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Lesson 2 of 21 · Fundamentals

The Conduction System

Electrical impulses follow a fixed highway from the SA node to the ventricular muscle. Knowing the order, and the intrinsic rate of each pacemaker, explains every escape rhythm and block.

01The pathway

  • SA node (high right atrium) fires and depolarizes the atria → P wave
  • AV node delays conduction ~0.1 s so the ventricles can fill → PR segment
  • Bundle of His → right and left bundle branches (left splits into anterior and posterior fascicles)
  • Purkinje fibers spread the impulse through the ventricles → QRS
The cardiac conduction system
Right atriumLeft atriumRVLVSA nodeAV nodeBundle of HisRight bundleLeft anterior fascicleLeft posterior fascicleSA node 60–100Atrial paths · AV node 40–60His bundleBundle branchesPurkinje 20–40

02Pacemaker hierarchy

Each tissue can fire on its own, but the fastest pacemaker wins. If a higher one fails, the next one 'escapes'.

  • SA node: 60–100 bpm
  • AV junction: 40–60 bpm (narrow QRS)
  • Ventricles: 20–40 bpm (wide QRS)
II
Junctional escape ~50 bpm: narrow QRS with an inverted P just before it.
II
Idioventricular escape ~35 bpm: wide, slow, no P waves.

Clinical pearl. The escape rate tells you where the rhythm comes from: narrow at 40–60 = junctional; wide at 20–40 = ventricular.

Key takeaways

  • SA → atria → AV node → His → bundle branches → Purkinje
  • AV nodal delay = PR segment
  • Escape rates: 60–100 / 40–60 / 20–40