exit block
Failure of a paced stimulus to depolarize cardiac tissue despite adequate electrical output from the device.
Full Definition
Exit block refers to a condition where the pacemaker generates and delivers an electrical stimulus, but the energy fails to propagate beyond the electrode-tissue interface to depolarize the surrounding myocardium. This phenomenon can occur due to excessive fibrosis around the lead tip, medication effects, electrolyte imbalances, or tissue necrosis. Unlike capture failure due to inadequate pacemaker output, exit block occurs despite normal or even elevated stimulus strength. The condition may be temporary (as seen with certain medications or electrolyte disturbances) or permanent (due to chronic fibrotic changes). Diagnosis requires careful evaluation of pacing thresholds, lead impedance measurements, and sometimes imaging studies to distinguish from lead fracture or displacement.
Usage
Usage note: Distinguish carefully from lead malfunction in clinical documentation.
In Context
- "Exit block developed six months post-implant, requiring lead revision despite normal impedance values." — Surgical case report
- "High-dose anti-arrhythmic therapy resulted in temporary exit block that resolved after medication adjustment." — Clinical management guideline