Conduction disease
Mobitz II AV block
P waves drop without warning while the PR interval stays fixed — infranodal disease, and a pacemaker indication.
What you see
- Constant PR interval on all conducted beats.
- Intermittent, unheralded failure of a P wave to conduct.
- The QRS is often broad, reflecting disease below the AV node.
- The pause containing the dropped beat is an exact multiple of the PP interval.
Why it looks like that
Block within the His bundle or the bundle branches, where conduction is all-or-nothing rather than decremental. The escape rhythm below such a block is slow and unreliable, which is why the arrhythmic risk is entirely different from Mobitz I.
What to do
- Permanent pacing, irrespective of symptoms.
- Temporary pacing or isoprenaline if there is haemodynamic compromise while awaiting it.
- Atropine may worsen it — speeding the sinus rate increases the number of impulses the diseased tissue fails to conduct.
The trap
Mobitz II progresses to complete heart block without warning, so 'asymptomatic' is not reassurance. The exam frequently pairs it with a broad QRS and a story of unexplained falls.
More in conduction disease
First degree AV block
A PR interval over 200 ms with every P wave still conducted.
Mobitz I (Wenckebach)
The PR interval lengthens beat by beat until one P wave fails to conduct.
Complete (third degree) heart block
P waves and QRS complexes marching independently, with a slow escape rhythm keeping the patient alive.
Left bundle branch block
Broad QRS with a notched R in the lateral leads — and repolarisation you can no longer interpret at face value.