Before interpreting an EP tracing, understand the normal route of electrical activation and the tissues that make that route possible.
What you should understand
SA nodeThe dominant physiologic pacemaker under normal conditions; activation spreads through the atria from this region.
AV nodeA specialized region with relatively slow, decremental conduction that connects atrial activation to the His-Purkinje system.
His-Purkinje systemThe His bundle, bundle branches and Purkinje network provide rapid conduction to coordinate ventricular activation.
Why this matters in the EP lab
Every diagnostic question in EP ultimately depends on sequence and timing. When activation appears earlier, later, reversed, blocked or unusually fast, the team uses those changes to localize the tissue or pathway involved.
EP tech habit
When a tracing changes, ask four questions: Where did activation start? What activated next? Did the sequence change? Did the timing change?
Learning objectives
SequenceTrace normal activation from the SA node through the atria, AV node, His-Purkinje system and ventricles.
Tissue behaviorRecognize that different tissues conduct and recover at different rates.
EP connectionUse sequence and timing changes as clues to where conduction is altered.
Conduction checkpoint
Question 1
Which structure normally introduces the most important physiologic delay between atrial and ventricular activation?
Question 2
If ventricular activation occurs unusually late after His activation, which conduction segment becomes especially relevant?
Trusted resources
Heart Rhythm Society — EP Lab Standards Laboratory processes, protocols, equipment, personnel, safety and quality. Open HRS resource →
ESC/EHRA — Heart Rhythm learning references Recommended reading and competency topics for invasive electrophysiology. Open ESC resource →
Educational scope: This lesson explains foundational concepts. It does not teach independent performance or interpretation of invasive EP procedures. Follow institutional training, current professional guidance, and manufacturer instructions.