Conduction Blocks Flashcards
Define Bradycardia?
Bradycardia is defined as a heart rate below 60 beats per minute, which can either be physiological or due to dysfunction of the conduction pathway (sinus node or atrioventricular (AV) node conduction dysfunction).
What is a heart block?
Heart block block is a cardiac electrical disorder defined as impaired (delayed or absent) conduction from the atria to the ventricles.
The severity of the conduction abnormality is described in degrees.
What is the aetiology of first degree heart block?
- LEV’s disease
- This is the result of proximal bundle branch calcification or fibrosisand is often described as senile degeneration of the conduction system.
- Ischaemic heart disease
- scar tissue from myocyte death blocks conduction pathway
- Myocarditis
- Hypokalaemia
What is the aetiology of second degree heart block?
- Natural aging process.
- Damage to the heart from surgery.
- Damage to the heart muscle from a heart attack.
- Other types of heart disease that damage the heart muscle.
- Low thyroid levels.
- Electrolyte abnormalities.
- Inflammatory or infectious heart conditions.
What is the aetiology of second degree heart block?
- Natural aging process.
- Damage to the heart from surgery.
- Damage to the heart muscle from a heart attack.
- Other types of heart disease that damage the heart muscle.
- Low thyroid levels.
- Electrolyte abnormalities.
- Inflammatory or infectious heart conditions.
What is the aetiology of third degree heart block?
- Coronary heart disease
- Infection
- Hypertension
What is the aetiology of third degree heart block?
- Coronary heart disease
- Infection
- Hypertension
What is the epidemiology of heart block?
The incidence of AV conduction abnormalities increases with age, corresponding with the age-related incidence of ischaemic heart disease.
What is the pathophysiology of heart block?
- A heart block does not mean that the heart stops beating altogether, because other ‘back-up’ systems can take over. However, it can result in an abnormally slow heartbeat (bradycardia), leading to the body’s organs and tissue becoming deprived of oxygen
Asymptomatic bradycardia does not usually require treatment. However, there are several different types of bradycardia which can be categorised as below:
Sinus node dysfunction may be associated with sinus bradycardia, tachycardia-bradycardia, and sick sinus syndrome.
Atrioventricular(AV) conduction disturbances can either be due to poor transmission of atrial depolarisation to the ventricles or a delay in atrial depolarisation.
What is first degree AV block?
- PR interval >0.2s
- Can be normal, e.g. in athletes. May represent pathology, e.g. inferior MI
- As an isolated finding often has little significance
First-degree AV block: occurs where there is delayed atrioventricular conduction through the AV node but every atrial impulse leads to a ventricular contraction.
Block at the level of the AV node manifesting as first-degree or type I second-degree AV block is usually a function of either high vagal tone or medication.
First-degree heart block; PR interval is approximately 200ms.
What is second degree AV block?
Second-degree AV block**(failure of conduction from atria to ventricles):
- Mobitz type 1 (Wenckebach)
Increasing PR interval culminating in a dropped QRS complex. The PR interval resets and the cycle repeats
- Due to reversible conduction block at the level of the AV node
- AV nodal cells progressively fatigue until they fail to conduct
- Can be normal, e.g. in athletes. May represent pathology, e.g. inferior MI
- As an isolated finding often has little significance
- Generally caused by AV node block and results in progressive PR interval prolongation until a P wave fails to conduct and a QRS is absent after the first P wave (dropped beat)
- Eventually ventricle’s pacemaker cells kick in
- The PR interval then returns to normal and the cycle repeats itself
- Patient often experiences light headedness, dizziness and syncope
**Mobitz type I / Wenckebach**: atrial inputs becomes gradually weaker until it does not pass through the AV node. After failing to stimulate a ventricular contraction the atrial impulse returns to being strong.
- Mobitz type II:
The PR interval remains constant, but with intermittent dropped QRS complexes (2:1, 3:1, etc)
- Due to disease of the His-Purkinje system → AV depolarisation does not result in appropriate ventricular depolarisation
- Usually due to structural heart damage, e.g. myocardial infarct
- High risk of sudden cardiac death
- Requires treatment
- Caused by a block at an intra-nodal level so the QRS is widened and QRS complexes are dropped without PR prolongation
- Fairly random dropped beats – no PR elongation
**Mobitz type II**: Usually due to disease of the His-Purkinje system which causes intermitted failure or interruption of AV conduction. This results in missing QRS complexes. There is usually a set ratio of P waves to QRS complexes.
Patients often experience chest pain, SOB, syncope and postural hypotension
**Mobitz type I shows an increasing PR interval, followed by a dropped beat. Mobitz type II shows a constant PR interval with an occasional dropped beat.**
**Mobitz type II with 2:1 block, i.e. every other beat is 'dropped'.**
What is third degree AV block?
- Complete absence of AV conduction. None of the supraventricular impulses are conducted to the ventricles
- P wave and QRS complex are completely dissociated
- Usually due to His-Purkinje disease
- High risk of sudden cardiac death
- Requires treatment
- In third-degree AV block, no atrial impulses could reach the ventricle.
- It can occur in the AV node or in the infranodal specialized conduction system.
- So it occurs when there is complete dissociation between atrial and ventricular activity. E.g. atrial BPM is 60, ventricular is 30 BPM
Third-degree AV block: complete heart block. This is no observable relationship between P waves and QRS complexes.
Complete heart block: no association between P-waves and QRS complexes.
What are the symptoms of heart block?
- Dizziness
- Fatigue
- Shortness of breath
- Syncope
What are the symptoms of heart block?
- Dizziness
- Fatigue
- Shortness of breath
- Syncope
What are the symptoms of first degree heart block?
- The least serious is 1st-degree heart block, which may not cause any symptoms.
- chest pain.
- shortness of breath.
- feeling very dizzy suddenly when standing up from a lying or sitting position – this is caused by having low blood pressure (hypotension)
