First degree long PR, second degree some dropped beats, third degree complete dissociation
Mnemonic
“Longer, longer, longer, drop, then you have a Wenckebach” and “If some Ps do not get through, then you have a Mobitz II”:
- First degree - PR over 200 ms, constant. Every P conducts. Usually benign
- Second degree Mobitz I (Wenckebach) - PR progressively lengthens until a beat is dropped. Nodal, usually benign, often vagal
- Second degree Mobitz II - constant PR with intermittent dropped beats. Infranodal and dangerous, progresses to complete block, needs pacing
- Third degree (complete) - atrioventricular dissociation. P waves and QRS complexes march independently, with an escape rhythm
“Mobitz I is a nuisance, Mobitz II is a pacemaker.”
In complete block the escape rhythm determines the risk: a narrow junctional escape at 40 to 60 is relatively stable; a broad ventricular escape at 20 to 40 is unreliable and demands urgent pacing.
Expansion
- First degree - PR interval over 200 ms, every P conducted. Usually benign
- Second degree, Mobitz I (Wenckebach) - progressive PR lengthening until a beat is dropped. Block is in the atrioventricular node; often vagal, usually benign
- Second degree, Mobitz II - constant PR with sudden non-conducted P waves. Block is below the node, in the His-Purkinje system; unstable and likely to progress
- Third degree (complete) - no relationship between P waves and QRS complexes; an escape rhythm maintains the ventricles
The clinically important divide is between Mobitz I and Mobitz II. The former usually needs no treatment; the latter, and complete block, carry a risk of asystole and generally require pacing.
In complete block, look for cannon a waves in the jugular venous pulse and a variable intensity first heart sound, both caused by atria contracting against closed atrioventricular valves.