AV blocks Flashcards
Cardiac conduction system pathway
Impulses are generated by the SA node (intrinsic pacemaker).
These travel through the atria, producing the P wave on the ECG.
The AV node receives these signals and delays the stimulus, before passing it onwards.
The stimulus passes down via the bundle of His, bundle branches, and purkinje fibres.
The ventricles become depolarised, producing the QRS complex on the ECG.
What is AV block?
Impaired communication between the atria and ventricles due to dysfunction of the atrioventricular node.
An excessively long PR interval or changing relationship between the P wave and QRS complex represent dysfunction of the AV node… AV block.
We can think of the PR interval as representing the function of the AVN, delaying the stimulus.
What are the different types of AV blocks?
1st degree
2nd degree - Wenckebach, Mobitz 2
3rd degree - CHB
1st Degree AV Block - features
Considered more to be a delay in conduction, rather than an outright block.
Normal cardiac conduction occurs in the atrium, resulting in a P wave on the ECG.
The cardiac conduction system is delayed at the A-V node, with the signal taking longer than usual to pass onward to the ventricles.
This conduction delay results in a prolonged PR interval:
>0.2 seconds (>200ms)
All atrial impulses successfully pass through to the ventricles, so all P waves are followed by a QRS complex.
Often asymptomatic.
Symptoms, diagnostics and treatments of 1st degree AV block
Asymptomatic in the majority of cases
Diagnosis is often incidental during routine follow up or pre-op appointments. This may be made via a 12-lead ECG, or often via Holter monitoring or during other procedures e.g. echocardiogram
Rarely requires treatment, providing the patient is healthy and asymptomatic.
Review of medications (beta blockers, CCBs, digoxin etc)
Rare cases of pacemaker implant have been documented.
Features of 1st degree AV block on ECG
All P waves are followed by a QRS complex.
Long PR interval (>200ms).
Normal P wave and QRS morphology.
Regular ventricular rate.
What is 2nd Degree AV Block
Refers to a disorder of the cardiac conduction system…
Causing intermittent failure of conduction of atrial impulses to the ventricles (I.E. Some P waves are not followed by QRS complexes)
2nd Degree AV block is split into 2 types:
- MOBITZ TYPE I (Wenckebach) - MOBITZ TYPE II
Type II being progressively more serious than type I.
what is Wenckebach?
Intermittent failure of the AV node, whereby it is unable to conduct as quickly as it receives impulses from the atria.
Normal cardiac conduction occurs in the atrium = P wave on the ECG.
Conduction is delayed at the AV node, with the signal taking longer than usual to pass onward to the ventricles.
When the next atrial impulse arrives, the AV node has already fallen behind, so each signal takes longer than the last to conduct to the ventricles.
This results in the PR interval gradually prolonging, beat after beat.
Eventually the AV node is so far behind that it fails to conduct the impulse.
This results in a P wave with no following QRS complex – a dropped beat.
The AV node is able to recover as it has dropped a beat, and the following beat is conducted more quickly, with the PR interval shortening once again.
The cycle starts again, with the AV node falling further behind until it drops a beat and is able to recover.
Wenckebach ECG features
Heart rate = Normal OR Slow.
PR Interval – Progressively lengthens until conduction fails completely causing a dropped beat. The cycle then repeats.
P Waves – Normal and Regular.
QRS Complex – Narrow.
Wenckebach causes
Often a normal variant and seen in those with a high vagal tone with no evidence of structural heart disease. Can result from inferior MI, medication toxicity, hyperkalaemia, cardiomyopathy or post cardiac surgery.
Symptoms, diagnostics and treatment of wenckebach
Possibly asymptomatic. Some patient may exhibit shortness of breath on exertion, dizziness, syncope or chest pain.
Diagnosis is often incidental during routine follow up or pre-op appointments. This may be made via a 12-lead ECG, or often via Holter monitoring or during other procedures e.g. echocardiogram.
May not require treatment providing the patient is healthy and asymptomatic.
Review of medications (beta blockers, CCBs, digoxin etc)
In rare instances a pacemaker may be implanted
Angioplasty if CAD is a suspected cause.
2nd degree AV block – Mobitz II
Intermittent failure of conduction of atrial impulses through the AV node to the ventricles
There is no gradual prolongation of the P-R interval, stable P-R interval when AVN does conduct
Often quoted as a ratio of P waves to QRS complexes (e.g. 2:1, 3:1, or 4:1 block)
Can progress to complete heart block, increased risk of morbidity and mortality
Mobitz type II
Intermittent failure of conduction of atrial impulses to the ventricles without preceding lengthening of the PR interval.
Every 2nd, 3rd or 4th impulse fails to reach ventricles.
PR Interval of conducted beats is constant.
Result of impaired conduction below the AV node.
Mobitz II ECG features
Heart rate normal or slow.
PR interval normal or prolonged.
P Waves – Normal and Regular.
QRS complex – Narrow; regular or irregular.
Number of P waves to every QRS are quoted in a ratio - 2:1, 3:1 OR 4:1.
Mobitz type II symptoms, diagnostics and treatment
Symptoms may include shortness of breath, dizziness or syncope, tiredness, chest pain.
Diagnosis may be made via a 12-lead ECG, or often via Holter monitoring or during other procedures e.g. echocardiogram.
Significant risk of progressing on to complete heart block.
Treatments may include:
Review/ withdrawal of medications (beta blockers, CCBs, digoxin etc)
Temporary pacing wire if the patient is unstable
Angioplasty if CAD is a suspected cause
Permanent pacemaker implant if no reversible cause is found.
3rd degree AV block – Complete heart block
No signals pass via the AV node.
Total lack of communication between the atria and ventricles – they are independent of one another (AV dissociation).
The atrial rate is still dictated by the SA node (usually between 60-100bpm).
The ventricular rate is dictated by a slower escape rhythm from a subsidiary pacemaker site (usually 20-40bpm) – usually broader QRS.
Usually results in haemodynamic compromise, high degree of mortality and morbidity.
3rd degree ECG features
Independent P wave and QRS activity.
Atrial rate: 60-100 BPM (NORMAL & REGULAR)
Ventricular rate: 25-40 BPM (SLOW, REGULAR & WIDE)
P-R Interval - NONE
Pay close attention to P waves falling at the same time as or very shortly after the QRS complex
Some T waves may look different – either broader and notched or tall and peaked
CHB symptoms, diagnostics, treatment
Symptoms include presyncope, syncope, shortness of breath, tiredness, confusion, chest pain, sudden death.
Diagnosis may be made via a 12-lead ECG, or via Holter monitoring. Rarely an incidental finding.
Treatments may include:
Review/ withdrawal of medications (beta blockers, CCBs, digoxin etc)
Urgent temporary pacing wire
Angioplasty if CAD is a suspected cause
Permanent pacemaker implant if no reversible cause is found.
AF with Complete Heart Block features
AF i.e. Fibrillation waves, no P waves
But … Regular, slow ventricular rate (<40bpm).
Causes of AV blocks
The mostcommon causeis idiopathic fibrosis and sclerosis of the conduction system.
CAD
Medically induced atrioventricular (AV) block
Electrolyte abnormalities (hypokalaemia, hypomagnesaemia)
Neuromuscular disorders
Infiltrative diseases e.g. sarcoidosis
Genetic - mutations in the SCN5A gene have been linked to familial AV block.
Increased vagal tone (Prevalence rates of up to 10% in young athletes).
2021 ESC guidelines pacing
- not recommended in pts with AVB due to transient causes that can be corrected and prevented
- Pacing is indicated in patients with SR with permanent or paroxysmal third- or second-degree type II, infranodal 2:1, or high degree AVB irregardless of symptoms
What is increased vagal tone?
Vagal tone refers to activity of the vagus nerve, the 10th cranial nerve, which is a key part of the parasympathetic nervous system, often referred to as the “rest and digest” system.
A higher vagal tone means the vagus nerve is more active, leading to:
Lower Heart Rate: Increased vagal activity slows down the heart rate.
Increased Heart Rate Variability (HRV): HRV measures the variation in time between heartbeats, and higher vagal tone is associated with greater HRV, indicating a more flexible and adaptable cardiovascular system
- common in athletes
