Cardiac Conduction System: Anatomy and Clinical Significance

Overview

The cardiac conduction system is a specialized network of modified cardiomyocytes capable of automaticity (spontaneous depolarization) and rapid impulse propagation. It ensures that the atria and ventricles contract in a coordinated, sequential manner to optimize cardiac output. Understanding its anatomy is essential for interpreting ECGs, managing arrhythmias, and performing cardiac interventions.

Components of the Conduction System

Sinoatrial (SA) Node

• Location: junction of superior vena cava and right atrium (crista terminalis), sub-epicardially
• Size: ~15 × 3 mm crescent-shaped structure
• Intrinsic rate: 60–100 bpm (fastest pacemaker — dominates rhythm)
• Blood supply: SA nodal artery from right coronary artery (55%) or left circumflex artery (45%)
• Innervation: vagal (slows) and sympathetic (accelerates) fibers

The SA node depolarizes spontaneously due to the funny current (If) — a mixed Na⁺/K⁺ inward current during diastole that gradually depolarizes the cell to threshold.

Internodal Pathways

Three preferential conduction pathways transmit impulses through the right atrium from the SA node to the AV node:

• Anterior internodal tract (Bachmann's bundle — to left atrium)
• Middle internodal tract (Wenckebach)
• Posterior internodal tract (Thorel)

Atrioventricular (AV) Node

• Location: floor of right atrium, triangle of Koch (bounded by: tendon of Todaro, septal leaflet of tricuspid valve, ostium of coronary sinus)
• Intrinsic rate: 40–60 bpm (escape pacemaker)
• Function: delays conduction by ~0.12 seconds to allow ventricular filling; acts as gatekeeper limiting atrial rate to ventricles
• Blood supply: AV nodal artery (90% from RCA posterior descending, 10% from LCX)
• AV nodal block: prolonged PR interval (1st degree), intermittent block (2nd degree Mobitz I/II), or complete block (3rd degree)

Bundle of His (Atrioventricular Bundle)

• Penetrates the central fibrous body (annulus fibrosus)
• Connects AV node to the ventricular conduction system
• Only normal electrical connection between atria and ventricles
• Bundle of Kent (accessory pathway in WPW syndrome) bypasses the AV node, causing pre-excitation (delta wave on ECG)

Bundle Branches

Right bundle branch (RBB):

• Thin, long, runs in the interventricular septum to the right ventricle and moderator band
• Susceptible to block (RBBB); often benign or associated with right heart disease

Left bundle branch (LBB):

• Short trunk divides into:
  • Left anterior fascicle: anterosuperior wall; thinner, more vulnerable
  • Left posterior fascicle: posteroinferior wall; thicker, better supplied

Purkinje Fibers

• Subendocardial network ramifying throughout ventricular walls
• Intrinsic rate: 20–40 bpm (last resort pacemaker)
• Rapid conduction velocity (~4 m/s) ensures near-simultaneous ventricular activation
• Activation: septum first (left to right), then apex to base, then base last

ECG Correlation

Conduction segment	ECG component
SA node depolarization	Not visible (isoelectric)
Atrial depolarization	P wave
AV nodal delay	PR interval (120–200 ms)
Ventricular depolarization	QRS complex (<120 ms)
Ventricular repolarization	ST segment + T wave

Blood Supply

Dual blood supply is a protective feature:

• AV node: typically dominant RCA (posterior descending artery)
• SA node: RCA or LCX
• Bundle branches: septal branches of LAD; this is why anterior STEMI frequently causes bundle branch block

Clinical Relevance

• Complete heart block (3rd degree AV block): P waves and QRS complexes completely dissociated; emergency pacemaker indication
• WPW syndrome: accessory pathway (Bundle of Kent) causes AVRT and delta wave; risk of sudden cardiac death in atrial fibrillation
• Sick sinus syndrome: SA node dysfunction causing inappropriate bradycardia, pauses, or tachycardia-bradycardia syndrome
• LBBB: complete left bundle branch block indicates myocardial disease; modifies STEMI criteria (Sgarbossa criteria)
• Radiofrequency ablation: targets accessory pathways and re-entrant circuits using anatomical landmarks

Study Points

• Trace the electrical impulse from SA node to Purkinje fibers naming each component.
• Explain the physiological basis of SA node automaticity (If current).
• Correlate each phase of the conduction system with ECG components.
• Describe the triangle of Koch and explain its surgical importance.
• Explain why AV nodal block is dangerous and how it presents clinically.