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AN22.1-7 | Heart & Pericardium — Part 3
The Conducting System — The Heart's Own Pacemaker (AN22.7)
Here's something that amazes students: the heart has its own built-in electrical system. It doesn't need the brain to beat — a transplanted heart (with all its nerves cut) still beats rhythmically. This is because of the conducting system — specialised cardiac muscle cells that generate and conduct electrical impulses.
The conducting system works in sequence, like a relay race:
1. Sinoatrial (SA) node — the heart's natural pacemaker. Located in the right atrium, at the junction of the SVC and the right auricle (near the crista terminalis). It generates impulses at 60–100 beats per minute. The SA node sets the heart rate.
- Blood supply: SA nodal artery — from the RCA (60%) or LCx (40%)
- Interatrial pathways — the impulse spreads through the atrial muscle, causing both atria to contract (atrial systole). Three internodal tracts (anterior, middle, posterior) carry the impulse preferentially to the AV node.
3. Atrioventricular (AV) node — located in the interatrial septum, just above the tricuspid valve (in the triangle of Koch — bounded by the coronary sinus, tendon of Todaro, and tricuspid annulus). The AV node has a critical function: it delays the impulse by about 0.1 seconds. This delay ensures the atria finish contracting before the ventricles begin — allowing complete ventricular filling.
- Blood supply: AV nodal artery — from the RCA (80%) or LCx (20%)
- Intrinsic rate: 40–60 bpm (if the SA node fails, the AV node takes over as a backup pacemaker)
4. Bundle of His — the only electrical pathway through the fibrous skeleton (remember: the fibrous skeleton insulates atria from ventricles). The bundle passes through the membranous part of the interventricular septum and immediately divides into:
- Right bundle branch — travels along the right side of the septum, reaches the anterior papillary muscle via the moderator band (septomarginal trabecula)
- Left bundle branch — divides into anterior and posterior fascicles, spreads across the left side of the septum
- Purkinje fibres — the terminal network that distributes the impulse rapidly across the ventricular myocardium, from endocardium (inside) to epicardium (outside). This ensures both ventricles contract almost simultaneously.
The complete sequence: SA node → atrial muscle → AV node (delay) → Bundle of His → right and left bundle branches → Purkinje fibres → ventricular muscle (ventricular systole)
The key concept: The SA node is the pacemaker, the AV node is the gatekeeper (with its crucial delay), and the His-Purkinje system is the rapid distribution network.
. Brave (ecgwaves.com). Used for educational purposes.
SELF-CHECK
A patient with an inferior MI (RCA occlusion) develops a very slow heart rate (35 bpm). Which part of the conducting system is most likely damaged, and why is the RCA responsible?
A. SA node; the RCA supplies the SA node in all hearts
B. AV node; the RCA supplies the AV node in approximately 80% of hearts
C. Bundle of His; the RCA is the sole supply to the interventricular septum
D. Purkinje fibres; the RCA supplies the ventricular endocardium
Reveal Answer
Answer: B. AV node; the RCA supplies the AV node in approximately 80% of hearts
The AV node is supplied by the AV nodal artery, which arises from the RCA in approximately 80% of hearts. An inferior MI (RCA occlusion) can therefore damage the AV node, causing heart block — the impulse from the atria cannot reach the ventricles properly, resulting in a dangerously slow heart rate. This is why inferior MIs are often associated with bradycardia.
CLINICAL PEARL
The 'widow-maker': Complete occlusion of the LAD artery (especially its proximal segment before the first diagonal branch) is nicknamed the 'widow-maker' because it supplies such a large territory — the entire anterior wall, the apex, and the anterior 2/3 of the septum. Sudden LAD occlusion causes massive anterior MI, often leading to ventricular fibrillation and sudden cardiac death. In India, where the 'golden hour' for reaching a catheterisation lab is often missed due to distance and delay, LAD occlusions are disproportionately fatal compared to Western countries. This is why the government's push for primary PCI centres (percutaneous coronary intervention — opening the blocked artery with a stent) in district hospitals is so critical. Every minute of delay in opening a blocked LAD costs myocardium — time is muscle.