Conduction disease
Complete (third degree) heart block
P waves and QRS complexes marching independently, with a slow escape rhythm keeping the patient alive.
What you see
- Complete AV dissociation: the PP and RR intervals are each regular but unrelated.
- The atrial rate exceeds the ventricular rate.
- A narrow escape at 40–60 suggests a junctional origin; a broad escape at 20–40 is ventricular and far less stable.
- P waves appear at varying points on the trace, and some are hidden in QRS complexes or T waves.
Why it looks like that
No atrial impulse reaches the ventricles at all, so a subsidiary pacemaker takes over at its own intrinsic rate. Where that pacemaker sits determines both the width of the complex and how reliable it is.
What to do
- Permanent pacemaker unless there is a clearly reversible cause.
- Reversible causes worth excluding: inferior MI, hyperkalaemia, Lyme carditis, drugs (beta-blockers, digoxin, calcium channel blockers).
- Atropine, then transcutaneous pacing or isoprenaline as a bridge; atropine rarely works in infranodal block.
The trap
Complete heart block during an inferior STEMI is usually nodal, narrow-complex and transient — it often needs nothing more than time and revascularisation. The same rhythm in an anterior STEMI means extensive septal infarction, carries a much worse prognosis, and needs pacing.
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.
Mobitz II AV block
P waves drop without warning while the PR interval stays fixed — infranodal disease, and a pacemaker indication.
Left bundle branch block
Broad QRS with a notched R in the lateral leads — and repolarisation you can no longer interpret at face value.