Conduction disease
Mobitz I (Wenckebach)
The PR interval lengthens beat by beat until one P wave fails to conduct.
What you see
- Progressive PR prolongation over successive beats.
- A dropped QRS after the most prolonged PR, giving grouped beating.
- The PR after the pause is the shortest of the cycle.
- The RR interval shortens progressively before the drop, because each PR increment is smaller than the last.
Why it looks like that
Fatigue within the AV node itself: each successive impulse arrives while the node is still partly refractory, until one finds it wholly refractory. Because the block is nodal rather than infranodal, the escape mechanism below it is usually reliable.
What to do
- Asymptomatic Wenckebach needs no treatment; it is often vagally mediated and common in athletes and at night.
- Review AV nodal blocking drugs.
- Pace only for symptoms — syncope, presyncope or symptomatic bradycardia.
The trap
Wenckebach with 2:1 conduction cannot be distinguished from Mobitz II on a single strip, because there are never two consecutive conducted beats to compare. A long rhythm strip, carotid sinus massage or exercise settles it — Mobitz I improves with exercise, Mobitz II worsens.
More in conduction disease
First degree AV block
A PR interval over 200 ms with every P wave still conducted.
Mobitz II AV block
P waves drop without warning while the PR interval stays fixed — infranodal disease, and a pacemaker indication.
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.