Conduction delays Flashcards
Describe first degree heart block
First-degree heart block involves a fixed prolonged PR interval (>200 ms).
Describe second degree AV block (type 1), also known as Mobitz type 1 AV block.
Typical ECG findings in Mobitz type 1 AV block include progressive prolongation of the PR interval until eventually the atrial impulse is not conducted and the QRS complex is dropped.
AV nodal conduction resumes with the next beat and the sequence of progressive PR interval prolongation and the eventual dropping of a QRS complex repeats itself.
Describe second degree AV block (type 2) is also known as Mobitz type 2 AV block.
Typical ECG findings in Mobitz type 2 AV block include a consistent PR interval duration with intermittently dropped QRS complexes due to a failure of conduction.
The intermittent dropping of the QRS complexes typically follows a repeating cycle of every 3rd (3:1 block) or 4th (4:1 block) P wave.
Describe 3rd degree heart block
Third-degree (complete) AV block occurs when there is no electrical communication between the atria and ventricles due to a complete failure of conduction.
Typical ECG findings include the presence of P waves and QRS complexes that have no association with each other, due to the atria and ventricles functioning independently.
In 3rd degree heart block how would you diferentiate between rhythm that origionate above and below the bundle of His?
- Narrow-complex escape rhythms (QRS complexes of <0.12 seconds duration) originate above the bifurcation of the bundle of His.
- Broad-complex escape rhythms (QRS complexes >0.12 seconds duration) originate from below the bifurcation of the bundle of His.
What is the significance of first degree heart block?
- A benign finding that is not associated with haemodynamic instability
- No specific treatment is required
Causes of first degree heart block?
- Normal variant
- Athletes
- Inferior MI (RCA supplies AVN)
- Electrolytes (hyperkalaemia)
- Myocarditis (Lyme disease)
- Drugs
- Beta blockers
- Digoxin
- Calcium channel blockers
Mobitz type 1
Pathophysiology
Significance
- Usually due to a reversible conduction block at the AVN
- AVN cells progressively fatigue until they fail to conduct an impulse
- Usually a benign rhythm with low risk of progression to complete heart block
Causes of Mobitz type 1
- Athletes
- Inferior MI (RCA supplies AVN)
- Myocarditis (Lyme disease)
- Drugs
- Beta blockers
- Digoxin
- Calcium channel blockers
Mobitz type 2
Pathophysiology
Significance
- His-Purkinjie cells intermittently fail to conduct an impulse. This is usually a structural issue
- Typicall patients will have a LBBB with intermittent failure of the right bundle
- There may be no pattern or a fixed block eg 2:1, 3:1
- Serious as it can cause haemodynamic compromise and tends to progress to complete heart block.
Causes of mobitz type 2
- Anterior MI (septal infarct)
- Myocarditis (Lyme disease)
- SLE
- Cardiac surgery (close to septum)
- Drugs
- Beta blockers
- Digoxin
- Calcium channel blockers
What is high grade AV block?
2nd degree AV block with P:QRS ratio of 3:1 or higher
Treatment of Mobitz type 2
- Admission
- Cardiac monitoring
- Temporary pacing
- Eventual permanent pacemaker
Definition of complete heart block (3rd degree heart block)
A severe bradycardia that arises due to complete absence of AV conduction
Pathophysiology of complete heart block
Significance
The atria cannot conduct impulses via the AVN, leading to cells distal to the block assuming pacemaker function. Their pacemaker rate is slower.
Severe risk of cardiac arrest - requires pacing, as per Mobitz type 2
Bradyarrhytmias: features which indicate risk of asystole
- Recent asystole
- Mobitz type 2 AV block
- Complete heart block (with broad QRS complex)
- Venticular pause >3 secs
Bradyarrhythmias interim measures
- Atropine 500micrograms IV (repeat to max of 3mg)
- Isoprenaline
- Adrenaline
- Alternative drugs eg aminophylline
OR
- Transcutaneous pacing
