Atrioventricular node
AV node · 40–60/min RCA ~90% ~100 ms delay
Compact node of slow-response cells in the triangle of Koch — bounded by the tendon of Todaro, the septal tricuspid leaflet and the coronary sinus ostium — at the base of the interatrial septum. It is the only normal electrical connection through the fibrous annulus and it deliberately delays the impulse about 100 ms so that atrial contraction finishes before the ventricles fire.
Traced from the start
Detail
- Intrinsic rate
- 40–60/min (junctional escape), normally suppressed by the SA node
- Conduction velocity
- ~0.05 m/s, the slowest tissue: L-type Ca2+-dependent slow-response action potentials with a long, rate-dependent refractory period — decremental conduction that shields the ventricles from atrial rates above ~200/min
- Delay
- ~100 ms, most of the PR interval (normal 120–200 ms)
- Arterial supply
- AV nodal artery from the RCA at the crux in ~85–90% (right-dominant hearts), from the LCx in ~10–15%; supplemented by LAD septal perforators
- Dual pathways
- Fast pathway (anterosuperior; rapid conduction, long refractory period) and slow pathway (posteroinferior along the tricuspid annulus near the coronary sinus; slow conduction, short refractory period)
- Autonomic / drugs
- Vagus (left vagus preferentially) slows conduction and lengthens refractoriness; adenosine (A1), beta-blockers, verapamil/diltiazem and digoxin all act here
When it goes wrong
Slowed nodal conduction from vagal tone, athletic training, beta-blockers, digoxin, verapamil, inferior MI or myocarditis
First-degree AV block: PR above 200 ms with every P conducted — benign, no treatment
Progressive fatigue of nodal cells (same causes; inferior MI, rheumatic carditis, post-cardiac surgery)
Mobitz I (Wenckebach): PR lengthens beat by beat until a P is dropped, grouped beating with shortening RR intervals; usually benign, narrow QRS, responds to atropine, rarely needs pacing
Re-entry between the slow and fast pathways after a premature atrial beat (slow–fast AVNRT, the commonest SVT in adults)
Regular narrow-complex tachycardia 140–250/min with P waves hidden in or just after the QRS (pseudo-R' in V1, pseudo-S in II, III, aVF); terminates with vagal manoeuvres or adenosine; slow-pathway ablation is curative
Complete block within the node (inferior MI, drug toxicity, congenital heart block)
Third-degree block with a junctional escape: narrow QRS at 40–60/min, often transient and atropine-responsive; far better prognosis than infranodal block
Atrial fibrillation filtered by intact decremental conduction
Ventricular rate ~110–180/min; rate control with a beta-blocker, diltiazem/verapamil or digoxin — every one of them acting on the node
Digoxin toxicity or nodal ischaemia enhancing junctional automaticity
Junctional rhythm 40–60/min (or accelerated junctional rhythm 60–100/min) with narrow QRS and absent or retrograde P; a regular rhythm appearing in known AF signals digoxin toxicity
Practise this structure
1 question in the bank tagged Atrioventricular Node.