What is the Cardiac Conduction System?
The cardiac conduction system is the heart's built-in electrical network that generates and spreads the signals which trigger each heartbeat. Specialized pacemaker cells and conducting fibers coordinate atrial and ventricular contraction so blood is pumped in the right order and rhythm.
The cardiac conduction system is a network of specialized cells — SA node, AV node, Bundle of His, and Purkinje fibers — that generates and conducts electrical impulses to make the heart beat in a coordinated rhythm.
- 1↓SA nodePacemaker in the right atrium fires ~60-100 signals per minute
- 2↓Atrial spreadImpulse spreads through both atria, triggering atrial contraction
- 3↓AV nodeDelays the signal about 0.1 s so the atria finish emptying first
- 4↓Bundle of HisCarries the signal into the interventricular septum
- 5↓Purkinje fibersRapidly spread the impulse through both ventricles
- 6Ventricular contractionVentricles contract together, ejecting blood
Step-by-step worked examples
A patient's SA node fires at 75 beats per minute with no conduction block. What is their heart rate and rhythm?
The SA node is the dominant pacemaker It fires 75 signals per minute, so every SA impulse produces one heartbeat Heart rate = 75 bpm, regular sinus rhythm
An ECG shows a PR interval (SA node to ventricles) of 0.20 s, at the upper normal limit. What does this reflect?
The PR interval is the time from atrial depolarization (P wave) to ventricular depolarization (QRS) Most of this delay happens at the AV node (about 0.1 s of the 0.12-0.20 s total) 0.20 s is still within the normal 0.12-0.20 s range, so conduction is normal but slow
If the SA node fails, the AV node takes over as pacemaker at 40-60 bpm instead of 60-100 bpm. Why is the new rate slower?
The SA node has the fastest intrinsic firing rate (60-100 bpm) so it normally sets the pace The AV node has a slower intrinsic rate (40-60 bpm) When the SA node fails, the AV node becomes the backup pacemaker at its own slower intrinsic rate
Flashcards
Quick quiz
Q1.Which structure is the heart's primary pacemaker?
Q2.What is the role of the AV node?
Q3.Which structures conduct the impulse fastest through the ventricles?
Q4.If the SA node fails, which structure typically becomes the backup pacemaker?
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Common mistakes
The AV node is the heart's main pacemaker. — Correct: The SA node is the primary pacemaker; the AV node is a backup with a slower intrinsic rate.
Electrical signals and blood flow travel through the same pathway. — Correct: The conduction system carries electrical impulses; blood flows through chambers and vessels — related but separate pathways.
The AV delay is a flaw in the system. — Correct: The AV node delay is essential — it lets the atria empty before ventricular contraction begins.
Purkinje fibers start the heartbeat. — Correct: The SA node initiates each heartbeat; Purkinje fibers only spread the signal through the ventricles at the end.
FAQ
What is the cardiac conduction system?
It's the heart's electrical network — SA node, AV node, Bundle of His, and Purkinje fibers — that generates and conducts the impulses controlling each heartbeat.
What is the order of the cardiac conduction pathway?
SA node → atria → AV node → Bundle of His → bundle branches → Purkinje fibers → ventricular muscle.
What is the function of the cardiac conduction system?
It coordinates the timing of atrial and ventricular contraction so the heart pumps blood efficiently and rhythmically.
What are examples of cardiac conduction system disorders?
Sick sinus syndrome (SA node failure), AV block (delayed or blocked AV conduction), and bundle branch block are common examples.