Second Degree Heart Block

There are two types of second degree heart block:

Type 1 (Mobitz I or Wenckebach)
Is when the PR interval lengthens each time and eventually after the longest PR interval there is a missing QRS complex, this cycle then repeats

The difference is more obvious if you compare the first PR interval in the cycle to the last.

Mobitz I is usually due to reversible conduction block at the level of the AV node:
- Malfunctioning AV node cells tend to progressively fatigue until they fail to conduct an impulse
- This is different to cells of the His-Purkinje system which tend to fail suddenly and unexpectedly (i.e. producing a Mobitz II block)

Causes of Mobitz I
- Drugs: beta-blockers, calcium channel blockers, digoxin, amiodarone
- Increased vagal tone (e.g. athletes) - hence can be a normal finding
- Inferior MI
- Myocarditis
- Following cardiac surgery (mitral valve repair, Tetralogy of Fallot repair)

Clinical Significance

Mobitz I is usually a benign rhythm, causing minimal haemodynamic disturbance
and with low risk of progression to third degree heart block

- Asymptomatic patients do not require treatment
- Symptomatic patients usually respond to atropine
- Permanent pacing is rarely required

Type 2 (Mobitz II) 
Is when there are non-conducted p-waves (not preceded by a QRS complex), 
there is no PR prolongation and the QRS complex is usually wide

Mobitz II is an “all or nothing” phenomenon whereby the His-Purkinje cellssuddenly and unexpectedly fail to conduct a supraventricular impulse

The PR interval in the conducted beats remains constant

The RR interval surrounding the dropped beat(s) is an exact multiple of the preceding 
RR interval (e.g. double the preceding RR interval for a single dropped beat, treble for two dropped beats, etc)

Mechanism

Mobitz II is usually due to failure of conduction at the level of the His-Purkinje system (i.e. below the AV node)

While Mobitz I is usually due to a functional suppression of AV conduction 
(e.g. due to drugs, reversible ischaemia), Mobitz II is more likely to be due 
to structural damage to the conducting system (e.g. infarction, fibrosis, necrosis)

Patients typically have a pre-existing LBBB or bifascicular block, and the 
2nd degree AV block is produced by intermittent failure of the remaining fascicle (“bilateral bundle-branch block”)

In around 75% of cases, the conduction block is located distal to the Bundle of His,
producing broad QRS complexes

In the remaining 25% of cases, the conduction block is located within the His Bundle itself, producing narrow QRS complexes

Causes of Mobitz II

- Anterior MI (due to septal infarction with necrosis of the bundle branches)
- Idiopathic fibrosis of the conducting system (Lenegre’s or Lev’s disease)
- Cardiac surgery (especially surgery occurring close to the septum, e.g. mitral valve repair)
- Inflammatory conditions (rheumatic fever, myocarditis, Lyme disease)
- Autoimmune (SLE, systemic sclerosis)
- Infiltrative myocardial disease (amyloidosis, haemochromatosis, sarcoidosis)
- Hyperkalaemia
- Drugs: beta-blockers, calcium channel blockers, digoxin, amiodarone

Clinical Significance of Mobitz II

Mobitz II is much more likely than Mobitz I to be associated with haemodynamic compromise, severe bradycardia and progression to 3rd degree heart block

Onset of haemodynamic instability may be sudden and unexpected, 
causing syncope (Stokes-Adams attacks) or sudden cardiac death

The risk of asystole is around 35% per year

Treatment: Mobitz II mandates immediate admission for cardiac monitoring, 
backup temporary pacing and insertion of a permanent pacemaker