Electrophysiologic Mechanism

Ectopic impulses arise from irritable atrial or ventricular foci. The timing of these impulses relative to the preceding beat depends on:

• automaticity
• triggered activity
• reentry timing
• refractory periods

A fixed coupling interval suggests a reentrant or triggered mechanism.
A variable coupling interval suggests enhanced automaticity.

The coupling interval influences:

• aberrancy
• compensatory pauses
• bigeminy/trigeminy patterns
• risk of R‑on‑T phenomena

ECG Appearance

Measured from the R wave of a normal beat to the R of the ectopic beat or P wave of a normal beat to the P wave of the ectopic beat

PVCs often show a fixed coupling interval

PVCs with variable coupling suggest ventricular parasystole

PACs may show variable coupling

Short coupling intervals increase the risk of R‑on‑T events

Common Misinterpretations

• Confusing coupling interval with cycle length
• Assuming all PVCs have fixed coupling
• Mislabeling variable coupling as “random” ectopy

Clinical Implications

Helps determine the mechanism of ectopy

Short coupling intervals may indicate higher arrhythmic risk

Useful in differentiating PACs from PVCs when morphology is ambiguous

Related Terms

Compensatory pause

Bigeminy

R‑on‑T phenomenon

Premature ventricular contraction