The conduction system: nodes, bundles and heart block

1. Sinus node: the pacemaker

The sinoatrial node lies under the epicardium where the superior vena cava joins the right atrium. It fires about 60–100 times a minute; the impulse spreads through both atria (the P wave) to the AV node.

Surgeon’s corner: Its artery comes from the right coronary artery in about 60% and the circumflex in about 40%: sinus bradycardia and pauses are common in inferior infarction. Symptomatic sinus node disease is a frequent indication for a pacemaker.

2. AV node: the delay

In the floor of the right atrium, the AV node sits at the apex of Koch’s triangle (formed by the coronary sinus ostium, the tendon of Todaro and the septal leaflet of the tricuspid valve), just below the membranous septum. It slows the impulse so the ventricles can fill: the PR interval, normally 120–200 ms.

Surgeon’s corner: First-degree AV block (PR > 200 ms) and Wenckebach block with a narrow QRS usually arise in the AV node and are benign; with a wide QRS the block may be infranodal. The AV nodal artery comes from the dominant artery at the crux — the right coronary artery in about 80–90%.

3. Bundle of His and second-degree block

The bundle of His is the only normal electrical bridge between atria and ventricles: it pierces the central fibrous body and runs on the crest of the muscular septum before dividing. Compare the two kinds of second-degree block in the panel: Mobitz I (PR lengthening, nodal) and Mobitz II (constant PR, sudden drop, below the node).

Surgeon’s corner: Mobitz II and complete heart block below the AV node are unstable: the escape rhythm is slow and unreliable — pacing. Atropine helps nodal block but not infranodal block, which it can worsen (by speeding the sinus rate): use isoprenaline or pacing.

4. Bundle branches

The right bundle branch runs as a thin cord down the right side of the septum and through the moderator band to the anterior papillary muscle. The left bundle branch fans out as a broad sheet on the left side of the septum. A blocked branch makes its ventricle depolarise late, through muscle: the QRS widens to 120 ms or more. Compare RBBB and LBBB in V1 and V6.

Surgeon’s corner: LBBB, RBBB or a paced rhythm with ongoing ischaemic symptoms is managed like a STEMI, whether or not the block is new (ESC 2023). LBBB with symptomatic heart failure and an ejection fraction ≤ 35% despite optimal medical therapy: cardiac resynchronisation therapy, strongest when QRS ≥ 150 ms (ESC 2021).

5. The fascicles and the axis

The left bundle divides into a thin anterior fascicle, running to the anterolateral papillary muscle, and a short, thick posterior fascicle to the posteromedial papillary muscle. Anterior fascicular block swings the axis left (−45° to −90°); posterior fascicular block, rare, swings it right.

Surgeon’s corner: The anterior fascicle has a single supply (LAD septal branches) and fails often; the posterior has a double supply and rarely does. RBBB with left anterior fascicular block is a bifascicular block: with syncope, look for intermittent complete block.

6. Complete heart block

When no impulse crosses, atria and ventricles beat independently: regular P waves march through a slow escape rhythm. A junctional escape (narrow QRS, 40–60/min) comes from near the His bundle; a ventricular escape (wide QRS, 20–40/min) from below — slower and less reliable.

Surgeon’s corner: Complete, high-degree or Mobitz II AV block not due to a reversible cause needs a permanent pacemaker even without symptoms (ESC 2021). If haemodynamically unstable, transcutaneous or transvenous pacing as a bridge. Exclude reversible causes first: inferior infarction (often transient — reperfuse and wait several days before deciding), drugs (beta-blockers, calcium-channel blockers, digoxin), hyperkalaemia, Lyme disease.