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Accessory pathway (bundle of Kent)

AV node

Congenital strand of working myocardium bridging the fibrous atrioventricular ring and inserting directly into ventricular muscle, bypassing the AV node and bundle of His. It conducts fast and without decrement, so atrial impulses reach the ventricle early (pre-excitation) and can form a re-entry loop with the normal conduction axis.

Traced from the start

  1. Sinoatrial node
  2. Right atrial myocardiumLeft atrial myocardium
  3. Accessory pathway (bundle of Kent)

Detail

Location
Left free wall ~50%, posteroseptal ~30–35%, right free wall ~10%, anteroseptal ~3–5%
Conduction
Fast and all-or-none — it does not filter rapid atrial rates the way the AV node does
ECG in sinus rhythm (WPW pattern)
PR under 120 ms, delta wave (slurred initial QRS), QRS above 110 ms, discordant ST–T changes; pseudo-infarct Q waves are common
Associations
Ebstein's anomaly (right-sided pathways), hypertrophic cardiomyopathy

When it goes wrong

Orthodromic AVRT — antegrade down the AV node, retrograde up the pathway (over 90% of AVRT)

Regular narrow-complex tachycardia 150–250/min with a retrograde P after the QRS; the delta wave vanishes during the tachycardia; vagal manoeuvres and adenosine terminate it

Antidromic AVRT — antegrade down the pathway, retrograde up the node

Regular broad-complex tachycardia indistinguishable from VT; treat as VT when in doubt

Atrial fibrillation conducted down the pathway (pre-excited AF)

Irregular broad-complex tachycardia often above 200/min with beat-to-beat QRS variation that can degenerate to VF — DC cardioversion if unstable; if stable, procainamide, ibutilide or flecainide; never adenosine, verapamil/diltiazem, beta-blockers or digoxin, because blocking the node drives every impulse down the pathway

Symptomatic WPW or a high-risk pathway (shortest pre-excited RR under 250 ms in AF)

Catheter ablation of the pathway is first-line and curative; untreated, a small risk of sudden cardiac death

Practise this structure

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