Conducting System Anatomy
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Direct answer
Every normal heartbeat originates at the junction of the superior vena cava and the right atrium, where the sinoatrial node — a 1-2 cm crescent at the upper end of the sulcus terminalis — fires 60-100 times a minute. The impulse reaches the atrioventricular node in Koch's triangle (tendon of Todaro, septal tricuspid leaflet, coronary sinus ostium), continues as the bundle of His through the central fibrous body, and splits in the interventricular septum into right and left bundle branches — the left dividing into anterior and posterior fascicles before fanning out as the subendocardial Purkinje network. The SA nodal artery comes from the right coronary in about 60%, the AV nodal in 80-90%.
What you must remember
- SA node: upper end of the crista terminalis at the SVC-RA junction; supplied by the SA nodal artery, from the right coronary artery in about 60% (the remainder from the circumflex).
- Koch's triangle: tendon of Todaro above, septal tricuspid leaflet below, coronary sinus ostium at the base — the AV node and slow pathway live here, making it the ablation target for AV nodal re-entrant tachycardia.
- Bundle of His: the only electrical bridge between atria and ventricles, piercing the central fibrous body; insulating fibrous skeleton elsewhere prevents accessory conduction.
- Left bundle fascicles: anterior fascicle to the anterosuperior septum and lateral wall, posterior fascicle to the posteroinferior wall — the basis of fascicular (hemiblock) patterns on ECG.
- AV nodal artery: from the right coronary artery in 80-90% of hearts, which is why inferior infarcts produce nodal (narrow-complex) AV block.
- Purkinje network: subendocardial fast-conducting cells delivering the impulse apex-ward, responsible for the rapid coordinated ventricular depolarisation seen as a narrow QRS.
- Autonomic gear: vagal efferents slow the SA node and delay AV conduction (bradycardia, athletic heart); sympathetic efferents do the opposite — the physiological basis of vasovagal syncope on one end and exercise tachycardia on the other.
Localising a block from anatomy to ECG
Work through three ward patients. The first, with an inferior wall infarct from a right coronary occlusion, develops second-degree AV block with narrow QRS complexes: the AV node is ischaemic because its artery is occluded, but the His-Purkinje system below is intact, so the block is usually transient and rarely needs pacing beyond temporary support. The second, with a large anterior infarct, develops complete heart block with wide escape complexes: left anterior descending septal branches have destroyed the bundle branches, and a permanent pacemaker is generally required.
The third has no infarct but regular narrow-complex tachycardia. The electrophysiologist maps dual slow and fast pathways in Koch's triangle forming a re-entrant circuit around the AV node; radiofrequency energy to the slow pathway near the coronary sinus ostium cures the arrhythmia while preserving conduction. Every step of that procedure is pure anatomy: the triangle's borders, the nearby coronary sinus, the risk to the His bundle if energy is delivered too superiorly.
Where students slip
The recurring slips are positional. The SA node is placed "anywhere in the right atrium" instead of the precise upper end of the sulcus terminalis at the SVC junction — a location that also explains why sinus node dysfunction follows SVC-atrial surgery. The AV node is drawn inside the interatrial septum rather than in the right atrial floor of Koch's triangle, and the tendon of Todaro is omitted from the triangle although it is the boundary examiners ask for by name. On supply, candidates quote the SA nodal artery as exclusively right coronary; the defensible statement is about 60% right coronary, the rest circumflex, while the AV nodal artery is right coronary in the large majority. Finally, the assertion that the left bundle has "two fascicles" needs the caveat that a midseptal group of fibres is often described as a third, though classical teaching retains the anterior-posterior pair for ECG logic.
Frequently asked questions
Where exactly is the sinoatrial node located?
At the junction of the superior vena cava and right atrium, along the upper end of the sulcus terminalis (crista terminalis), just deep to the epicardium.
What are the boundaries of Koch's triangle?
The tendon of Todaro superiorly, the septal leaflet of the tricuspid valve inferiorly, and the coronary sinus ostium at the base — containing the AV node and its approaches.
Which artery supplies the AV node, and why does its occlusion cause heart block?
The AV nodal artery, arising from the right coronary artery in 80-90% of hearts, so inferior infarcts commonly produce transient narrow-complex AV nodal block.
What is the blood supply of the bundle branches?
Chiefly septal perforators of the left anterior descending artery with collateral contribution from the posterior descending artery, which is why anterior infarcts cause wide-complex complete block.
Why does the ECG show a narrow QRS in nodal block but a wide QRS in infra-His block?
Nodal block delays the impulse above the branching bundle, so ventricular activation still uses both bundle branches; infra-His block forces one ventricle to activate from the other through muscle, broadening the QRS.