temporary pacing or isoprenaline may be required if the patient is haemodynamically compromised due to bradycardia.
A permanent pacemaker is usually inserted if there are no reversible causes identified.
What are the complications of a 2nd degree type 2 AV block?
Patients are at risk of progressing to symptomatic complete AV block, in which the escape rhythm is likely to be ventricular and thus too slow to maintain adequate systemic perfusion.
Patients are also at risk of developing asystole.
What is 2:1 or 3:1 (advanced) block?
where every second or third P wave conducts to the ventricles
What is third-degree heart block?
complete heart block that occurs when all atrial activity fails to conduct the ventricles due to complete dissociation between atrial and ventricular complexes
How does third-degree heart block present on ECG?
there is no relationship between P and QRS waves
Where do narrow-complex escape rhythms typically occur?
(QRS complexes of <0.12 seconds duration) originate above the bifurcation of the bundle of His. A typical heart rate would be >40bpm.
where do broad-complex escape rhythms originate?
(QRS complexes >0.12 seconds duration) originate from below the bifurcation of the bundle of His. These escape rhythms produce slower, less reliable heart rates and more significant clinical features (e.g. heart failure, syncope).
Give 2 congenital causes of complete heart block:
1) structural heart disease
2) autoimmune conditions such as SLE
Give 3 drugs that can induce complete heart block:
1) digoxin
2) Ca2+ channel blockers
3) beta blockers
Give 2 non-ischaemic heart disease associated causes of complete heart block:
1) aortic stenosis
2) idiopathic dilated cardiomyopathy
Give 2 infections that can cause complete heart block:
1) Lyme disease
2) infective endocarditis
Give 2 ischaemic heart disease causes of complete heart block:
1) ischaemic cardiomyopathy
2) acute MI
What does a narrow QRS (<0.12) originating from the His bundle tell you about the nature of a complete heart block?
the block lies more proximally in the AV node
How should acute narrow QRS complete heart block be treated?
atropine
How should chronic narrow QRS complete heart block be treated?
pacemaker
What does a broad QRS (>0.12) tell you about the nature of complete heart block?
the escape rhythm originates below the His bundle and therefore the region of block lies more distally in the His-Purkinje system
Give 2 causes of the broad complex complete heart block in the elderly?
- Degenerative fibrosis
- Calcification of the distal conducting system
What is the most common cause of broad complex complete heart block in middle aged people?
Lenegre’s disease
What is Lenegre’s Disease?
progressive sclerodegenerative disease of the cardiac conduction system
How is broad complex complete heart block treated?
pacemaker
