Conduction disease
First degree AV block
A PR interval over 200 ms with every P wave still conducted.
What you see
- PR interval greater than 200 ms — more than one large square.
- Every P wave is followed by a QRS; the ratio stays 1:1.
- The PR interval is constant from beat to beat.
Why it looks like that
Conduction through the AV node is slowed but never fails. Increased vagal tone, AV nodal blocking drugs, ischaemia and infiltrative disease all do it; in athletes it is usually physiological and disappears on exercise.
What to do
- Usually none. Review rate-limiting drugs and check potassium.
- Marked prolongation (over 300 ms) can cause symptoms if atrial systole falls too close to the preceding ventricular one.
- Follow up if it coexists with bifascicular block, which suggests trifascicular disease.
The trap
It is easy to dismiss, but first degree block plus right bundle branch block plus left anterior hemiblock is 'trifascicular block' — and in a patient with syncope that combination is a pacemaker discussion, not a reassurance.
More in conduction disease
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.
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.