First-degree AV block: prolonged PR, all beats conduct
First-degree AV block is defined by a PR interval greater than 200 ms (5 small boxes) with every P wave followed by a QRS complex — no beats are dropped. The conduction is delayed but not blocked. This is usually a benign finding and commonly occurs with: increased vagal tone (athletes, during sleep), inferior MI, AV node dysfunction from aging, medication effects (digoxin, beta-blockers, calcium channel blockers), and Lyme carditis.
First-degree AV block alone rarely causes symptoms or hemodynamic compromise. However, in the context of existing conduction system disease (bundle branch block), a newly prolonged PR interval may indicate worsening infranodal disease. In inferior MI, first-degree block progression to higher-degree block warrants close monitoring.
Complete (third-degree) heart block: AV dissociation and escape rhythms
Complete (third-degree) heart block is complete failure of AV conduction — no atrial impulses conduct to the ventricles. The atria and ventricles beat independently: P waves march at the sinus rate, QRS complexes march at the escape rhythm rate, and the two are completely dissociated.
The escape rhythm source determines QRS width and rate, and the hemodynamic risk. Junctional escape (block at the AV node level): rate 40–60 bpm, narrow QRS, more reliable and stable. Ventricular escape (block at infranodal level): rate 20–40 bpm, wide QRS, less reliable, more dangerous — pauses and abrupt asystole are risks.
Causes: Inferior MI (AV nodal artery ischemia, often reversible), anterior MI with septal infarction destroying the bundle branches (serious, often permanent), Lyme carditis, cardiac sarcoidosis, idiopathic degeneration of conduction system (Lev's disease, Lenègre's disease), digoxin toxicity, and calcific aortic valve disease extending to the conduction system.
Management: transcutaneous pacing for hemodynamic compromise while arranging transvenous pacing. Atropine has limited effect on infranodal complete heart block. Permanent pacemaker is indicated for persistent complete heart block not from a reversible cause.