The present case report illustrates a mechanism that can interrupt cardiac resynchronization therapy (CRT) and lead to recurrence of dyspnea during exercise. It points out that CRT requires a regular cardiac history, analysis of device-stored data and profound knowledge about function and especially timing intervals of these systems in order to utilize the full potential of this therapy.
Keywords: Cardiac resynchronization therapy; Left ventricular sensing; Loss of resynchronization; Refractory periods; Upper tracking limit.